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International Journal of Heat and Mass Transfer
Journal Prestige (SJR): 1.498
Citation Impact (citeScore): 4
Number of Followers: 275  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 0017-9310 - ISSN (Online) 0017-9310
Published by Elsevier Homepage  [3155 journals]
  • Heat transfer methodology of microreactor based on Bandelet finite element
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Bin Zhao, Yi Ren, Diankui Gao, Lizhi Xu, Yuanyuan Zhang In order to study the heat transfer rule of microreactor in depth, the Bandelet finite element model is established using Bandelet basis function as interpolating function. Firstly, the mathematical model and property of Bandelet transformation are analyzed. Secondly, the heat transfer Bandelet finite element model of microreactor is constructed. Finally, the effectiveness of Bandelet finite element method is verified through comparing analysis between simulation and experimental results, the computing precision and accuracy of heat transfer of microreactor can be improved based on the proposed method. In addition, the relationship between Nusselt number and Reynolds number is analyzed. The inlet velocity has obvious effect on the heat transfer of microreactor. The influence of main geometric sizes concluding height and diameter of micro-pin-fin on heat transfer performance of microreactor is obtained. The analysis results can offer favorable basis for optimization of microreactor and enrich the theoretical system of heat transfer of microreactor.
  • Laser cooling arc plasma effect in laser-arc hybrid welding of 316L
           stainless steel
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Zhongyan Mu, Xin Chen, Zengchao Zheng, Anguo Huang, Shengyong Pang Current laser-plasma interaction theory supports that the plasma energy e.g. electron temperature would increase by the effect of inverse bremsstrahlung (IB) absorption, when a laser beam passed through the plasma. However, in this paper, we found an interesting laser cooling arc plasma effect (LCAPE) during kilo-Watt fiber laser-TIG hybrid welding. Based on theoretical modelling and experiments, we observed that a temperature decrease of more than 5000 K at the tail of the argon plasma occurred under different process parameters during hybrid welding of 316L stainless steel. We proposed the LCAPE is caused by the laser-induced metal vapor. The mechanism mainly includes the convection cooling and enhanced radiation of the arc plasma by the metal vapor. Our findings could broaden the theory of laser-plasma interaction and provide a theoretical reference to the modulation and control of plasma in industries.
  • Criteria of pressure and thermal damage during laser irradiation of port
           wine stains: Which is dominant to vascular lesions'
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Hao Jia, Bin Chen, Dong Li Port wine stains (PWSs) are congenital dermal vessel proliferations mainly treated with laser therapy. The complete removal of the vessel lesions is rarely achieved because of a lack of discriminatory analysis of the two competitive laser damages to blood vessels, namely, pressure damage and thermal damage. Unlike complete vessel constriction, which is caused by thermal damage that can be measured by temperature-related integral Ω, vessel rupture results from pressure damage, which has been seldom studied. In this study, the rupture potential index based on wall pressure (RPIP) was calculated as the ratio of locally acting pressure to the pressure threshold. RPIP > 1 and Ω > 103 were adopted as benchmarks to judge pressure damage (vessel rupture) and thermal damage (complete vessel constriction), respectively. A computational fluid dynamics simulation was carried out to provide the temperature and pressure field in the PWS vessel model during irradiation by 595 nm pulsed dye laser (PDL) or 1064 nm Nd:YAG laser. Numerical results showed that for the 595 nm laser, vessels constantly underwent rupture. The area of high RPIP determined the degree of rupture by predicting the large and multiple rupture locations of the vessel. By contrast, for the 1064 nm laser, complete constriction was the main damage type. To a single vessel of 100 μm diameter, the optimized laser parameters were E = 10 J/cm2 with tp = 6 ms for 595 nm PDL and E = 180 J/cm2 with tp = 6 ms 1064 nm for Nd:YAG laser.
  • In-line tube-bank heat exchangers: Arrays with various numbers of
           thermally participating tubes
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): J.M. Gorman, E.M. Sparrow, J. Ahn This investigation presents numerical results for both laminar and turbulent flow and heat transfer for an in-line tube bank ranging in size from 1 to 20 tube rows over a Reynolds number range of 100–1000. Both the longitudinal and transverse pitches were fixed at 1.5D. It was demonstrated that the most useful heat transfer results were expressible as a total-tube-bank-averaged Nusselt number value, which is in contradiction to other investigations. For sufficiently lengthy tube banks, the existence of a fully developed regime characterized by an axially unchanging array-average Nusselt number was identified. It was found that the highest array-average heat transfer coefficients occurred in the initial portion of the tube bank, also in contradiction with information conveyed in some of the literature. A special case in which only a single tube in the array was thermally active was investigated in deference to experiments conducted under that condition. The present results obtained by numerical simulation compared favorably to existing experimental data.
  • Assessment of laminar-turbulent transition models for Hypersonic
           Inflatable Aerodynamic Decelerator aeroshell in convection heat transfer
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Yatian Zhao, Chao Yan, Hongkang Liu, Yupei Qin Hypersonic Inflatable Aerodynamic Decelerator (HIAD) has shown its great potential for future planetary explorations. However, the HIAD surface deflections could both promote boundary-layer transition early and augment heating levels sharply, which poses challenges for survivability of a Thermal Protection System (TPS). The goal of this work is to assess different transition models for the prediction of hypersonic transitional flows over scalloping deformed HIAD surface and to seek the critical factor for their capabilities to provide a reference for the application and advancement. Three representative transition models are considered: γ-Reθ, k-ω-γ and kT-kL-ω. The results show that the undulating surface causes flow separations and reattachments in valleys and strong crossflow along the leeward. k-ω-γ model can correctly predict both the shapes and locations of the transition onsets. The transition zone predicted by γ-Reθ model is much small, while kT-kL-ω is incapable of transition prediction for such a complex and irregular configuration. Moreover, this study reveals that the crossflow instability plays a dominant role in the transition. The crossflow Reynolds number Recf, whose distributions are approximately consistent with the transition zone, could be a feasible crossflow strength indicator for the transition onsets on the undulating surface. Once k-ω-γ model excludes effects of crossflow instability, it predicts incorrect transition results for the deformed surface of HIAD. Besides, a detailed analysis shows that both the crossflow instability mode and the first disturbance mode in valleys are the major contributors to the transition on the leeward surface. Near the leeward ray, the transition is mainly determined by the first disturbance mode.
  • Development of vacuum impregnation equipment and preparation of
           mass/uniform shape-stabilized phase change materials
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Seong Jin Chang, Seunghwan Wi, Jongki Lee, Sumin Kim Various industries require production equipment for uniform mass-production short periods of time, such as in the construction industry. This study presents vacuum impregnation equipment that is capable of mass-producing uniform, shape-stabilized PCM (MUSPCM). The vacuum impregnation equipment can manufacture MUSPCM of various diameters without additional filtering and crushing process because the mixing ratio of the SSPCM was analyzed beforehand through a TGA analysis. As the result, MUSPCM using vacuum impregnation equipment can save 32.5 times based on production of 15.0 kg compared to conventional vacuum impregnation. In addition, the analysis of the thermo-physical properties and long-term stability of the MUSPCM were characterized via DSC and TGA analysis. The results indicate that a large amount of uniform MUSPCM were produced, and the long-term phase stability was excellent. Therefore, the developed vacuum impregnation equipment is expected to be useful in various industries, including construction.
  • Flow regime identification and classification based on void fraction and
           differential pressure of vertical two-phase flow in rectangular channel
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Vikrant Siddharudh Chalgeri, Ji Hwan Jeong Better understanding of two-phase flow in narrow rectangular channels is important because it is considerably different from that in round tubes. Two-phase flow in narrow rectangular channels is invariably employed in most of the research reactors that use plate-type nuclear fuels around the world, high-heat-flux compact heat exchangers, and high-performance micro-electronics. In this study, an objective attempt was made to identify and classify various flow regimes in co-current air–water two-phase flow for vertical upward and downward direction flow inside a narrow rectangular channel. The void fraction and differential pressure data of the flow were measured. Flow regimes were identified based on visual observation, mean void fraction, and differential pressure data. A high-speed camera was used to capture images, and the void fraction was measured and analyzed by the electrical impedance method. In addition, digital image analysis was conducted, and differential pressures were measured using differential pressure transducers. Four flow patterns were identified for the vertical upward flow, and seven flow patterns were identified for the vertical downward flow in a narrow rectangular channel.
  • The effect of acoustic wave on the stability of stationary convective flow
           of binary fluid in a horizontal layer subjected to horizontal temperature
           and concentration gradients
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): T.P. Lyubimova, R.V. Skuridin The effect of acoustic wave propagating in the direction of temperature gradient on a stability of stationary plane-parallel flow of binary fluid, generated by horizontal temperature and concentration gradients in a horizontal layer is investigated for two types of thermal boundary conditions: perfectly conductive and adiabatic boundaries. The study is performed analytically for the longwave perturbations and numerically for finite-wavelength perturbations. Stability maps for fixed Prandtl number value Pr=0.01 and different values of Schmidt number and acoustic Reynolds number are obtained. The most dangerous instability modes and critical perturbation structure are discussed. Comparison with the case where acoustic wave is absent is performed. It is shown, that longwave perturbations are always stabilized by acoustic wave. Numerical investigation demonstrates, that acoustic wave also stabilizes the flow with respect to finite-wavelength perturbations when negative concentrational Grashof number is higher in modulus than certain value depending on acoustic wave intensity.
  • Onset of buoyancy driven convection in an inclined porous layer with an
           isobaric boundary
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Michele Celli, Antonio Barletta The linear stability analysis of a fluid saturated porous layer is carried out. The porous layer is inclined to the horizontal and is infinitely wide. One boundary of the layer is permeable while the other one is impermeable. The two boundaries are subject to different temperatures, so that convective instability may arise when such a temperature difference exceeds a threshold value. The basic state whose stability is studied consists of a single cell with no net mass flow rate. The critical values of the governing parameters are computed numerically and are presented as functions of the inclination angle. The threshold relative to the horizontal layer recovers the results already present in the literature. The inclination angle comes out to be a stabilising parameter such that the vertical layer cannot become unstable.
  • Enhancement of pool boiling heat transfer in water on aluminum surface
           with high temperature conductive microporous coating
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Juan C. Godinez, Dani Fadda, Jungho Lee, Seung M. You The effects of an aluminum high temperature conductive microporous coating (Al-HTCMC) on the nucleate boiling heat transfer (NBHT) coefficient and critical heat flux (CHF) are studied in saturated distilled water at 1 atm. Aluminum powders with three different mean particle diameters (dm = 11, 24, and 66 µm) are used in the fabrication of the Al-HTCMC. For each mean particle diameter, an optimal coating thickness to yield the highest NBHT coefficient is determined. The optimized Al-HTCMC thickness is found to result in comparable NBHT coefficients regardless of the particle diameter. Pool boiling tests with a plain aluminum surface are used for comparison. The coated and plain aluminum surfaces are treated equally before the pool boiling tests to establish a Boehmite oxidation nano layer on the aluminum surfaces. Following the Boehmite treatment, the contact angle is unmeasurable (∼0°) with the Al-HTCMC surface and 12° with a plain aluminum surface. Then, pool boiling tests are performed and reveal comparable CHF (1725–1850 kW/m2) values with or without the Al-HTCMC. However, the Al-HTCMC is shown experimentally to improve the NBHT coefficient by a factor of five as the wall superheat is reduced by from 31 K to 6 K just before CHF. The results obtained are also compared to similar work using an HTCMC layer on a copper surface to demonstrate the performance of the Al-HTCMC.
  • A simplified method for calculating the heat rejection from a rectangle
           droplet sheet
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Chen Zeng, Sichao Tan, Shouxu Qiao, Fulong Zhao, Tao Meng To calculate the thermal characteristics of the rectangle liquid droplet radiator more accurately, a novel method, which estimated the characteristics of each droplet, was introduced. The shape factors of two and three droplets were theoretically deduced and verified by the Monte Carlo method. It was found that the shape factor of two droplets depends only on the ratio of the distance between the two droplets to the radii of droplets. According to this characteristic, the shape factor of two droplets was calculated and fitted to be a formula, which makes it convenient to calculate the shape factor in the following analysis. Based on the superposition rule of the shape factor, a shape factor matrix was introduced to calculate the effect of surrounding droplets when analyzing the heat transfer of the droplet sheet. The temperature distribution of the droplet sheet was calculated based on the shape factor matrix, which obtained using the combination of theoretical method and Monte Carlo method. The results showed that the temperature difference among the droplets on a same width-thickness plane was negligible. Therefore, a simplified model was developed based on this characteristic. The discrepancies of the temperature distributions obtained by the original model and the simplified model were found to be less than 0.35%, which demonstrates the feasibility of the simplified model to analyze the thermal characteristics of the rectangle droplet radiator. By further analyzing the heat transfer characteristics of the droplet sheet, proposals were made for design of the rectangle liquid droplet radiators.
  • Mass transfer from a soluble wall into gas-liquid slug flow in a capillary
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): M.C.F. Silva, J.B.L.M. Campos, J.D.P. Araújo The mechanism of wall-liquid mass transfer of a solute in micro-scale systems has a huge relevance in many practical scenarios with particular interest for medical devices. A possible enhancement on this kind of phenomenon through the application of slug flow regime was studied with CFD techniques. Different flow conditions were considered to enable the inspection on the distinct hydrodynamics that may occur on the Taylor bubble surroundings in micro-scale. The VOF methodology was used to track the gas–liquid interface and the mass and hydrodynamic fields were simultaneously solved.The effects of the bubble passage on the mass transfer from a finite soluble wall to the flowing fluid were analyzed for each flow condition, and the corresponding mass transfer coefficients were quantified.Overall, this numerical work indicates that the flow due to the presence of one Taylor bubble leads to a moderate increase of the wall-liquid mass transfer coefficients. This increase can be enhanced if, instead of one, a continuous flow of bubbles is considered. The abrupt variation on the wall shear stress induced by the bubble movement is important to promote the referred increase.
  • A numerical study on the buoyancy effect around slanted-pin fins mounted
           on a vertical plate (Part-I: Laminar natural convection)
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Yeong Woong Oh, Yoon Suk Choi, Man Yeong Ha, June Kee Min A numerical study on the natural convection heat transfer of slanted-pin fins mounted on a vertical wall has been conducted for the laminar flow regime. Assuming the air is the ideal gas, three-dimensional governing equations of flow and heat transfer was solved using the periodic boundary condition in the horizontal lateral direction with the SIMPLE algorithm. The effect of radiation heat transfer was considered using the discrete ordinate method based on the evidence of the code validation through a comparison with experimental data. The effects of the fin-inclination angle ranging from −45° to + 45° and the aspect ratio of the rectangular-fin pins for 0.25–4.0 were examined for a modified Rayleigh number range of 5.2 × 1010–1.3 × 1010 under constant heat flux conditions. For the positively-inclined fins, the enhancement of the heat transfer performance on the heated plate and fin-side surface was captured, which is similar to previous forced convection studies. For negative inclination angles, however, it was observed that the penetration of cold air from the quiescent region affects the heat transfer coefficient distribution on the top side of the fins in the vertical fin interspacing. As a result, in the present calculation, the negatively-inclined fins showed the best heat-transfer performance under a natural convection condition. Details on the buoyant-flow and heat transfer characteristics, such as the distributions of the local- and average-heat transfer coefficient, are quantitatively summarized.
  • Estimation of the time-dependent convective boundary condition in a
           horizontal pipe with thermal stratification based on inverse heat
           conduction problem
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): W.W. Han, H.B. Chen, T. Lu The time-dependent convective boundary condition on the inner wall are the essential causes of flow and heat transfer that induce thermal fatigue in pipes with thermal stratification. A three-dimensional transient inverse heat conduction problem (IHCP) was developed in this work for estimating simultaneously the multi-variables of the time-dependent convective boundary condition. Conjugate gradient method (CGM) is chosen as the optimization method. The known parameter in the IHCP is the measuring temperature on the outer wall. Experiments under different seven operating conditions have been taken for the validation of the method. Based on the estimated results, the influence of volume flow rate of cold water on the conductive heat transfer coefficient near the inner wall in the test section and the temperature distribution of the pipe wall were discussed.
  • Correlations for prediction of the bubble departure radius on smooth flat
           surface during nucleate pool boiling
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Xueli Wang, Zan Wu, Jinjia Wei, Bengt Sundén Based on a modified force balance model, new correlations were proposed for the prediction of vapor bubble departure radius in saturated and subcooled pool boiling under atmospheric pressure. To predict the departure radius, the wall temperature and contact angle are two important input parameters. Instead of the static contact angle, the present correlations use the dynamic advancing contact angle at root of the bubble base at the moment before bubble detachment (i.e., the maximum dynamic advancing contact angle) to calculate the bubble departure radius. The results show that for the bubble departure radius obtained in this study, the developed correlation can predict all the data points within a maximum error of 3.8% in both normal earth gravity and 0.01ge reduced gravity. Moreover, for data sets in the literature including 1g saturated boiling, 1g subcooled boiling, saturated boiling in reduced gravity, and subcooled boiling in reduced gravity, it is also demonstrated that compared with the thirteen existing correlations, the proposed correlations exhibit a big improvement in predicting bubble departure radius.
  • Numerical investigation of wafer drying induced by the thermal Marangoni
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Changkun Li, Dewen Zhao, Jialin Wen, Xinchun Lu Marangoni drying induced by organic vapor is an important process in integrated circuit manufacturing to obtain an ultra-clean surface, but its application is limited by the utilization of flammable vapor. Thermal Marangoni drying induced by a temperature gradient is a safe and environmentally friendly technology. However, the regulation of thermal Marangoni-driven flow is complicated, and current understanding of the thermal Marangoni drying mechanism is inadequate. In this work, we present a coupled model of thermal Marangoni drying that combines the two-phase flow, heat transfer, and water vapor transfer in air. The evolution of entrained water film thickness, dynamics of Marangoni-driven flow, and evaporative cooling are numerically simulated. The results show that the achieved minimum residual thickness of the entrained water film is thinned more than tenfold compared with that of the wafer withdrawn without the thermal Marangoni effect. The temperature rise and Marangoni stress grow dramatically in the film and meniscus at the initial time, and then they remain as almost invariant after achieving the dynamic equilibrium between the heating and evaporative cooling. The convective water vapor transfer in air and the reduction of entrained water film thickness improve the evaporative flux, which in turn suppresses the increase in the thermal Marangoni effect until the dynamic equilibrium is attained. Furthermore, the higher heat source, lower wafer thermal conductivity and smaller wafer thickness can enhance drying performance. The investigation of drying dynamics will contribute to a comprehensive understanding of the thermal Marangoni drying process and provide guidance to its industrial applications.
  • A reduction of thermal conductivity of non-periodic Si/Ge superlattice
           nanowire: Molecular dynamics simulation
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Chun Wei Zhang, Hai Zhou, Yong Zeng, Lei Zheng, Yue Lin Zhan, Ke Dong Bi The thermal conductivity (TC) of non-periodic Si/Ge superlattice nanowire (SLNW) is investigated by non-equilibrium molecular dynamics simulation (NEMD). It is found that the TC of non-periodic Si/Ge SLNW can be significantly reduced at room temperature. Compared to the minimum TC of periodic Si/Ge SLNWand pure Si nanowire, the TC of non-periodic Si/Ge SLNW is further decreased to be around 47.4%, 4.4% of that of periodic Si/Ge SLNW and pure Si nanowire respectively. By introducing 20% Ge atom doping, the TC of non-periodic Si/Ge SLNW with 10-unit cell (UC) thickness is reduced by 38%. The reduction of TC of non-period Si/Ge SLNW is first due to the destruction of phonon coherent transport. Additionally, the change of periodic length can cause shift of density of state (DOS), which makes the interface of Si/Ge with different lengths play roles to scatter the phonon with different wavelengths. Therefore, the distribution of thickness from atomic to nano scale can decrease the TC by scattering phonon with more wavelengths. The results provide an efficient way for future thermoelectric applications.
  • New retrofit method to improve the thermal performance of natural draft
           wet cooling towers based on the reconstruction of the aerodynamic field
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Xuehong Chen, Fengzhong Sun, Youliang Chen, Ming Gao The heat transfer deterioration that occurs in the inner rain zone weakens the thermal performance of natural draft wet cooling towers (NDWCTs). Existing NDWCT retrofit methods including the air deflectors and the cross wall have limited effects on this deterioration. In this paper, we propose a new retrofit method in which air ducts are installed in the rain zone and air deflectors are installed around the air inlet to improve the total tower thermal performance. To clarify the effect and mechanism of our retrofit method, a hot test for a NDWCT model is performed under various crosswind velocities, and a 3D numerical model for a NDWCT with air deflectors and air ducts is established and validated. Using the proposed method, the thermal performance of a NDWCT is substantially improved with less crosswind sensitivity. It is found that the flow diversion efficiency of the air deflectors weakens the adverse impact of the ambient crosswind on air inflow of the tower, and the additional ambient air introduced through the air ducts enhances the heat transfer in the central rain zone. Compared with the single effect of the air deflectors or the cross wall, the combined effect of the air ducts and air deflectors is more efficient in improving the thermal performance of NDWCTs.
  • Experimental investigation on pressure oscillations of steam jet
           condensation through multi-holes
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Quanbin Zhao, Weixiong Chen, Lutao Wang, Daotong Chong, Junjie Yan The pressure oscillation characteristic of steam jet condensation in water was really important for the safety operation of related devices. In present study, the pressure oscillation characteristic of steam condensation through two and three holes would be investigated, and the character of dominant oscillation frequency with operation conditions (steam mass flux and water temperature) would be investigated. Meanwhile, the impact of geometrical factors (hole pitch and number) would also be analyzed. The experimental results indicated that the dominant oscillation frequency would decrease with water temperature increasing. Then as steam mass flux increased, the dominant frequency had a peak value for three holes nozzle, which always appeared when the steam mass flux varied from 250 to 300 kg·m−2·s−1. The dominant frequency increased as the hole pitch increased, and the dominant frequency difference between hole pitches gradually grew smaller as the water temperature raised. Meanwhile, the dominant frequency would decline as hole number increased. Besides, a experimental correlation considering the effect of hole number and pitch was applied to predict dominant oscillation frequency, while the maximum deviation was less than 15%.
  • Steady secondary flow in a turbulent mixing layer
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): V.B. Zametaev, A.R. Gorbushin, I.I. Lipatov Turbulent mixing layer between the moving 2D viscous incompressible fluid and fluid in rest is studied. The characteristic Reynolds number of the flow is assumed to be large and the layer thickness being small. To analyze the problem, the method of multiple scales was applied, which allowed to find and investigate the steady secondary flow inside the turbulent mixing layer. Self-induced entrainment of fluid from the external stream is the main flow in this case, which ensures the supply of kinetic energy from the maximum speed zone to the turbulence generation zone. Secondary steady solutions were found analytically for the longitudinal velocity component. The found solutions were compared with the available experimental data. Understanding of turbulence nature has a principal value for mass and heat transfer in different flows.
  • Comparison of steady and transient flow boiling critical heat flux for
           FeCrAl accident tolerant fuel cladding alloy, Zircaloy, and Inconel
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Soon K. Lee, Maolong Liu, Nicholas R. Brown, Kurt A. Terrani, Edward D. Blandford, Heng Ban, Colby B. Jensen, Youho Lee Steady and transient (with a heating rate of 685 °C/s) internal-flow CHF (Critical Heat Flux) experiments were conducted under atmospheric pressure at a fixed inlet temperature (40 °C or 60 °C) and mass flow (300 kg/m2 s) on Fe-13Cr-6Al alloy, Inconel 600 and Zircaloy-4 tube samples. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., roughness, wettability, and oxide scale morphology) on the occurrence of CHF. Despite notable changes of wettability, roughness, and oxide layer characteristics on samples that had already been subjected to CHF, measured flow CHF remained unchanged throughout repeated experiments for tested materials. This demonstrates that the surface effects on flow CHF are limited in the test conditions. In the steady-state flow boiling condition, Fe-13Cr-6Al alloy demonstrated a 22% and 14% increase in CHF compared to Zircaloy-4 and Inconel 600, respectively. Compared to the 2006 Groeneveld CHF lookup table, Fe-13Cr-6Al alloy gives a 13% increase in the tested flow boiling condition. Material properties are considered primarily responsible for the observed CHF differences among the tested materials. The surface thermal economy parameter (ρcp3/2k) is proposed as an explanation for the observed CHF differences; this parameter is related to material’s ability to avoid an irreversible dry spot formation. The apparent disagreement of Zircaloy-4 CHF with both the look up table predictions and Inconel 600 shows the limitation of departure of nucleate boiling (DNB) evaluations that do not consider cladding materials. The transient Fe-13Cr-6Al CHF is 39% and 23% higher than the lookup table prediction and the steady-state condition experimental result, respectively.
  • Experimental and numerical investigation on shell-side performance of a
           double shell-pass rod baffle heat exchanger
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Xinting Wang, Yunmin Liang, Yue Sun, Zhichun Liu, Wei Liu In this paper, the heat transfer and flow performance of a double shell-pass rod baffle heat exchanger (DS-RBHX) is investigated experimentally. Likewise, a single shell-pass rod baffle heat exchanger (SS-RBHX) is set as the control. Water serves as the working fluid both in the shell side and tube side. Experimental results indicate that the overall heat transfer coefficient of the DS-RBHX is higher than that of the SS-RBHX for all measurements. As the shell-side volume flow rate varies from 2.8 to 15.2 m3/h, the shell-side heat transfer coefficient and pressure drop of the DS-RBHX increase by 33.5–54.0% and 34.0–74.3%, respectively. From the perspective of the comprehensive performance, the shell-side heat transfer coefficient of the DS-RBHX is 14.4–24.3% higher than that of the SS-RBHX under the same shell-side pressure drop. Consequently, it is proved that the DS-RBHX has better comprehensive performance compared with the SS-RBHX. On the basis of experimental results, numerical studies are conducted to analyze the shell-side behaviors of the DS-RBHX further. According to numerical results, three kinds of guide shells, arranged at the end of the sleeve, are proposed to reduce the flow dead zone in the shell-side outlet zone. The behaviors of DS-RBHXs with the guide shell (DS-RBHX-GSs) are obtained numerically. The results show that all three guide shells improve the heat transfer performance of the shell-side outlet zone, particularly in the outer side. Moreover, the guide shell of the DS-RBHX-GS2 has more significant effects than the others.
  • Experimental investigation of liquid nitrogen cavitating flows in
           converging-diverging nozzle with special emphasis on thermal transition
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Tairan Chen, Hui Chen, Wendong Liang, Biao Huang, Le Xiang The objective of this paper is to investigate the dynamic evolution of unsteady liquid nitrogen cavitating flows in a wide range of free-stream conditions and propose a thermal parameter to evaluate and predict the transition process of two typical cavitation dynamics in liquid nitrogen. The dynamic evolutions of liquid nitrogen cavitating flows in a converging–diverging (C-D) nozzle with a throat height of 2.0 mm under a wide range of free-stream conditions were experimentally investigated. Experiments were carried out in liquid nitrogen with the temperature range from 68 K to 86 K, the pressure in the tanks is within the range of 30–300 kPa. The results show that two typical cavitation dynamics, namely the quasi-isothermal mode and the thermo-sensitive mode were observed under similar cavitation number and Reynolds number with the increasing throat temperature. The cavitation dynamics transits from the quasi-isothermal mode to the thermo-sensitive mode with the increasing throat temperature, and the transition temperature (transition mode) is approximately at 77–78 K. In the quasi-isothermal mode, the shedding cavity with clear interface collapses immediately after shedding. The cavity area increases with the increasing temperature under similar cavitation number and Reynolds number. In the transition mode, the magnitude of cavity area, the time duration of the shedding process, as well as the cavitation aggressiveness reaches the maximum values. In the thermo-sensitive mode, the shedding cavity turns to be mushy and frothy, and the mushy interface collapses slowly after shedding. The cavity area decreases with the increasing temperature under similar cavitation number and Reynolds number. When the thermodynamic effects completely dominate the change of the cavitation dynamics in the thermo-sensitive mode, the cavitation process becomes more stable. The shedding cavity collapses more slowly, while moves more quickly. The thermal parameter C-factor could quantitatively evaluate and predict dynamics transition from the quasi-isothermal mode to the thermo-sensitive mode in liquid nitrogen cavitating flows. The transition mode (transition temperature) should be prevented from causing the maximum cavitation aggressiveness in liquid nitrogen apparatus or system.Graphical abstractGraphical abstract for this article
  • Distribution of gas-liquid two-phase slug flow in parallel micro-channels
           with different branch spacing
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Yanchu Liu, Shuangfeng Wang An experimental study was conducted in order to investigate the phase distribution of gas-liquid slug flow in six parallel micro-channels in presence of different branch spacing of 0.8 mm(2d), 4 mm(10d) and 12 mm(30d), respectively. The parallel micro-channels was composed of a header with hydraulic diameter of 0.48 mm and six branch channels with hydraulic diameter of 0.40 mm, all with rectangle cross sections. The entire test section was machined by PMMA to facilitate flow visualization. Nitrogen and 0.03 wt% sodium dodecyl sulfate (SDS) solution at ambient pressure and room temperature were used as the test fluids. A flow-regime map of bubbly, slug, slug-annular and annular was generated, covering the range of gas and liquid inlet superficial velocities of 0.28 ≤ JG ≤ 33.3 m/s and 0.008 ≤ JL ≤ 2.52 m/s, respectively. The phase distribution experiments of slug flow were conducted. The fluid dynamics of two-phase flow splitting in parallel micro-channels was captured by high speed recording technique. It was found that the phase distribution characteristics of two-phase flow in parallel channels highly depend on the inlet flow conditions and the distance between channels. Besides, the effect of branch spacing took on distinct characteristics under different inlet flow conditions. At low mass flux and quality, the increase of branch spacing can facilitate the liquid phase to flow into channels at the rear part of the header, while the first three channels are more supplied with liquid as the branch spacing increasing at high mass flux and quality. Specially, an improvement of gas distribution was also observed with the increase of the branch spacing. Finally, a correlation capable of predicting the liquid phase distribution of slug flow in parallel micro-channels was developed.
  • Discovering the active subspace for efficient UQ of molecular dynamics
           simulations of phonon transport in silicon
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Manav Vohra, Sankaran Mahadevan This paper develops an efficient methodology for both forward and inverse problems in uncertainty quantification with respect to molecular dynamics simulation. Specifically, our objectives are to investigate the impact of uncertainty in the Stillinger-Weber (SW) potential parameters on NEMD-based predictions of bulk thermal conductivity of silicon (forward problem), and perform a Bayesian calibration of these parameters using experimental data (inverse problem). However, both analyses typically require tens of thousands of model evaluations and therefore, relying purely on atomistic simulations would be impractical. The common strategy of building a surrogate model in the space of the uncertain parameters is also unaffordable due to the need for many training evaluations using atomistic simulations. Therefore, computational effort is minimized in this paper by reducing the dimensionality of the input space of the surrogate model by first computing the so-called active subspace. The active subspace is found to be 1-dimensional, indicating enormous scope for dimension reduction and computational savings. A surrogate model is then built in the 1-dimensional subspace to help quantify the variability of the bulk thermal conductivity, and is shown to have reasonable accuracy. The active subspace is also used to perform efficient global sensitivity analysis (GSA) of the SW parameters. Finally, we use the active subspace-based surrogate model for fast calibration of SW parameters in a Bayesian setting.
  • Experimental analysis of the thermohydraulic performance of graphene and
           silver nanofluids in automotive cooling systems
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Edwin Martin Cárdenas Contreras, Guilherme Azevedo Oliveira, Enio Pedone Bandarra Filho This paper presents an experimental investigation of the thermohydraulic performance of nanofluids, composed of graphene and silver nanoparticles with a binary mixture of equal parts of water and ethylene glycol (50:50 vol%) as a base fluid, in automotive radiators. The nanofluids were prepared by high pressure homogenization method with volumetric concentrations of 0.01%, 0.05% and 0.1%. The thermophysical properties were measured experimentally and compared with correlations and others results of similar research found in the literature. The nanofluids were tested in an automotive radiator installed in a wind tunnel, simulating the operation of an automotive cooling system. The experiments were conducted at mass flow rates between 0.08 and 0.11 kg/s, with coolant inlet temperatures between 55 and 85 °C. The air velocity on the radiator was kept constant at 2.1 m/s. The heat transfer rate and the pumping power of the fluids tested were determined under the test conditions stipulated. With regard to the pumping power at high temperatures and mass flow rates, the nanofluids showed increases up to 4.1%. The silver nanofluids produced an increase up to 4.4% in the heat transfer rate, while the graphene samples demonstrated a decrease in thermohydraulic performance when compared with the base fluid.
  • Analysis of the ideal gas flow over body of basic geometrical shape
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Sergei V. Ryzhkov, Victor V. Kuzenov An approximate mathematical model of the heat transfer processes in the laminar and turbulent boundary layers, which occur near the surface of an aircraft moving at the hypersonic speed in the Earth’s atmosphere, is derived. This mathematical model makes it possible to calculate the convective heat transfer on the surface of typical structural elements of modern perspective aircrafts. 2D versions of the calculations of convective heat fluxes for bodies of simple geometric shapes are performed.
  • Experimental investigation on the characteristics of melt jet breakup in
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Hui Cheng, Jiyun Zhao, Jun Wang In the present study, an experimental apparatus is set up to investigate the characteristics and mechanism of melt jet breakup in water in the premixing phase of fuel-coolant interaction during nuclear reactor severe accident. Wood’s metal is used as the simulant material. The melt jet breakup experiments are conducted under different jet diameters, melt temperatures, water temperatures and penetration velocities. The breakup process is recorded by a high speed video camera. The characteristics of the breakup of the leading edge and the jet column are analyzed in detail. It is concluded that Rayleigh-Plateau instability has the dominant effect on the melt jet column breakup in water.
  • Experimental study on distribution parameter characteristics in vertical
           rod bundles
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Ting-pu Ye, Liang-ming Pan, Quan-yao Ren, Wen-xiong Zhou, Tao Zhong The drift-flux model is important for rod bundle two-phase flow analysis. A 5 × 5 rod bundles experiment has been performed under air-water two-phase flow conditions. The superficial liquid velocity ranges from 0.00 to 1.50 m/s while the superficial gas velocity ranges from 0.02 to 6.00 m/s. The void fraction has been measured by the impedance meter. Four typical drift-flux models developed for rod bundles have been compared with present data. By analyzing the experimental data and visualization, the distribution parameter may be influenced by the flow recirculation and turbulence significantly. In order to consider these two effects, a critical liquid velocity has been defined to determine which effect is dominant as the increase of liquid velocity, and a new critical void fraction correlation related to the flow-regime transition from cap-bubbly to cap-turbulent flow is incorporated to determine the distribution parameter. The new drift-flux model developed from existing models shows a good prediction capability.
  • A numerical study on the buoyancy effect around slanted-pin fins mounted
           on a vertical plate (Part-II: Laminar mixed convection)
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Yeong Woong Oh, Yoon Suk Choi, Man Yeong Ha, June Kee Min This paper presents a numerical study on the mixed convection heat transfer of slanted-pin fins mounted on a vertical heated wall for the laminar flow regime, which is an extension of the authors’ accompanying study (Oh et al. 2018) on the natural convection. Using the ideal gas assumption for the air, three-dimensional governing equations for fluid flow and heat transfer were calculated by imposing a periodic boundary condition in the horizontal lateral direction with the SIMPLE algorithm. The discrete ordinate method was also included to calculate the effect of radiation. The effects of the fin-inclination angle were investigated in a modified Richardson number range of 500–2500 by varying the Reynolds number at a fixed Grashof number. It was found that as the forced convection increases, the fin-inclination angle showing the best heat transfer performance changes from negative to positive fin angles. The critical Richardson number for this transition and the corresponding flow and heat-transfer characteristics in the mixed convection regime are summarized.
  • Heat transfer enhancement in a loop thermosyphon using nanoparticles/water
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): V. Kiseev, O. Sazhin This research focuses on two-phase thermal control systems, namely loop thermosyphons (LTS) filled with nanofluids and their use as LED cooling devices. The behavior of the fluid in the thermosyphons and the mechanisms explaining the possible impact of nanoparticles on the thermal properties of the working fluid, as well as the processes in the LTS, are addressed. Nanoparticle distribution in the nanofluid, methods of preparing nanofluids and the nanofluid degradation processes (aging) are studied. The results are obtained from a set of experiments on thermosyphon characteristics depending on the thermophysical properties of the working fluid, filling volume, geometry and nanoparticle mass concentrations. The impact of nanofluids on the heat-transfer process occurring inside the thermosyphon is also studied. The results indicate the strong influence of nanoparticles on the thermal properties of the thermosyphons, with up to a 20–25% increase in the heat transfer coefficient. It is shown that this effect is due to the aggregation of nanoparticles and the formation of a micro/nano relief on the vaporization surface. Additionally, a method of calculating the hydrodynamic limit of the LTS is proposed, which allows for estimation of the maximum heat that can be transferred by means of the LTS. The nanofluids are shown to be effective means for enhancing heat transfer in two-phase thermal management systems.
  • Ledinegg instability-induced temperature excursion between thermally
           isolated, heated parallel microchannels
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Todd A. Kingston, Justin A. Weibel, Suresh V. Garimella Two-phase flow through heated parallel channels is commonly encountered in thermal systems used for power generation, air conditioning, and electronics cooling. Flow boiling is susceptible to instabilities that can lead to maldistribution between the channels and thereby heat transfer performance reductions. In this study, the Ledinegg instability that occurs during flow boiling in two thermally isolated parallel microchannels is studied experimentally. A dielectric liquid (HFE-7100) is delivered to the parallel channels using a constant pressure source. Both channels are uniformly subjected to the same power, which is in increased in steps. Flow visualization is conducted simultaneously with pressure drop, mass flux, and wall temperature measurements to characterize the thermal-fluidic effects of the Ledinegg instability. When the flow in both channels is in the single-phase regime, they have equal wall temperatures due to evenly distributed mass flux delivered to each channel. Boiling incipience in one of the channels triggers the Ledinegg instability which induces a temperature difference between the two channels due to flow maldistribution. The temperature difference between the two channels grows with increasing power until boiling incipience occurs in the second channel. The wall temperatures of both channels then reduce significantly as the flow becomes more evenly distributed. The experimentally observed temperature excursion between the channels is reported here for the first time and provides an improved understanding of the thermal performance implications of the Ledinegg instability in thermally isolated parallel channels.Graphical abstractGraphical abstract for this article
  • A low-Mach methodology for efficient direct numerical simulations of
           variable property thermally driven flows
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): A.D. Demou, C. Frantzis, D.G.E. Grigoriadis Thermally driven flows can be approximated as constant property flows only when temperature differences are relatively small. In that case, the Oberbeck-Boussinesq approximation holds and the set of governing equations is simplified. For larger temperature differences, the variation of the fluid properties with temperature cannot be ignored and in the case of gasses the governing equations take the low-Mach form. This significantly complicates the numerical solution of variable property against constant property flows because of the emergence of a variable coefficient Poisson equation for the pressure.In the present study, a numerical methodology for the direct numerical simulation of thermally driven low-Mach flows is presented. In this framework, a pressure-splitting scheme is utilised to transform the variable coefficient Poisson equation for the pressure into a constant coefficient Poisson equation, improving the efficiency of the numerical solution. The consistent boundary conditions for the pressure are derived and presented. Furthermore, the proposed methodology is validated for the natural convection of air inside a differentially heated cavity, for a wide range of temperature differences, both within and outside the limits of applicability of the Oberbeck-Boussinesq approximation. Finally, to demonstrate the potential of this methodology, the three-dimensional natural convection of air inside a differentially heated cavity is simulated for a Rayleigh number Ra=2.0×109 and for temperature differences ΔT=50 K, 100 K, and 200 K. It is found that, with increasing temperature difference, the symmetry around the centre of the cavity that characterises the Oberbeck-Boussinesq solution is lost. In addition, the laminar-turbulent transition point on the heated and cooled walls changes position, moving to a more upstream position on the heated wall and a more downstream position on the cooled wall.
  • Mathematical model of injection of liquid carbon dioxide in a reservoir
           saturated with methane and its hydrate
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): M.K. Khasanov, M.V. Stolpovsky, I.K. Gimaltdinov Based on the equations of mechanics of multiphase media, a mathematical model of the injection of liquid carbon dioxide into a natural reservoir saturated with methane and its gas hydrate is constructed. Numerical solutions for two different flow regimes are obtained and investigated. In the first regime, methane is replaced by carbon dioxide in the gas hydrate without the release of free water. In the second mode, gas hydrate decomposes into methane and water, and further formation of CO2 gas hydrate from water and carbon dioxide occurs. Numerical calculations show that the first mode is realized at low values of permeability and injection pressure, as well as high values of the temperatures of injected carbon dioxide, initial temperature of the reservoir and pressure at the right boundary of the reservoir. For each regime, the conditions under which the pressure in the liquid carbon dioxide filtration region can drop below the equilibrium boiling point of CO2 are investigated. Numerical calculations show that this is possible at low injection pressures and pressures at the right boundary of the reservoir. Critical diagrams were constructed that determine the “injection pressure-pressure on the right-hand boundary of the reservoir”, “injection pressure-permeability”, “injection pressure-injection temperature” and “injection pressure-initial temperature of the reservoir” in the region of existence of four qualitatively different flow regimes.
  • Lattice Boltzmann equation for mass transfer in multi solvent systems
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): J.H. Lu, H.Y. Lei, C.S. Dai The key problem for simulating mass transfer in multi solvent systems is the concentration jump at the phase interface described by Henry’s law. In the present paper, after analyzing the shortages of Fick’s law in describing mass transfer in multi solvent systems, the revised Fick’s law that can be applied to mass transfer in multi solvent systems is proposed. The corresponding governing equation can describe the concentration jump directly without any modification and doesn’t need to transform the scalar or parameters. To solve the governing differential equation based on the revised Fick’s law, a multi-relaxation-time (MRT) lattice Boltzmann equation (LBE) is constructed. Five test cases containing both the transient and steady mass transfer problems in multi solvent systems with straight or curved interfaces are conducted to validate the proposed MRT LBE. The results show that proposed MRT LBE is easy in implementation and capable of simulating both steady and transient mass transfer in multi solvent systems. In addition, the comparison indicates that the MRT LBE could have better accuracy than single-relaxation-time (SRT) LBE by keeping an optimal relation between two relaxation times.
  • Effect of inclination on heat transfer coefficient during flow boiling in
           a mini-channel
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Thibaut Layssac, Stéphane Lips, Rémi Revellin An experimental study of heat transfer during R245fa flow boiling is carried out in a 1.6 mm inner diameter circular channel (198 original data points). The test section is composed of a sapphire tube coated with ITO, which enables a total transparency of the evaporator in the visible spectrum. The acquisition of the outer wall temperature field is ensured by means of an infrared camera and the heat transfer coefficient is calculated by means of a local thermal model. The effect of the inclination on the heat transfer is presented and discussed for various vapour qualities and mass velocities and a saturation temperature of 81 °C, corresponding to a Bond number of 4.1. For each experimental condition, the mean heat transfer coefficient is calculated for various inclinations from the vertical downward flow (−90°) to the vertical upward flow (+90°). The variation of the heat transfer coefficient are first presented and analyzed in the horizontal case. The heat transfer coefficient variations with the flow parameters are interpreted in terms of nucleate and convective boiling and these observations are confirmed by the visualisation of the nucleation phenomenon in the evaporator. The inclination has almost no effect on the heat transfer and this lack of effect is finally interpreted in terms of shift between the annular to intermittent flow pattern transition coupled with the nucleate to convective boiling transition.
  • Analytical investigation on homogeneous nucleation of bi-component fuels
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Xi Xi, Hong Liu, Chang Cai, Ming Jia, Hongchao Yin A modified model of bubble point pressure for binary mixtures based on the simplified Wilson equation was proposed to study the homogeneous nucleation of binary fuel mixture. The model was verified for four binary systems and well agreed with the available experimental results. On the basis of the bubble point pressure model, the mole fraction of each component in the vapor phase was obtained. The homogeneous nucleation model combined with the classical nucleation theory and the bubble point pressure model was used to predict the critical bubble radius and the nucleation rate of dimethyl ether (DME)/n-decane and DME/n-dodecane binary systems. The influence of blending ratio on homogeneous nucleation was discussed under several ambient pressures. The superheat limit temperature (SLT) was also calculated for the tested binary systems. The results showed that even a small amount of light component could obviously reduce the temperature needed to trigger the homogeneous nucleation. Consequently, the intensities of homogeneous nucleation and flash boiling could be also greatly improved. The calculation revealed that depressurization could promote the performance of homogeneous nucleation and flash boiling of the binary mixtures.
  • Investigation on three mixing enhancement strategies in transverse gaseous
           injection flow fields: A numerical study
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Lang-quan Li, Wei Huang, Ming Fang, Yi-lei Shi, Zhi-hui Li, Ao-ping Peng Numerical investigation of some mixing enhancement strategies based on the traditional transverse injection technique proposed in recent years was carried out by means of the three-dimensional Reynolds-average Navier–Stokes (RANS) equations coupled with the two equation k-ω shear stress transport (SST) turbulence model. The numerical approaches employed in the current study were validated against the available two- and three-dimensional data in the open literature, and they can be used with confidence to achieve a better understanding of mixing augmentation mechanisms in transverse injection flow fields with various mixing enhancement strategies, namely the pulsed jet, the air jet and the ramp. Results obtained in this study provide important insight into complex flow phenomena. Various fundamental mechanisms dictating the intricate flow characteristics for the transverse injection flow field with various mixing enhancement strategies, including the circulation, vertical structures, velocity vectors and shock wave systems, have been analyzed systematically. The performance parameters of the transverse injection flow fields, such as the mixing length, the fuel penetration depth and the stagnation pressure loss have been compared. Different mixing enhancement strategies have their advantages and disadvantages, and the combination of various mixing enhancement strategies may be a promising injection strategy for better mixing performance of the transverse injection flow field. A stronger streamwise vorticity is the main reason for the mixing enhancement. However, the mixing length and the stagnation pressure loss of the transverse injection flow field show opposite trends from each other with a variance in the intensity of streamwise vorticity.
  • Improved wettability of graphene nanoplatelets (GNP)/copper porous
           coatings for dramatic improvements in pool boiling heat transfer
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Aniket M. Rishi, Satish G. Kandlikar, Anju Gupta Graphene nanoplatelets (GNP) are known for their excellent thermal and mechanical properties making them suitable candidates for a variety of engineering applications. In this work, a novel GNP/Cu porous coating obtained via a multistep electrodeposition technique is presented and tested for their efficacy in improved critical heat flux (CHF) and heat transfer coefficient (HTC). An array of hierarchical porous coatings was obtained by systematically increasing the GNP concentration in the electrodeposition bath and were found to be superhydrophilic with very high wicking rates. Our pool boiling tests indicate that 2% GNP/Cu (wt/vol.) surfaces yielded a CHF of 286 W/cm2 and a heat transfer coefficient of 204 kW/m2-°C, representing an improvement of 130% in CHF and 290% in HTC compared to pristine copper surfaces. The reported CHF and HTC represent the highest values reported in the literature till date for pool boiling on a plain surface. This enhancement in heat transfer properties is attributed to the hierarchical pores that serve as the nucleation sites and influence the overall bubble dynamics that is responsible carry the heat between liquid and vapor phases. The porous surfaces also improved the surface wickability and wettability that further promoted nucleation and microlayer evaporation.Graphical abstractGraphical abstract for this article
  • Development of a novel quasi-3D model to investigate the performance of a
           falling film dehumidifier with CFD technology
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Tao Wen, Yimo Luo, Weifeng He, Wenjie Gang, Liyuan Sheng The present study proposed a novel quasi-3D model for simultaneous simulation of heat and mass transfer process in a falling film dehumidifier with penetration mass transfer theory and CFD technology. Different from the existing 2D model, the new model took the shrinkage of falling film into consideration by introducing variable film wettability. Besides, an experimental system with a single channel falling film dehumidifier was designed and built up to validate the developed quasi-3D model. Results indicated that the shrinkage model could predict the flow pattern accurately, and the relative different between calculated and experimental wetting ratio was less than 4%. Combined with the shrinkage model, the CFD model was proved to be reliable to simulate the dehumidification process with the relative deviation of absolute moisture removal less than 10%. It was found that the non-wetting of falling film and mass transfer resistance in the air side hindered the dehumidification performance. Consequently, super-hydrophilic coating and curved plate were proposed to enlarge the wetting ratio and decrease the mass transfer resistance in the air side. By adopting the nano-TiO2 coating, the wetting ratio of dehumidifier increased from 81% to 97.8%, which led to a relative improvement of 15.2% for absolute moisture removal. In addition, the employment of curved dehumidifier could enhance the dehumidification performance by 70%.
  • Experimental investigation on convective heat transfer of ferrofluids
           inside a pipe under various magnet orientations
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Jin Wang, Guolong Li, Hengxuan Zhu, Jing Luo, Bengt Sundén Some experimental tests were conducted to reveal the enhancement of the ferrofluid heat transfer under a permanent magnetic field. This research aims to investigate the effect of various external magnetic fields on convective heat transfer characteristics of the ferrofluid (magnetic nanofluid). Comparison of theoretical predictions and experimental data were conducted to validate the rationality of the test results, and a good agreement with less than 10% deviations was found. The deviations from experimental data decrease with an increase of the Reynolds number (Re) from 391 to 805. Results from the case with 5 cannulas indicate that a continuous increase in the magnetic flux density (by increasing the quantity of the magnets) can improve the heat transfer enhancement significantly. The ferrofluids with a magnetic cannula shows heat transfer enhancements of 26.5% and 54.5% at Re = 391 and 805, respectively.
  • Molecular dynamics simulations on the heat and mass transfer of
           hypercrosslinked shell structure of phase change nanocapsules as thermal
           energy storage materials
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Xinjian Liu, Zhonghao Rao Four different trihydroxy crosslinkers were respectively used to construct molecular models of hypercrosslinked polyurethanes as shell materials of phase change nanocapsules for thermal energy storage so as to investigate the heat and mass transfer properties. The impacts of microscopic characteristics of crosslinkers on glass transition temperature, thermal expansion and diffusion, interaction energy, thermal conductivity, mechanical properties under tensile and shear stress as well as some of thermal properties after packing water molecules into the hypercrosslinked polyurethanes were simulated and analyzed based on equilibrium or non-equilibrium molecular dynamics methods. The results showed that the detailed molecular structure of crosslinkers, namely the positions of hydroxyl groups, the geometric features of skeleton and even involved element type, will determine the structural characteristics of the hypercrosslinked polyurethanes, such as the formed micro cavities, the dangling functional groups as well as stiffness of short chain, and then affect the heat and mass transfer as well as mechanical properties. Furthermore, the hypercrosslinked polyurethanes possess relatively high glass transition temperature, better heat transfer performance and excellent mechanical properties compared to the polyurethanes with low crosslink density.
  • Single-phase heat transfer of multi-droplet impact on liquid film
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Gangtao Liang, Tianyu Zhang, Liuzhu Chen, Yang Chen, Shengqiang Shen Multi-droplet impact on a thin liquid film covering a heated wall is numerically studied using a three-dimensional model with an implement of a random disturbance subjected to Gaussian distribution. Results show that successive impact results in higher splashing threshold for the trailing droplet than the leading one due to radial flow inside the residual film. Single-phase heat transfer coefficient in the impinged region is remarkably higher than the undisturbed film region, caused by enhanced convective heat transfer. A heat transfer blind spot situated under the central liquid sheet in simultaneous impact is identified, at which relatively high local temperature is observed and heat is more difficult to be rejected due to flow stagnation there. Cooling of the heated wall in successive impact is mainly achieved by contacting of the trailing droplet with the heated wall, rather than mixing of cold droplets and warm film.
  • Structural effects in partially-wetting thin evaporating liquid films near
           the contact line
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Adriana Setchi, Yanqi Chen, Jiapeng Yu, Hao Wang Evaporating thin liquid films at triple-phase lines play important roles in phase-change processes. Many researchers have tried to model a steady one on an isothermal surface using continuum mathematical models. Usually they choose to solve the profile from a nanoscale position where the film thickness is slightly larger than the adsorbed film. This position is vaguely defined and due to the lack of experimental information, assumptions have to be made about the initial film profile especially the slope and curvature. In the present work, we apply the boundary conditions at the microscale and solve the film profile back through the nanoscale. The vagueness and assumptions are therefore avoided. To achieve comprehensive description for the very thin part near the adsorbed film, the structural force component and the short-range Born repulsion component are included in the disjoining pressure expression besides the dispersion component. We try out five different forms of dispersion components and five different forms of structural components. It is found that only when dispersion component, short-range Born repulsion component, and one specific form of structural component are working together, a complete film profile from microscale to nanoscale can be obtained including the thin film beyond the apparent contact line, and the obtained profile is qualitatively consistent with the most recent experimental report. The results indicate that an oscillatory structural force of exponential decay dominates when the film thickness is between 2 and 25 molecular diameters.
  • Mixed convection heat transfer correlations in shallow rectangular
           cavities with single and double-lid driven boundaries
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): A. Louaraychi, M. Lamsaadi, M. Naïmi, H. El Harfi, M. Kaddiri, A. Raji, M. Hasnaoui Mixed convection in single and double-lid driven horizontal rectangular cavities filled with a Newtonian fluid and subjected to uniform heat flux along their vertical short sides is studied numerically and analytically. The finite volume method with the SIMPLER algorithm is used to solve the full governing equations for which the Boussinesq approximation is adopted while the analytical approach lies on the parallel flow assumption, valid in the case of shallow enclosures. A good agreement between the two approaches is observed within the explored ranges of Peclet and Rayleigh numbers. The effects of such parameters on the flow and heat transfer characteristics are analyzed for both kinds of driven cavities. The zones characterizing the dominance of natural and forced convections as well as when the two phenomena compete (mixed convection) are delineated. It is found that the transition from one dominated regime to another depends on the ratio Ra/Pe3.
  • Characterization of pyrolysis and combustion of rigid poly(vinyl chloride)
           using two-dimensional modeling
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Joshua D. Swann, Yan Ding, Stanislav I. Stoliarov A quantitative understanding of an intumescent material’s reaction to fire is largely an unsolved challenge in the field of fire science. To advance fire modeling, a systematic methodology to fully parameterize a comprehensive pyrolysis model for charring and intumescent materials is presented. Thermogravimetric analysis, differential scanning calorimetry and microscale combustion calorimetry were employed to characterize the kinetics and thermodynamics of thermal decomposition and heats of complete combustion of gaseous pyrolyzates. A multi-step reaction mechanism, consisting of sequential steps, was constructed to capture the physical transformations and chemical reactions observed in all milligram-scale experiments. Controlled Atmosphere Pyrolysis Apparatus II gasification experiments were conducted on 0.07 m diameter disk-shaped samples to parameterize the thermal transport within the undecomposed material and developing char layer. A recently expanded fully verified and validated numerical framework, ThermaKin2Ds, was employed to inversely model the gasification experimental results. The model accounted for spatially non-uniform swelling of the sample and associated changes in the radiant heat exposure. Rigid poly(vinyl chloride), a widely used intumescent material, was analyzed in this work. The resulting two-dimensional model was shown to reproduce the gasification experimental unexposed surface temperatures and mass loss rates with a mean error of 3.9% and 12.6%, respectively. A preliminary analysis of the char pore structure was also conducted to determine the pore size distribution and char porosity. The resulting porosity based on the density of graphite and the porosity based on image analysis (including only the pores that are greater than 1 × 10−4 m in diameter) was found to be 0.96 and 0.53, respectively.
  • A DEM-based heat transfer model for the evaluation of effective thermal
           conductivity of packed beds filled with stagnant fluid: Thermal contact
           theory and numerical simulation
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Lei Chen, Cong Wang, Marigrazia Moscardini, Marc Kamlah, Songlin Liu The Effective thermal conductivity (keff) is one of the key thermal properties for packed beds in the presence of a stagnant fluid. In this study, a thermal discrete element model (DEM) based on the original Cheng-Yu-Zulli analytical model for mono-sized packed beds has been improved and implemented especially for mixed beds of different particle sizes or materials. In order to perform the DEM simulation for packed beds, a thermal contact theory considering three heat transfer mechanisms (solid contact conduction, solid-fluid-solid conduction and radiation) was derived and applied in the network of Voronoi cells obtained by radical Voronoi tessellation of the relevant beds. The numerical model was validated through a comparison with experimental results already reported in literature and a good prediction for the effective thermal conductivity was obtained for both mono-sized and multi-sized packed beds in a wide range of solid-to-fluid conductivity ratio. The model also showed a good performance to study the heat flow distribution as well as the coupled thermo-mechanical behavior of packed beds.
  • Translational velocity of a charged oil droplet close to a horizontal
           solid surface under an applied electric field
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Chengfa Wang, Yongxin Song, Xinxiang Pan, Dongqing Li This work numerically investigated the translational velocity of a negatively charged oil droplet in the water near a negatively charged solid surface under a DC electric field. The equilibrium separation distance between the charged oil droplet and the charged solid surface is calculated under different parameters and the translational motion of the droplet near the solid surface is simulated via a three-dimensional mathematical model. The results indicate that the velocity of the droplet is higher under a larger zeta potential of the solid surface and a smaller zeta potential of the droplet. When the absolute value of the negative zeta potential of the droplet is larger than that of the solid surface, the droplet will move in opposite direction of the electric field. It is also found that the droplet translational velocity increases with the decrease of the separation distance between the oil droplet and the solid surface.
  • Design of the structure of battery pack in parallel air-cooled battery
           thermal management system for cooling efficiency improvement
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Kai Chen, Mengxuan Song, Wei Wei, Shuangfeng Wang In this paper, the cell spacing distribution of the battery pack in the parallel air-cooled BTMS is designed to improve the cooling efficiency of the system. The flow resistance network model is used to calculate the airflow rates in the cooling channels. A modification factor is introduced to reduce the error of the model. The effectiveness of the model is verified by the computational fluid dynamics (CFD) calculation, and the CFD method is validated by the experimental air-cooled system with aluminum blocks. Combining with the improved flow resistance network model, an optimization strategy is adopted to optimize the battery cell spacings for the homogenization of the airflow rates among the cooling channels. Then an adjustment coefficient is introduced to adjust the airflow rates in the cooling channels for more uniform cell temperatures. The results of typical numerical cases indicate that the cooling efficiency of the BTMS is improved remarkably after the cell spacing optimization using the developed strategy. Compared to the original BTMS, the maximum temperature of the battery pack for the optimized BTMS is reduced by about 4.0 K, and the maximum cell temperature difference is reduced by more than 69% for various inlet airflow rates. Compared to the optimized BTMS in previous study, the maximum cell temperature difference for the present optimized BTMS is reduced by more than 25% for various inlet airflow rates.
  • Flow regimes during condensation from superheated vapor
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Jiange Xiao, Pega Hrnjak Two-phase flow during condensation in smooth horizontal round tubes of R245fa, R1233zd(E), R1234ze(E), R134a, R32 from superheated vapor is visualized and presented in this paper. Flow regimes under different mass fluxes, heat fluxes, saturation pressures, specific enthalpies and tube sizes (1, 4, 6 mm) are identified. The paper describes to the flow regime transitions according to the visualizations. The driving force behind the annular-stratified flow transition is identified to be the force balance between shear, gravity and surface tension. The mechanism that dictates the annular-intermittent flow transition is the comparison between wave-height and the tube size. The slip ratio, which generates the Kelvin-Helmholtz instability, is considered to be the reason of transition from stratified-wavy to the fully-stratified flow. The more complicated scenarios where characteristics of different flow regimes coexist are detailed and methods for simplification are provided. The results are also compared to two different flow regime maps. The flow regime map that does not consider the non-equilibrium effects does not provide information beyond bulk quality 1 and 0. Additionally, it does not capture the annular entrance during condensation either. The flow regime map with non-equilibrium taken into account addresses issues above while having its own defects. For instance, it is highly empirical and some transition lines do not properly reflect experimental observations. A more mechanistic flow regime map is recommended.
  • Investigation of the interfacial heat transfer coefficient of sheet
           aluminum alloy 5083 in warm stamping process
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Kunmin Zhao, Daxin Ren, Bin Wang, Ying Chang The interfacial heat transfer coefficient (IHTC) between die and blank is an important boundary condition needed for numerical simulation of warm stamping of sheet aluminum alloys. The heat transfer behaviors of sheet aluminum alloy AA5083 are investigated through cylindrical-die stamping experiments. The methods for calculating IHTC such as the heat balance method and Beck's nonlinear estimation method are introduced. The IHTC during blank transportation and stamping-cooling stages are calculated and the influencing factors are analyzed. Both contact pressure and initial temperature positively influence the IHTC and the relationship can be expressed by a compound power function. Lubricant is found to have heat insulation effect beneficial to stamping and cooling in addition to its main function of lubrication. Finally, the heat transfer behaviors under one side contact the other side free and one side contact the other side insulated situations are investigated.
  • A first look at the performance of nano-grooved heat pipes
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Yigit Akkus, Chinh Thanh Nguyen, Alper Tunga Celebi, Ali Beskok Passive thermal spreaders utilizing liquid/vapor phase-change mechanism such as heat pipes, have been widely used in the macro-scale thermal management of electronic devices for many years. Micro-fabrication techniques enabled the fabrication of micro-scale grooved heat pipes on semiconductors. Recent advances in fabrication techniques, on the other hand, enabled production of nano- and ångström-scale capillaries and cavities, which renders the manufacturing of nanoscale heat pipes possible. In the present study, we have simulated nanoscale heat pipes composed of nano-grooves using molecular dynamics and evaluated their performance based on different operating parameters such as the filling ratio and heat load. Moreover, evaluation of size effect on the thermal performance is made by comparing proportionally scaled heat pipes. Simulation results reveal that efficient operation of nano-grooved heat pipes depends not only on the proper selections of filling ratio and heat load, but also on the geometrical parameters such as cross sectional dimensions and aspect ratio of the groove. The modeling strategy used in this study opens an opportunity for computational experimentation of nanoscale heat pipes.
  • Experiments on skin friction reduction induced by superhydrophobicity and
           Leidenfrost phenomena in a Taylor-Couette cell
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): M. Soltani Ayan, M. Entezari, S.F. Chini Film boiling and superhydrophobic surfaces may each decrease the skin drag by creating a vapor/air layer encapsulating the body. However, film boiling requires enormous power, and air plastron on superhydrophobic surfaces is not stable. The combined utilization of superhydrophobicity and film boiling is promising as on superhydrophobic surfaces the film boiling (Leidenfrost) temperature is only a few degrees higher than the boiling point. In a recent literature, a Taylor-Couette (T-C) cell was proposed to measure the combined effect of superhydrophobicity and film boiling on skin drag. However, rotation of the inner cylinder of the TC cell creates turbulent Taylor vortices, makes the vapor layer unstable and creates uncertainty in the results. Whereas if the outer cylinder rotated, flow pattern would become azimuthal laminar with weak Ekman vortices. We modified the T-C cell, accordingly; and found that by decreasing the surface wettability (i.e. apparent contact angle), Leidenfrost temperature, and the minimum heat flux to reach the film boiling decrease as well. Furthermore, by increasing the shear Reynolds number from 0.8×104 to 3.2×104, for both heated and unheated superhydrophobic surfaces, skin drag reduces. The heated one results in 67% decrease in skin drag. It is worth mentioning that the heat flux to create this 67% skin drag reduction is less than 2W/cm2, which is 25 times less than the minimum heat flux to create film boiling on a regular aluminum surface.
  • Experimental and theoretical study of pool boiling heat transfer and its
           CHF mechanism on femtosecond laser processed surfaces
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Bin Liu, Jie Liu, Yonghai Zhang, Jinjia Wei, Wenjun Wang In the present study, the pool boiling heat transfer of micro/nano hierarchically structured surfaces, as well as that of a smooth surface in gas dissolved FC-72 (the subcooling is 1 K) was studied. Femtosecond laser processing was used to fabricate the structured surfaces. It was found that for the surfaces with small processing spacing (LS30 and LS70, where the number after LS specifies the spacing in μm), the critical heat flux (CHF) showed almost no increase, while the heat transfer coefficient (HTC) was enhanced noticeably compared to that of a smooth surface (SS). For LS100, LS200, LS200-2 (compared to LS200, LS200-2 has the same processing spacing but a much higher peak-to-valley height), LS400 and LS800, both the CHF and HTC were enhanced remarkably compared to those of SS. The maximum HTC enhancement was obtained for LS70, with the HTC being 5.87 times larger than that of SS. The most remarkable increase in the CHF was achieved for LS200-2, with an improvement of 91% relative to that of SS. The liquid supply mechanism at the CHF of the micro/nano hierarchically structured surfaces was investigated. A modified model taking into account the coalesced bubble departure frequency, Jakob number and capillary wicking effects was proposed for CHF prediction. The CHF data from this study and the literature were used to validate the model, and it was found that the predicted results agree quite well with the experimental data within ±8%.
  • Heat losses associated with the upward flow of air, water, CO2 in
           geothermal production wells
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Tran X. Phuoc, Mehrdad Massoudi, Ping Wang, Mark L. McKoy This present work reports on the heat losses associated with the upward flow of hot air, water, and CO2 in a production well. The heat losses considered here are the frictional heat loss, the gravitational heat loss, the Joule-Thomson effect, and the conduction heat loss to the surrounding rock formation. These heat losses were characterized using the mass flow rate, the operation time as variable parameters while the surrounding formation was assumed to have constant properties with a linear geothermal gradient. The results show that, the frictional heat loss was small while the influences of the others depended on the flow conditions and the operation time. For water, the Joule-Thomson effect was a heating effect and its magnitude was comparable to that due to the gravitational effect. The conduction heat loss was dominant for all situations. For air as well as CO2, the Joule-Thomson effects were the cooling effects. For low mass flow rates, the conduction heat loss was the dominant heat loss during the initial stages of the operation and the combined heat losses due to the gravitational and the Joule-Thomson effects became dominant during the later times. For high mass flow rates, throughout the operation life time, the gravitational and the Joule-Thomson effects were the dominant heat losses that control the temperature of air and CO2 in the production well.Graphical abstractGraphical abstract for this article
  • Heat transfer characteristics of obliquely dispensed evaporating falling
           films on an elliptic tube
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Yee-Ting Lee, Sihui Hong, Chaobin Dang, Liang-Han Chien, Li-Wang Chang, An-Shik Yang This study examines the evaporation heat transfer characteristics of obliquely dispensed falling films over a horizontal elliptic tube. The theoretical model applied the volume-of-fluid (VOF) method to simulate the spreading development of falling liquid films for resolving the distributions of velocity, temperature, volume fraction in conjunction with the convective heat transfer coefficient. Specifically, a user defined function (UDF) was formulated to treat the evaporative effect at the liquid film surface to the environment. The predicted heat transfer coefficients and film thickness distributions around the circular and elliptic tube surfaces were validated by the experimental data from the available literatures. Based on the test results via comparing different flow models, the SST k-ω model provides the best capabilities in terms of the better prediction accuracy and less calculation time. CFD simulations were then extended using the validated computational model to explore the effects of inlet liquid mass flow rate and obliquely dispensed angle on the liquid film distribution and heat transfer characteristics over the horizontal elliptic tube. At a low liquid flow rate of 0.093 kg/m-s, the liquid films failed to fully enfold the tube surface with the appearance of dry areas, causing the substantial decline of heat transfer effectiveness. The condition at a high liquid flow rate of 0.186 kg/m-s achieved the average heat transfer coefficient up to 4.18 kW/m2 K, indicating good thermal performance similar to literature reports. Reasonable average heat transfer coefficients can be attained for the variation of dispensed angle within 30°.
  • Role of porosity and solid-to-fluid thermal conductivity ratio on
           turbulent combined heat and mass transfer in a porous cavity
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Paulo H.S. Carvalho, Marcelo J.S. de Lemos Knowledge on the effects of porosity and thermal conductivity ratio on double-diffusive transport are much needed for optimum design and analysis of a number of engineering equipment. So far, the open literature seems to lack specific investigations on the effects of those two parameters on overall heat and mass transfer in cavities. This work contributes to such much-needed study on double-diffusive convection in a porous square cavity. Turbulent flow regime and aiding drive cases were considered. Governing equations were time- and volume averaged. Turbulence was handled with a macroscopic two-equation model. The thermal non-equilibrium hypotheses was employed to analyze energy transport across the enclosure. Mass transport assumes a binary mixture with solute characterized by its mass fraction. Equations were discretized with the control volume method numerically relaxed using the SIMPLE method. Here, two situations are investigated regarding the effect of porosity. First, porosity is varied along with permeability. Second, permeability is fixed while porosity takes different values. Results indicated that reducing both porosity and permeability induced flow recirculation and increased overall heat and mass transfer, leading to higher levels of turbulent kinetic energy. Such effects are less pronounced when permeability was kept constant while varying porosity. Further, increasing the thermal conductivity ratio substantially affected flow recirculation in the cavity, enhancing, ultimately, turbulence and mass transfer.
  • Fluid flow, heat transfer and entropy generation analyses of turbulent
           forced convection through isotropic porous media using RANS models
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Mehrdad Torabi, Mohsen Torabi, Mohammad Eftekhari Yazdi, G.P. Peterson Turbulent fluid flow, heat transfer and entropy generation through isotropic porous media were investigated utilizing two RANS models, i.e., RNG k-ε and SST k-ω. Circular, longitudinal elliptical and transverse elliptical cross-sectional configurations were analyzed with symmetrical boundary conditions for the upper and lower lines and periodic boundary conditions for the back and front lines. Evaluation of the temperature contours indicated two low-temperature regions behind the solid rods of the transverse elliptical cross-sectional configuration exist. For a given specific normalized pressure gradient, dimensionless turbulence kinetic energy, Nusselt number, and heat transfer efficiency, the RNG k-ε model resulted in more reliable results for the dimensionless turbulence kinetic energy at low Reynolds numbers when compared with the SST k-ω model. Using the thermal analysis of the three cross-sectional configurations investigated, the longitudinal elliptical resulted in higher heat transfer efficiencies when compared with the circular and transverse elliptical cross-sectional configurations. When the influence of the turbulence effects were included in the entropy generation rate, a second law of thermodynamics analysis indicated that the longitudinal elliptical cross-sectional configuration resulted in a lower entropy generation rate when compared with the circular and transverse elliptical cross-sectional configurations. Combining the results of the first and second law analyses for the three cross-sectional configurations, it is apparent that isotropic porous media consisting of the longitudinal elliptical cross-sectional configuration can simultaneously result in a high heat transfer efficiency and low entropy generation rate.
  • Fe 3 O 4 H 2 O +nanofluid+in+an+annulus+subject+to+thermal+radiation&rft.title=International+Journal+of+Heat+and+Mass+Transfer&rft.issn=0017-9310&">CVFEM analysis for Fe 3 O 4 – H 2 O nanofluid in an
           annulus subject to thermal radiation
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): A.S. Dogonchi, M. Waqas, S.M. Seyyedi, M. Hashemi-Tilehnoee, D.D. Ganji Colloidal nanoparticles suspensions (nanofluids) are the materials of consideration for thermal engineering due to their typically enhanced heat transportation characteristics in comparison to base liquid. Nanoliquids have utilizations in transportation, solar absorption, nuclear systems chilling, friction reduction and energy storage etc. Besides, magnetic nanoliquids are utilized in the cancer therapeutics via implementation of drug delivery and cancer imaging. Thus, in view of such utilizations, here modeling and simulations are presented to scrutinize the natural convective Fe3O4-water nanoliquid flow in an annulus between a triangle and a rhombus enclosures. Thermal radiation aspect is considered for formulation. CVFEM is implemented for computations of numerical outcomes. Impacts of embedding variables on the flow and heat transfer features have been perused. Furthermore a correlation for average Nusselt number is established in terms of energetic parameters. The obtained results portray that average Nusselt number rises subjected to Rayleigh number, radiation parameter and volume fraction of nanofluid while it diminishes when Hartmann number is increased.
  • A modified heat transfer correlation for flow boiling in small channels
           based on the boundary layer theory
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Yang Zhang, Hongbo Tan, Yanzhong Li, Siyu Shan, Yuman Liu Flow boiling in small channels has attracted much attention due to its promising application in compact phase change heat exchangers. In this paper, a flow boiling heat transfer database of light hydrocarbons in horizontal small channels is established. It is found that the heat transfer coefficient (HTC) for propane in the channel with 1.0 mm diameter depends on heat and mass fluxes in either the low or high quality region, which is different from the corresponding HTC in conventional channels. To explore the special phenomenon in small channels, a new model is proposed based on boundary layer theory by considering the function of bubble growth in flow boiling. The relationship between the boundary layer thickness and the bubble diameter is emphatically discussed. Based on the theoretical model, a new dimensionless parameter (Rtd) is introduced to evaluate the effect of channel dimension on the flow boiling heat transfer. Rtd reveals that the bubble growth is enhanced in both the low and high quality regions due to the flow confinement of the small channel. With the aid of Rtd, a modified heat transfer correlation is developed based on the database. Within the whole database, the overall standard error and γ30 of the modified correlation are 18.9% and 91.3%, respectively. The prediction performance is superior to those of existing correlations.
  • An improved DNB-type critical heat flux prediction model for flow boiling
           at subcritical pressures in vertical rifled tube
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Haiyan Xie, Dong Yang, Wenyu Wang, Li Wan, Chunmei Li Based on the near-wall bubble coalescence model and the structural characteristics of the rifled tube, improvements and optimization of the model for rifled tube were conducted. DNB-type CHF phenomenon in the rifled tube under subcritical and near-critical pressures was investigated by numerical calculation. A new CHF prediction model of flow boiling in rifled tube with low qualities was established and compiled by FORTRAN language. The model used the Staub model which considering the influence of the bubble contact angle to calculate the bubble detachment diameter. The surface friction coefficient of the rifled tube was calculated by the classical Kohler and Kastner formula. And a new critical void fraction of the wall bubbly layer αb correlation was proposed based on a large number of CHF experimental data. The present model was verified by the CHF look-up values and CHF experimental values. Based on the CHF experimental data of the rifled tube, three classic CHF prediction models and the present CHF model were evaluated, and it was found that the present modified CHF model has the highest prediction accuracy for the rifled tube, which is the best agreement with the experimental values, indicating that the present model is appropriate for DNB-type CHF prediction of the rifled tube. In this paper, the effects of pressure, mass flux and inlet fluid subcooled enthalpy on CHF were studied. It was found that, the effect of pressure and mass flux on CHF is obvious in the low quality region, with the increase of vapor quality, the influence of pressure and mass flux on CHF weakens.
  • Nucleate pool boiling of R-245fa at low saturation temperatures for
           hydrogen precooling applications
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Gilberto Moreno, Bidzina Kekelia, Hariswaran Sitaraman, Sreekant Narumanchi, Kevin Bennion Experiments were conducted to measure the pool boiling performance of saturated R-245fa at low saturation temperatures (−30 °C, −40 °C, and −50 °C). Horizontally oriented cylindrical heat sources were used as the test samples and were intended to simulate a small section of a hydrogen-filled tube. Measurements evaluated the effects of saturation temperature and two microporous coatings on heat transfer coefficients (HTC), critical heat flux (CHF), and boiling incipient superheat. Results show that boiling performance decreases as the saturation temperature reduces—a result of less active nucleation sites and higher vapor coverage of the heated surface at lower temperatures. Microporous coatings increased HTCs and CHF values, but did not have a significant effect on reducing the boiling incipient superheat. The experimental results were used to develop correlations for nucleate pool boiling on plain and microporous surfaces that can be used to predict HTCs and CHF values at low saturation temperature conditions (down to −50 °C).
  • On the energy harvesting and heat transfer ability of a ferro-nanofluid
           oscillating heat pipe
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): J. Gabriel Monroe, Swati Kumari, John D. Fairley, Keisha B. Walters, Matthew J. Berg, Scott M. Thompson A unique thermal-to-mechanical-to-electrical energy conversion process is demonstrated via thermally-excited, pulsating ferro-nanofluid within a solenoid-equipped oscillating heat pipe (i.e., ferrofluid-OHP or FF-OHP). The FF-OHP was charged with an aqueous cobalt ferrite ferro-nanofluid, comprised of custom-synthesized CoFe2O4 nanoparticles surface-modified with citric acid for increased suspensibility. Annular bias magnets were placed directly above and below the FF-OHP solenoid to temporarily magnetize the internal, oscillating ferrofluid. During FF-OHP operation, a measured peak-to-peak voltage of ∼2 mV was measured across the solenoid due to electromagnetic induction. When filled with ferro-nanofluid, the OHP heat transfer was enhanced (relative to pure water) by ∼58% with bias magnets and ∼71% without bias magnets. A maximum effective thermal conductivity of 12.9 kW/m·K was achieved in the FF-OHP at ∼470 W of heat input. With the bias magnets installed (i.e., harvesting configuration), the FF-OHP effective thermal conductivity was ∼11% lower than when the bias magnets were not present, and this is attributed to an increase in ferrofluid viscosity due to particle magnetization in the bias field. The FF-OHP/solenoid harvesting process is a novel means for accomplishing thermal-to-electrical energy conversion while maintaining high heat transfer capabilities and extreme temperature functionality.
  • Effect of halloysite nanotubes on shape stabilities of polyethylene
           glycol-based composite phase change materials
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Sarinthip Thanakkasaranee, Jongchul Seo A series of shape-stabilized phase change materials (PCMs), composed of polyethylene glycol (PEG) and halloysite nanotube (HNT) (PEG/HNT), were prepared using a melt-extrusion technique. Chemical and morphological structures, shape-stabilities, thermal properties, and thermal stabilities of the PEG/HNT composites were investigated. The composite properties were strongly dependent on the weight ratio of HNT to PEG. Transmission electron microscopy and Brunauer–Emmett–Teller analyses showed that PEG was perfectly adsorbed into the HNT pores and covered the surfaces of HNT, which helped prevent leakage of melted PEG during a phase change from solid to liquid. Among the various PEG/HNT composites, the PEG/HNT composites with HNT contents in the range 30–50% exhibited good shape-stabilities. Moreover, the rate of heat transfer increased with the HNT content. Based on these results, we expect that the PEG/HNT composites can be used in food packaging materials to counteract the negative effects such as migration induced by unwanted temperature changes.
  • A theoretical study of molten pool behavior and humping formation in full
           penetration high-speed gas tungsten arc welding
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Xiangmeng Meng, Guoliang Qin A clear insight into the full penetration humping (FP-humping) formation in high-speed gas tungsten arc welding (GTAW) is difficult owning to the complex and multi-coupled transport phenomena in molten pool. In this paper, sensitivity analysis and dimensional analysis are combined to study the molten pool behavior and FP-humping formation. Firstly, sensitivity analysis and an experimentally-verified numerical model are used to clarify the effect of driving forces on characteristic molten pool behaviors and defect formation quantitatively. The results show that both arc pressure and arc shear stress have considerable effects on promoting defect formation, and their significances are in the same order. The surface tension shows predominant role to suppress defect formation. Subsequently, dimensional analysis based on Buckingham π-theorem is performed to derive some physically meaningful dimensionless variables. The dimensionless humping frequency is a linear function of a dimensionless group containing characteristic molten pool variables and material properties. The reasonability of dimensional analysis result is tested by additional numerical data, and there is a good agreement. This study clarifies the physical origin of FP-humping defect in GTAW, and may also provide some fundamental guidelines for its suppression.
  • Heat transfer and hydrodynamics of air assisted free water jet impingement
           at low nozzle-to-surface distances
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Aspen W. Glaspell, Victoria J. Rouse, Brian K. Friedrich, Kyosung Choo In this study, heat transfer and hydrodynamics of air-assisted free water jet impinging a flat plate surface are experimentally investigated. The effects of the nozzle-to-surface distance (H/d = 0.02–0.51) and volumetric quality (β = 0.3–0.8) on the stagnation Nusselt number, stagnation pressure, and hydraulic jump diameter are considered. The results show that the normalized stagnation Nusselt number and hydraulic jump diameter drastically increase with decreasing the nozzle-to-surface distance, since the stagnation pressure increases due to the jet deflection effect. Based on the experimental results, new correlations for the stagnation Nusselt number and pressure are developed as a function of the nozzle-to-surface distance.
  • Experimental investigation on the sputtering and micro-explosion of
           emulsion fuel droplets during impact on a heated surface
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Chunze Cen, Han Wu, Chia-fon Lee, Longjie Fan, Fushui Liu Micro-explosion has been concerned for decades due to its potential effect on fuel atomization and combustion. In this work, the characteristics of sputtering and micro-explosion induced by water-diesel emulsion fuel droplets impact on a heated surface under different water content (from 5% to 20% with an interval of 2.5%) and surface temperature conditions was investigated by a high speed camera synchronized with backlit technique. It was found that the secondary droplets was still heated by the environment to induce sputtering. It was observed the secondary droplets induced by sputtering would trigger further sputtering. Higher water content and higher temperature are necessary to induce sputtering and micro-explosion. The variation of an emulsion fuel droplet impacts on the surface from 100 °C to 320 °C was investigated. Typical Leidenfrost phenomenon was observed when the temperature was above the range when temperature was more than 300 °C. Various types of bubbles depending on the specific temperature value will generate when the surface temperature is below the range, while no bubble generates for neat diesel droplet.
  • Multi-fidelity model based optimization of shaped film cooling hole and
           experimental validation
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Hao Zhang, Yifei Li, Ziyu Chen, Xinrong Su, Xin Yuan An optimization framework is developed for the film cooling hole shape design to achieve highest film cooling effectiveness with affordable cost. A key to this design method is the development and applications of a multi-fidelity model (MFM) to improve the efficiency of surrogate model construction. To construct multi-fidelity model, empirical correlations from existing literature are utilized as low-fidelity data and Reynolds-Averaged Navier-Stokes (RANS) simulations provide high-fidelity data. Compared with single-fidelity surrogate models, multi-fidelity model developed in this work combines vast existing knowledge about shaped film cooling hole and three dimensional numerical simulation. As a result, the computation cost has been substantially decreased by 64.5% while accuracy is maintained with similar level, which provides a firm basis for the optimization. Considering three geometric parameters as design variables, i.e. laidback angel, lateral angle, and hole length-diameter ratio, the shaped hole is optimized with genetic algorithm (GA) combined with sequential quadratic programming (SQP) algorithm. After the optimization, an experiment campaign is further carried out to validate the optimization result using pressure sensitive paint (PSP) technology. With current multi-fidelity model, the spatially averaged film cooling effectiveness has been improved by 39% compared to the baseline, which is an already well-designed shaped hole and the improvement is also verified by experiment. This work verifies the effectiveness of current multi-fidelity model for the design and optimization of shaped film cooling hole for better performance.
  • Accurate and inexpensive thermal time-of-flight sensor for measuring
           refrigerant flow in minichannels
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Allison J. Mahvi, Bachir El Fil, Srinivas Garimella Thermal flow meters are good candidates for measuring flow rates in small diameter channels because they can be integrated into compact geometries and consist of only a few simple components. A new thermal time-of-flight (TOF) flow sensor is developed here for measuring liquid refrigerant velocities in minichannels. The sensor consists of a small heater and two downstream thermocouples. The heater is pulsed to induce a temperature rise in the fluid, and the time for the temperature pulse to reach the downstream thermocouples is measured. Insights from the data and a simplified one-dimensional transient model are used to calibrate the device. The calibration relates the time of flight to both the average refrigerant velocity and local temperature gradients in the fluid. The calibrated TOF device can detect refrigerant velocities between 1 and 20 mm s−1, measuring 98.7% of the data to within ±3% of the full scale (±0.6 mm s−1).
  • Wave-wise falling film in liquid desiccant dehumidification systems: Model
           development and time-series parameter analysis
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Ronghui Qi, Chuanshuai Dong, Li-Zhi Zhang The wavy surface of a falling film has a significant effect on the heat/mass transfer of liquid desiccant dehumidification systems. This paper developed and analytically solved a 2D theoretical model for predicting the time-series surface waves of falling film desiccant dehumidification process. The influences of operating conditions, physical properties of desiccant and air, vapor pressure and thermocapillary forces due to interfacial mass transfer, and the interaction between the desiccant and working plates were considered. The model was validated by experiments with an acceptable error of 10.9% for the frequency, 6.59% for the amplitude and 14% for the contact area changing rate. The actual changing rate of liquid/air contact area with time could be predicted with this model, which was 20–90% higher than the flat film assumptions at a solution mass flow rate from 0.0126 to 0.043 kg/s. Then, parameter analyses of fluctuating desiccant film were numerically performed. Although increasing the temperature or decreasing the concentration of a liquid desiccant increased the wave intensity, the mass transfer driving force was also reduced, which is not suitable for practical systems. However, if the surface tension dropped to the original 10–50% by surface modification or surfactant addition, then the contact area could increase by approximately 10–40% due to the surface wave enhancement. In addition, this method is cost efficient and has little impact on the system structure. Therefore, with this model, it is theoretically possible to evaluate the time-series characteristics of a wave-wise liquid/air interface with heat and mass transfer, which could improve the evaluation accuracy of falling film liquid desiccant dehumidification systems. Thus, this study effectively helps the prediction and optimization of practical dehumidification systems, and other applications related to gas/liquid flow.
  • Effect of micro-roughness shapes on jet impingement heat transfer and
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Prashant Singh, Mingyang Zhang, Shoaib Ahmed, Kishore R. Ramakrishnan, Srinath Ekkad With recent advancements in the field of additive manufacturing, the design domain for development of complicated cooling configurations has significantly expanded. The motivation of the present study is to develop high performance impingement cooling designs catered towards applications requiring high rates of heat removal, e.g. gas turbine blade leading edge and double-wall cooling, air-cooled electronic devices, etc. In the present study, jet impingement is combined with strategic roughening of the target surface, to achieve high heat removal rates. Steady state experiments have been carried out to calculate the heat transfer coefficient for jet impingement onto different target surface configurations. The jet-to-jet spacing (x/d = y/d) was varied from 2 to 5, and jet-to-target distance (z/d) was varied from 1 to 5. The target surface configurations featured cylindrical, cubic and concentric shaped roughness elements, fabricated through binder jetting process. The baseline case for the roughened target surface was a smooth target. Heat transfer and pressure drop experiments were carried out at Reynolds numbers ranging from 2500 to 10,000. Further, numerical simulations were carried out to model flow and heat transfer for all configurations at a representative Reynolds number. Through our experiments and numerical results, we have demonstrated that the novel “concentric” roughness shape was the best in terms of fin effectiveness and Nusselt numbers levels, amongst the investigated shapes. The concentric-shape roughened target resulted in fin effectiveness up to 1.6, whereas the cubic- and cylindrical-shape roughened targets yielded in fin effectiveness up to 1.4 and 1.3, respectively. Further, it was experimentally found that the addition of micro-roughness elements does not result in a discernable increment in pressure losses, compared to the impingement on the smooth target surface. Hence, the demonstrated configuration with the highest heat transfer coefficient also resulted in highest thermal hydraulic performance.
  • Computational examination of two-phase microchannel heat transfer
           correlations with conjugate heat spreading
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Bryan E. Burk, Torben P. Grumstrup, Taylor A. Bevis, Jack Kotovsky, Todd M. Bandhauer Microchannel flow boiling is an attractive thermal management strategy for ever-growing volumetric heat dissipation demands associated with electronic systems. Due to difficulties related to measurement at the microscales, the majority of researchers have chosen to study relatively simple situations with uniform heat flux. However, many applications involve local hotspots which give rise to highly non-uniform heat flux and temperature gradients due to heat spreading. This necessitates the consideration of conjugate heat transfer for accurate analysis.The current work is aimed at investigating conjugate heat transfer in a two-phase microchannel array. Experimental data was collected on R134a flow boiling heat transfer for very small hydraulic diameter (1 kW cm−2) applied via platinum strip heaters with a footprint size of 1 cm × 1 mm. The collected experimental data was then combined with detailed computational modeling utilizing finite element modeling with COMSOL Multiphysics and MATLAB to examine the applicability of five published heat transfer correlations for use in determining local heat transfer coefficients. A two-phase correlation from Agostini and Bontemps, developed for markedly different test parameters, provided the best computational agreement with an RMS temperature difference from experiment of 3.3 °C and a predicted peak perimeter-averaged heat transfer coefficient of 116 MW m−2 K−1. Modeling confirms the presence of highly non-uniform local heat flux and correspondingly non-uniform local heat transfer coefficient. The results of this study make clear the need for better micro-scale two-phase correlations developed to predict local heat transfer coefficients at these small scales and high local heat fluxes.
  • Dropwise condensation heat transfer on superhydrophilic-hydrophobic
           network hybrid surface
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Xianbing Ji, Dongdong Zhou, Chao Dai, Jinliang Xu Superhydrophilic-hydrophobic (SHPi-HPo) network hybrid surface was designed to investigate the condensation heat transfer using stainless steel as substrate material. The SHPi-HPo surface was comprised of superhydrophilic network grooves and hydrophobic regions. Hydrophobic (HPo) surface was prepared with fluorocarbon coating using polytetrafluoroethylene (PTFE) as the matrix resin and micro-nano silicon dioxide (SiO2) as additive to control surface roughness. Three kinds of SHPi-HPo surfaces were tested, having a grid spacing of 1.5, 2.5 and 3.5 mm and named as SHPi-HPo-1, SHPi-HPo-2 and SHPi-HPo-3, respectively. To study the effects of the wall subcooling, steam mass flux, cooling water temperature, cooling water mass flow rate and grid spacing, a series of experiments were conducted and a high speed camera was used to visualize the condensation process. The results show that SHPi-HPo surface can well control condensate droplet diameters and its condensation heat transfer performance is better than that of smooth hydrophilic (HPi) and HPo surfaces. This is attributed to SHPi-HPo surface sucking away droplets in time and limiting the growth of large condensate droplets through the superhydrophilic grooves. At wall subcooling ΔTw = 6.3 K, the heat transfer coefficient of SHPi-HPo-2 surface is 2.7 and 3.4 times that of HPi and HPo surfaces, respectively. For SHPi-HPo surface, there is optimum grid spacing between superhydrophilic grooves to enhance condensation heat transfer. Among three SHPi-HPo surfaces, the heat transfer coefficient of SHPi-HPo-2 surface has the best condensation heat transfer performance, about 0–10% higher than that of SHPi-HPo-1 surface, and at ΔTw = 9 K, the heat transfer coefficient is 1.7 times that of SHPi-HPo-3 surface.
  • Heat transport for evaporating droplets on superhydrophilic, thin,
           nanoporous layers
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Claire K. Wemp, Van P. Carey Previous studies of droplet spreading on superhydrophilic nanostructured surfaces have demonstrated that enhanced wetting and wickability of such surfaces can produce more rapid and extensive spreading of liquid droplets. There is also ample evidence from prior studies that these types of superhydrophilic nanostructured surfaces can exhibit enhanced vaporization heat transfer performance in for droplet evaporation and pool boiling processes. This study specifically explored the mechanisms of droplet vaporization on superhydrophilic nanoporous thin layers, at a fundamental level, for conditions in which liquid viscous forces and pore capillarity forces dominate, and liquid inertia forces are low. The investigation, summarized here, experimentally explored the droplet evaporation heat transfer performance on superhydrophilic, nanoporous, thin layers on a heated metal substrate. A thermal growth process was used to fabricate a layer of ZnO nanopillars a few microns thick on a copper substrate. The resulting nanoporous layer exhibited ultra-low contact angles and high wickability. The experiments indicate that, on these surfaces, a deposited droplet undergoes an initial rapid spreading phase followed by a slower vaporization phase, which shrinks the droplet until evaporation is complete. Although droplet evaporation without bubble nucleation is the primary focus of this work, the experiments also examined the added effects of bubble nucleation on droplet evaporation at high surface superheats. Based on experimental observations and data, a model of heat transport during the evaporation process was developed. The model predictions are shown to agree well with droplet evaporation time measurements for droplets of different initial sizes and a range of surface temperatures. The experiments and model indicate that the surfaces tested enhance droplet evaporation heat transfer performance by as much as a factor of three compared to an ordinary copper surface at the same conditions. The model is also used to explore the potential for more extreme augmentation of droplet evaporation heat transfer by further enhancement of surface wetting and wickability. The results of this study provide a clearer picture of the interconnection of droplet spreading mechanisms and evaporation heat transfer for these circumstances.
  • Design of Cassie-wetting nucleation sites in pool boiling
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Yi Liu, Jiaqi Tang, Linxuan Li, Yi Nok Shek, Dongyan Xu This work reports a novel approach for the design and fabrication of controllable nucleation sites in pool boiling, with the aim to study the correlation of boiling curves and number of nucleation sites. Each nucleation site was designed to be a cavity with hydrophobic micropillar arrays. The dimensions of micropillars were carefully selected so that Cassie wetting can be guaranteed. Therefore, the designed structure allows to trap vapor and evolve as an effective nucleation site in pool boiling environment. Compared to traditional nucleation sites with hole-shaped cavities, these Cassie-wetting nucleation sites own unique advantages including easiness of activation and reliability of enduring harsh boiling perturbations. A correlation of boiling curves and number of nucleation sites is obtained by systematically examining the pool boiling performance of surfaces with different numbers of nucleation sites.
  • Dual steady flow solutions of heat and pollutant removal from a slot
           ventilated welding enclosure containing a bottom heating source
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Dong-Dong Zhang, Yang Cai, Di Liu, Fu-Yun Zhao, Yuguo Li Air conditioning and ventilation in buildings are major sources of energy consumption, particularly in large industrial buildings with significant pollutant and heat sources. Unfortunately, air flow motions in these slot-ventilated large building spaces are currently poorly understood, particularly concerning their special flow behaviours – multiple steady flows, i.e., identical boundary conditions but different initial conditions or load perturbations may lead to two or more flow solutions. Multiple steady enclosure flow behaviours essentially complicate the convective transport of air, heat and species, which has been vividly analyzed by streamlines, heatlines and masslines. In the present study, the flow mechanisms and transitions driven by combined natural and forced convections in an industrial building space for a welding process will be investigated through the numerical methodology of computational fluid dynamics. The research has taken into consideration the effects of ambient air temperature, indoor heating loads, and welding shifting position on multiple flow motions. The parameters governing the problem are the Reynolds number (103 ≤ Re ≤ 107) and the Grashof number (107 ≤ Gr ≤ 1013) and it is observed that the multiple-steady-regions can be maintained for a range of values of Gr/Re2, 150 
  • An enhanced evaporator model for working fluid phase length prediction,
           validated with experimental thermal imaging data
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Dhruvang Rathod, Ujjwal Belwariar, Bin Xu, Mark Hoffman This paper presents the modeling and validation of a counter-crossflow heat exchanger used to extract thermal energy from Heavy-Duty Diesel (HDD) engine exhaust. The finite volume evaporator modeling methodology is enhanced for both accurate working fluid temperature and phase length estimation, facilitating improved offline waste heat recovery simulation and accurate control-oriented model development.Transient model calibration and validation experiments were performed on a stand-alone flow bench. Heated gas was passed through the evaporator, replicating different engine exhaust gas conditions. In contrast to other studies, thermal imaging data served to identify the working fluid liquid, mixed and vapor phase lengths within the evaporator. The FVM modeling methodology was enhanced based on the thermal imaging data to accurately predict the working fluid phase lengths. Once calibrated, working fluid phase lengths predicted by the proposed model were validated against thermal imaging from additional transient experimental flow bench data sets. In comparison to the baseline finite volume evaporator model, the enhancements proposed herein showed a 43% mean improvement in predicting the vapor phase boundary during transient operation.Graphical abstractGraphical abstract for this article
  • Quantification of gas-assisted evaporation in minichannels with negative
           wall superheats
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Hung-Yi Wu, Ben-Ran Fu, Chin Pan The present study quantifies the gas-assisted evaporation of ethanol in two parallel minichannels with negative wall superheats using helium as the auxiliary gas. At the exit of minichannels, an innovative gas–liquid separation and vapor condensation device is designed to measure the ethanol evaporated. The experimental results demonstrate that both the gas-assisted evaporation and mixing effects are significant mechanisms responsible for the significant enhancement of heat transfer with the introduction of helium gas in ethanol flow. For the present study, the maximum heat transfer enhancement of 275% is demonstrated under the condition of ethanol flow rate of 0.028 m/s, helium flow rate of 0.833 m/s, and wall superheat of −3.2 °C. Under such a condition, the gas-assisted evaporation contributes about 40% of total heat transfer rate. Flow visualization and exit quality measurement reveal that the phase change mechanism is dominated by convective evaporation in the region with negative wall superheat, while it is dominated by convective boiling under the conditions with positive wall superheat. The evaporation efficiency increases with decrease in ethanol flow rate, increases in helium flow rate and wall superheat. An empirical correlation for the evaporation efficiency is, thus, developed.
  • A 3D discrete element-finite difference coupling model for predicting the
           effective thermal conductivity of metal powder beds
    • Abstract: Publication date: April 2019Source: International Journal of Heat and Mass Transfer, Volume 132Author(s): Hang Zhang, Yizhen Zhao, Fu Wang, Dichen Li The effective thermal conductivity (ETC) of metal powder is a basic physical property that has an important role in industrial design for metallurgical, energy, chemical, and other types of application. To obtain this information, a numerical model based on the coupled discrete element-finite difference (DE-FD) method is proposed to precisely and efficiently predict the ETC of metal powders. A random packaged aluminum powder unit influenced by the effects of gravity is piled up concerning the particle size distribution of the real powder. Moreover, a sandwich structure model is proposed to simulate the thermal dynamic behavior during directional heat transfer. Based on the amount of heat passing through the system, the effective thermal conductivity was precisely calculated in an indirect way and the results of the simulation were consistent with corresponding experimental results. Relative error was within ±5%. Thus, the proposed numerical model may be useful in predicting the ETC of the different powder beds filed using various types of interstitial gas media within a wide range of temperatures.
  • Natural convection heat transfer of nanofluid in a cavity under an
           inhomogeneous electric field
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Yi-Ying Bao, Jia-Hui Huang, Yan-Jun Chen, Zhen-Hua Liu Natural convective heat transfer of nanofluid consisted of transformer oil and Al2O3 nanoparticles in a cavity under a non-uniform high electric field intensity was experimental investigated. The main objective is looking for a coupling enhanced method by nanofluid and electric field. It is found that when the effect of gravity is ignored, the electric field forces applied on the working fluids induce an electric convection and thence induce significant heat transfer enhancement. This enhancement effect increases rapidly with the increase of electric field intensity, the nanoparticles mass concentration of nanofluid and the heat flux. When the effect of gravity exists, a coupled convection is caused by both gravity and electric field force together. For pure transformer oil, the influences of electric field force and gravity on the heat transfer almost offset each other. However, for nanofluid, the impact of the electric filed force is dominant. The study results revealed the enhanced mechanism of natural convection heat transfer of nanofluid under electric field and provided some useful technical support for practical application.
  • Study on nucleation position and wetting state for dropwise condensation
           on rough structures with different wettability using multiphase lattice
           Boltzmann method
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Mingjie Li, Christian Huber, Wenquan Tao, Jinjia Wei Dropwise condensation on rough structures with different wettability are numerically simulated using Lattice Boltzmann method. The latent heat during condensation process together with the energy equation is taken into consideration in this paper by solving the temperature distribution equation with a source term for the phase change. Typical droplet nucleation positions and wetting states are found out at different surface wettability through numerical results. It is concluded that with the increase of strength coefficient of the fluid-solid interaction or surface hydrophobicity, the nucleation position rises from the bottom to top of the pillar, and the wetting state of droplet changes from the wetting Wenzel state to the nonwetting Cassie one.
  • Influence of substrate temperature on Marangoni convection instabilities
           in a sessile droplet evaporating at constant contact line mode
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Tian-Shi Wang, Wan-Yuan Shi Marangoni instabilities of a sessile droplet of 0.65 cSt silicone oil evaporating at constant contact line mode are experimentally investigated in a wide range of the substrate temperatures (Tw) from those lower than room temperature (Ta) to those higher than it. For Tw > Ta, the Bénard-Marangoni convection cells are observed and the cell patterns vary from the quasi-steady state to irregular oscillatory state with the droplet evaporation. For Tw  Ta. When Tw is higher than 14.3 °C three kinds of convection mode, i.e. the travelling hydrothermal waves, the coexistence of hydrothermal waves and irregular oscillatory Bénard-Marangoni convection, and the irregular oscillatory Bénard-Marangoni convection, occur successively with droplet evaporating. The critical contact angles for the incipience of these Marangoni instabilities are determined.
  • Experimental study on thermosyphon boiling in 3-D micro-channels
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Jia-Hui Huang, Shuang-fei Li, Zhen-Hua Liu An experimental study on thermosyphon boiling in 3-D micro-channels with inclined angle changed from 90° to 0° was carried out, to develop a new passive cooling technology for 3-D chip cooling. The specific micro-channel structure of actual 3-D chip was simulated as evaporating section of thermosyphon, and the critical heat flux and heat transfer coefficient of 3-D micro-channel thermosyphon boiling in different inclined angles were studied in detail. Experiments were carried out using two kinds of working liquids of deionized water and R113. The length and gap (the distance between two heated walls) of test channels with a rectangular section were in the range of 30–100 mm and 30–50 μm, respectively. Stainless steel wires with different numbers were evenly laid in the channel along the flow direction for forming change in width of micro-channel. The width of rectangular section changed from 0.4 mm to 4 mm. The study results show that the boiling characteristics in 3-D micro-channels thermosyphon can be predicted through the extension of the correlations used for 2-D micro-channels. The present 3-D micro-channel thermosyphon boiling is a promising passive technology for 3-D chip cooling.
  • Effect of gas turbine endwall misalignment with slashface leakage on the
           blade endwall aerothermal performance
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Kaiyuan Zhang, Zhigang Li, Jun Li Effect of the slashface leakage on the gas turbine blade endwall film cooling and heat transfer performance under three endwall misalignment modes (aligned, cascade and dam) is numerically investigated using the method of solving the three-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with the SST k-ω turbulence model. The numerical calculations are conducted with four coolant momentum flux ratios (I) of 0.48, 0.96, 1.42 and 1.87 under three different endwall misalignment modes. The computational results indicate that for the aligned mode, the fore part endwall (z/Cax 
  • Capillary viscous flow and melting dynamics: Coupled simulations for
           additive manufacturing applications
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Michael Blank, Prapanch Nair, Thorsten Pöschel The rate of melting of a solid and the rate of deformation of the resulting melt due to capillary forces are comparable in additive manufacturing applications. This dynamic structural change of a melting solid is extremely challenging to study experimentally. Using meshless numerical simulations we show the influence of the flow of the melt on the heat transfer and resulting phase change.We introduce an accurate and robust Incompressible Smoothed Particle Hydrodynamics (ISPH) method to simulate melting of solids and the ensuing fluid-solid interaction. We present validations for the heat transfer across the free surface and the melting interface evolution, separately. We then present two applications for this coupled multiphysics simulation method — the study of rounding of an arbitrarily shaped particle during melting and the non-linear structural evolution of three spheres undergoing agglomeration. In both the studies we use realistic transport and thermal properties for the materials so as to demonstrate readiness of the method for solving engineering problems in additive manufacturing.
  • Development of novel plate heat exchanger using natural graphite sheet
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Pouya Jamzad, John Kenna, Majid Bahrami Natural flake graphite sheets have superior physical properties such as: high in-plate thermal conductivity (300–600 W·m−1·K−1), light weight (2.1 g·cm−3), negligible coefficient of thermal expansion and resistivity to corrosion under high temperatures. With the new roll-embossing process, fabrication of graphite sheets with different patterns has become fast and economical. These properties make natural graphite an excellent candidate for thermal applications, such as heat exchangers (HEXs), as an alternative to metallic alloys. This study presents a new fabrication method for chevron-type plate HEXs, as a proof-of-concept demonstration, using flat embossing process. To measure the performance of the proposed graphite plate heat exchanger, a custom-designed water-water experimental testbed is designed based on ANSI/AHRI Standard 400. The heat transfer rate and the pressure drop of the fabricated graphite plate heat exchanger are compared to a conventional stainless-steel chevron HEX with similar plate dimensions and number of plates. Compared with the commercially available unit, the proposed graphite plate heat exchanger shows identical thermal performance and a 26% higher pressure drop, due to its narrower channel design. This provide a promising platform for future optimized designs of efficient, corrosion-resistance, and cost-effective graphite-based heat exchangers for a number of industrial applications.
  • Study on friction and wear of Cellulose Nanocrystal (CNC) nanoparticle as
           lubricating additive in engine oil
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): N.W. Awang, D. Ramasamy, K. Kadirgama, G. Najafi, Nor Azwadi Che Sidik A novel Cellulose Nanocrystals (CNC) nanoparticles were proposed as green additive as green additives for improving tribological properties of lubricants. Enhanced tribological performance was measured using piston–skirt liner tribometer under variable load, speed and temperature; and varying concentrations of nanoparticles in lubricating oil. Study on a worn surface on the plate was characterized by SEM and EDX. This study shows that the mixing of CNC nanoparticles in engine oil significantly reduces the friction and wear rate and hence improves the lubricating properties of engine oil. Base oil containing 0.1% CNC demonstrates excellent tribological properties including the lowest COF and the strongest wear resistance under all lubrication conditions. An elemental content in EDX analysis reveals that Carbon and Aluminum were the most elements present.
  • Effects of thermal conductivity of airframe substrate on the dynamic ice
           accretion process pertinent to UAS inflight icing phenomena
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Linkai Li, Yang Liu, Zichen Zhang, Hui Hu An experimental investigation was conducted to quantify the dynamic ice accretion and the unsteady heat transfer process over the ice accreting surfaces of composite-based airframes widely used for light-weight, Unmanned-Aerial-Systems (UAS), in comparison to those over the surfaces of metal-based airframes used by conventional manned aircraft, in order to elucidate the underlying icing physics specifically pertinent to UAS inflight icing phenomena. Two airfoil/wing models with the same airfoil shape, but made of different materials (i.e., thermoplastic material with the thermal conductivity being only ∼0.2 W/m⋅K to represent typical UAS airframe substrates vs. Aluminum with the thermal conductivity being ∼200 W/m⋅K widely used for conventional manned aircraft). The two test models were mounted side-by-side inside an Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT) under the same wet glaze or dry rime icing condition. During the icing experiment, while a high-speed imaging system was used to record the dynamic ice accretion process over the surfaces of the test models, an infrared thermal imaging system was also used to map the corresponding surface temperature distributions over the ice accreting airfoil surfaces. It was found that, upon the impacting of the airborne, super-cooled water droplets in ISU-IRT, ice would start to accrete rapidly on the surfaces of the test models with a significant amount of the latent heat of fusion being released associated with the phase changing of the impacted super-cooled water mass over the airfoil surfaces. The thermal conductivity of the airframe substrate was found to affect the dynamic ice accretion and unsteady heat transfer processes over the ice accreting surfaces significantly. With the two test models being exposed under the same icing conditions, the released latent heat of fusion was found to be dissipated much slower over the surface of the thermoplastic model, due to the much lower thermal conductivity of the thermoplastic substrate. In comparison with those on the surface of the Aluminum model, the slower dissipation of the released latent heat of fusion on the surface of the thermoplastic model was found to cause higher surface temperatures and greater “heated” regions near the airfoil leading edge, more obvious surface water runback over the airfoil surface, and formation of more complex rivulet-shaped ice structures at further downstream locations beyond the direct impinging zone of the super-cooled water droplets.
  • A permeable-membrane microchannel heat sink made by additive manufacturing
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Ivel L. Collins, Justin A. Weibel, Liang Pan, Suresh V. Garimella Microchannel heat sinks are capable of removing dense heat loads from high-power electronic devices with low thermal resistance, but suffer from high pressure drops due to the small channel dimensions. Features that reduce the pressure drop, such as manifolds, increase fabrication complexity and are constrained by traditional subtractive manufacturing approaches. Additive manufacturing technologies offer improved design freedom and reduced geometric restrictions, expanding the types of features that can be produced and integrated into a heat sink. In this work, a novel permeable membrane microchannel (PMM) heat sink geometry is proposed and fabricated using direct metal laser sintering (DMLS) of an aluminum alloy (AlSi10Mg). In this PMM design, the cooling fluid is forced through thin, porous walls that act as both conducting fins and membranes that allow flow through their fine internal flow features for efficient heat exchange. The design leverages the ability of this fabrication process to incorporate complex, arbitrarily curved structures having internal porosity to enhance heat transfer and reduce pressure drop across the heat sink. The PMM heat sink geometry is benchmarked against a low-pressure-drop manifold microchannel (MMC) heat sink. A reduced-order model is used to explore the relative performance trends between the designs. Both heat sinks are experimentally characterized at flow rates of 50–500 mL/min using deionized water as the working fluid. At a constant pumping power of 0.018 W, the permeable membrane microchannel design offers both lower thermal resistance (17% reduction) and lower pressure drop (28% reduction) compared to the manifold microchannel heat sink.
  • Analytical heat transfer model for laterally perforated-finned heat sinks
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Mohammad Reza Shaeri, Richard W. Bonner An analytical model is proposed to predict the average Nusselt numbers of laterally perforated-finned heat sinks (LA-PFHSs) with high aspect ratios in forced convection laminar flows. The model is developed based on the experimental data acquired from testing air-cooled heat sinks including square cross-sectional perforations distributed equidistantly along the length of the fins. The experiments were conducted using three different perforation sizes and five different porosities at each perforation size. The accuracy of the experiments was validated by comparing the experimental pressure drops and heat transfer coefficients of the heat sink without perforation with those obtained from the widely accepted correlations in the literature. The developed model in this study predicts the Nusselt number as a function of Reynolds number, Prandtl number, fin and perforation geometrical parameters, porosity, and the distances between perforations. The model showed excellent predictions for the Nusselt numbers of all LA-PFHSs tested in this study to be within ±12% of the experimental data and a mean absolute error of 4.90%. This study is the first attempt in the literature to develop an analytical model based on experimental data for investigating heat transfer in LA-PFHSs.
  • Effect of shape modification on heat transfer and drag for fluid flow past
           a cam-shaped cylinder
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Sunil Chamoli, Tingting Tang, Peng Yu, Ruixin Lu In this study, a cam-shaped cylinder immersed in the cross-flow is investigated numerically, which is made of two circular cylinders joined by their two common tangents. The equivalent diameter of the cam-shaped cylinder Deq is constant, which is maintained by adjusting the diameters of and the distance between the front and rear cylinders. The present study systematically investigates the effects of shape transformation of a base circular cylinder into different shapes of the cam-shaped cylinder on the flow and heat transfer characteristics. The cam-shaped cylinders with different relative diameter ratios of cylinder 1 (D1/Deq = 0.2–0.84), and different relative diameter ratios of cylinder 2 (D2/Deq = 0.5–0.9) for the two Reynolds numbers (Re = 100, and 200) are considered. The functional dependence of drag coefficient (CD) and lift coefficient (CLmax) on D1/Deq and D2/Deq are examined. CD decreases with an increase in Re and a decrease in D2/Deq, respectively, while the Strouhal number (St) increases with a decrease in D2/Deq. The average Nusselt number (Nu‾) increases with an increase in Re and decreases with a decrease in D2/Deq. The change in Nu‾ is almost negligible for a low D1/Deq. Different regimes are proposed for the design parameters selection for high Nu‾ and low CD. In the regime of constrained design and flow conditions, the present cam-shaped cylinders show their superiority over the other bluff bodies in terms of enhanced heat transfer and reduced drag.
  • Acoustic analysis on the dynamic motion of vapor-liquid interface for the
           identification of boiling regime and critical heat flux
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Seok Bin Seo, In Cheol Bang In this study, the feasibility of identifying boiling regimes through acoustic emission (AE) measurement is tested, and the relationship between AE features and boiling dynamics is investigated. Conventional pool quenching experiments are performed using a spherical test section to investigate the entire boiling regime from film boiling to nucleate boiling. The AE measurements and accompanying optical visualization enable analyses of the AE features of each boiling regime, which are characterized by statistical and spectral parameters. The recorded boiling dynamics provide the history of vapor–liquid interface motion to verify the characteristics of AE features. The present work analyzes different acoustic features generated from different boiling regimes including nucleate boiling, transition boiling, boiling crisis, and film boiling. Each boiling regime is characterized by its measured AE signal with respect to various parameters. At the same time, optical visualization of the boiling process accompanies the AE measurements. A comparison of the wave information in vapor–liquid interface motion with AE features proves the feasibility of AE measurement for clear identification of boiling regimes and offers an insight into understanding boiling dynamics with respect to vapor behaviors.
  • The diffusion-controlled remelting/resolidification process in
           directionally solidified Sn-Mn peritectic alloy
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Peng Peng, Xinzhong Li, Yanqing Su, Jingjie Guo, Hengzhi Fu The Sn-40at.%Mn peritectic alloy were directionally solidified in a temperature gradient at different growth velocities. It was demonstrated that the diffusion-controlled remelting/resolidification process occurred on the secondary dendrite arms in the mushy zone of Sn-40at.%Mn peritectic alloy. Theoretical analysis has shown that the diffusion-controlled solute transport which occurred between secondary dendrite arms was produced by both the temperature gradient zone melting (TGZM) and the Gibbs-Thomson effects (capillary effect). An analytical model was established to describe the diffusion-controlled solute transport during the remelting/resolidification process. The coupling influences of the TGZM and Gibbs-Thomson effects on the remelting/resolidification process during peritectic solidification was analyzed in terms of the specific surface area (SV) of dendrites. It was found that the remelting/resolidification process by the Gibbs-Thomson effect was retarded by peritectic reaction, while that by the TGZM effect was accelerated by peritectic reaction. Since the diffusion-controlled solute transport by the TGZM effect is dominant as compared with that by the Gibbs-Thomson effect in this work, the diffusion-controlled remelting/resolidification process of dendrites is accelerated during peritectic solidification in a temperature gradient.
  • Heat transfer model for horizontal flows of CO2 at supercritical pressures
           in terms of mixed convection
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Tae Ho Kim, Jin Gyu Kwon, Joo Hyun Park, Hyun Sun Park, Moo Hwan Kim When a material crosses its critical point or a pseudocritical temperature at which the specific heat of the material at a constant pressure is maximal, the thermal and hydraulic properties vary significantly. Buoyancy, induced by a great density variation of near-wall fluid, results in asymmetric heat transfer coefficients at the top and bottom walls of a horizontal circular channel. However, flow acceleration, which occurs in the flow direction because of significant density variations, has an identical effect on heat transfer regardless of the flow direction. For this reason, only the acceleration effect can be investigated in terms of a turbulent shear stress variation for horizontal flows. Therefore, a study on the buoyancy effect for horizontal flows is required with respect to not the shear stress variation but different aspects between the top and bottom walls.In this study, semi-empirical and empirical heat transfer models were proposed based on a mixed convection. The models were evaluated using experimental data. The semi-empirical model has a mean absolute difference (MAD), the average error, of 21.73% for the top wall and 22.35% for the bottom wall. However, the empirical model has a MAD of 10.00% for the top wall and 10.44% for the bottom wall. The proposed models significantly improve the prediction accuracy of the Nusselt number at each wall, as well as for the average Nusselt number compared to the previous correlations.
  • New effective thermal conductivity model for the analysis of whole thermal
           storage tank
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Min Ho Kim, Yong Tae Lee, Jiwon Gim, Abhishek Awasthi, Jae Dong Chung In this paper, a new effective thermal conductivity model is proposed and used to numerically investigate a whole tank designed for latent-heat thermal energy storage (LHTES). The tank was filled with phase-change materials (PCM) inside 9 × 9 × 20 spherical capsules. Previous studies have focused on the performance of only one capsule under the assumption that one capsule could represent the performance of the whole tank. The analysis of phase change involves much complexity, even for one capsule; thus it is challenging to analyze a whole tank due to the tremendous amounts of calculation time and memory capacity required. The new effective thermal conductivity model includes the effect of the natural convection in molten PCM, and estimates charging/discharging performance in the full scale system with reduced calculation time. This new effective thermal conductivity model can appropriately predict dissimilar melting behavior depending on the unique position of each capsule in the tank. The model was validated by comparison with experimental data, and also by rigorous numerical analysis including natural convection.
  • 3D simulations of pool boiling above smooth horizontal heated surfaces by
           a phase-change lattice Boltzmann method
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Xiaojing Ma, Ping Cheng Pool boiling heat transfer from smooth horizontal hydrophilic and hydrophobic heaters having a finite thickness under constant bottom wall temperature conditions is simulated numerically with a three-dimensional liquid-vapor phase-change lattice Boltzmann method. For a small heater, single bubble dynamics and temporal variations of heater’s top wall temperature in contact with the bubble are investigated. It is found that top wall temperature drops during initial bubble growth period at low wall superheats on a hydrophilic heater due to the microlayer evaporation process. Because of a residual bubble is left on the top of the heater after bubble departure, no wall temperature drop is found on a hydrophilic surface at high wall superheats nor on a hydrophobic surface at any wall superheats. 3D pool boiling curves in dimensionless forms for smooth horizontal heaters of infinite extent from nucleation to critical heat flux through transition boiling to stable film boiling are presented. It is found that 2D and 3D simulated heat fluxes are identical in nucleate boiling and stable film boiling regimes, although transition boiling heat fluxes differ greatly. It is also found that the 3D simulated CHF is in better agreement with existing theories and correlation equations than 2D simulated CHF. Detailed 3D multiple bubbles dynamics above hydrophobic and hydrophilic surfaces and temporal variations of dry spots distribution on these surfaces at CHF are presented. Effects of wettability and thickness of the heater on boiling curves for smooth heaters are investigated. It is founded that only wettability play predominate roles in nucleate boiling regime from smooth superheated surfaces while all factors influence the transition boiling regime (including maximum and minimum heat fluxes) greatly.
  • Numerical study of deformation and breakup of a multi-core compound
           droplet in simple shear flow
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Tri-Vien Vu, Truong V. Vu, Dang Thanh Bui In this paper, we present the numerical simulation results of deformation and breakup of a multi-core compound droplet. The method used is a two-dimensional front-tracking method with a modification for multi-layer droplets. The droplet is placed at the center of the domain whose top and bottom boundaries move in the opposite directions to create the shear flow. We vary the values of various parameters in specific ranges to investigate their effects on the deformation and breakup of the multi-core compound droplet in shear flow. We mainly focus on the two-core compound droplets. Basing on the numerical results, we reveal various patterns (i.e. modes) of deformation and breakup: non-breakup types 1 and 2, breakup types 1, 2, 3 and 4. For the first non-breakup type (type 1), the outer interface is deformed with two inner droplets accumulating at the center whereas in the other non-breakup type (type 2), the inner droplets move to the two furthest ends. The non-breakup type 1 generally changes to the breakup type 4, at some critical parameters, in which the compound droplet breaks up into simple droplets at its ends with a smaller daughter compound droplet in between. The non-breakup type 2 transits to the breakup type 1 or type 2, depending on the flow conditions, with the formation of two smaller daughter compound droplets at the ends. The remaining breakup mode (type 3) is a mixed breakup mode that is a combination of the breakup types 2 and 4. These deformation and breakup patterns are mapped onto a Re–Ca diagram. We also propose some other diagrams based on the variations of the inner droplet location. In addition, we investigate the effect of the number of the inner droplets encapsulated in the compound droplet on its deformation and breakup.
  • A compliant microstructured thermal interface material for dry and
           pluggable interfaces
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Jin Cui, Jicheng Wang, Justin A. Weibel, Liang Pan Thermal interface materials (TIMs), such as thermal pastes and pads, can successfully enhance contact thermal conductance by filling the gaps caused by the surface nonflatness and roughness. However, there is still an unaddressed demand for TIMs which can be applied to pluggable or reworkable interfaces in electronic systems, such as in opto-electronic transceiver modules. Reducing the contact thermal resistances at these interfaces has become increasingly important as device power density increases. These applications require dry contact interfaces that can offer the required thermal conductance under a low pressure and endure repeated mechanical compression and shear. We present a compliant metallized finned zig zag micro-spring array, as a low-cost dry TIM, that allows conformal interface contact at low pressures (∼10–100 s of kPa) by effectively accommodating surface nonflatness at a rate of a few µm per kPa. Experimental characterization of the mechanical compliance and thermal resistance confirm that this dry TIM can achieve conformal thermal contact between nonflat mating surfaces under low pressures. The total insertion thermal resistance of this dry TIM, even when mating to nonflat surfaces, is comparable to that of a polished and flat metal-to-metal contact. Mechanical compression and shear cycling tests are performed to assess the durability.
  • Effect of sintered microporous coating at the evaporator on the thermal
           performance of a two-phase closed thermosyphon
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Yeonghwan Kim, Dong Hwan Shin, Jin Sub Kim, Seung M. You, Jungho Lee In this study, the effects of sintered microporous coating at the evaporator on the thermal performance of a two-phase closed thermosyphon (TPCT) are experimentally investigated using distilled water as a working fluid. The TPCT comprises evaporator, adiabatic, and condenser sections, which have an inner diameter of 25 mm and lengths of 300, 300, and 325 mm, respectively. Copper microparticles are sintered on the inner surface of the evaporator to form a 500-μm-thick microporous structure. Boiling and condensation heat transfer coefficients on the microporous coated TPCT are obtained at filling ratios (FRs) ranging from 0.25 to 1.0 and inclination angles ranging from 90° to 5°. The boiling heat transfer coefficient at the microporous coated evaporator is enhanced owing to an increase in the number of nucleation sites and thin film evaporation in the liquid film formed by capillary force through the porous structure, especially at a low FR. The condensation heat transfer is also affected by the microporous coated evaporator due to the increased amount of vapor generation at the low FR. The overall thermal resistance of the microporous coated TPCT is reduced by 51% at an FR of 0.35 and 30% at an FR of 0.75. The TPCT used in this study shows the best thermal performance at inclination angles between 15° and 30° and FRs between 0.2 and 0.4.
  • A fully-developed boundary condition for the random walk particle tracking
    • Abstract: Publication date: March 2019Source: International Journal of Heat and Mass Transfer, Volume 131Author(s): Aaron M. Lattanzi, Xiaolong Yin, Christine M. Hrenya Random walk particle tracking (RWPT) is an effective and flexible approach to resolve scalar transport in direct numerical simulations (DNS) of single and multi-phase flows. It is often part of a hybrid scheme where the flow field is recovered by a separate hydrodynamic solver and the scalar field is resolved by RWPT. Since RWPT method tracks the displacement of passive Brownian tracers, rather than discretizing the advection–diffusion equation, development of boundary conditions for RWPT is not trivial. Namely, rules imposed upon a single tracer at the boundary must, after averaging, recover the desired continuum scale boundary condition. Here we develop means for imposing a fully-developed outflow boundary condition for the RWPT method. The technique developed here utilizes a semi-reflecting barrier at the outflow boundary. Tracers that reach the boundary plane are either reflected back into the domain or allowed to vacate the domain. We show that the semi-reflecting barrier developed here converges to the classic reflective boundary and open boundary in the asymptotic limit of low and high Peclet number, respectively. The new outflow boundary condition is verified against boundary layer (BL) theory for flow past a hot plate. The temperature field extracted at the outflow boundary is observed to be in agreement with BL solutions.
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