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Experimental Thermal and Fluid Science
Journal Prestige (SJR): 1.271
Citation Impact (citeScore): 4
Number of Followers: 25  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 0894-1777
Published by Elsevier Homepage  [3177 journals]
  • Study of the influence of water vapour and carbon dioxide dilution on
           flame structure of swirled methane/oxygen-enriched air flames
    • Abstract: Publication date: Available online 30 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): A. Vandel, J.P. Chica Cano, S. de Persis, G. CabotAbstractIn this work, the effect of dilution with water vapour H2O and carbon dioxide CO2 on the structure and stability of a methane/enriched air premix flame, confined and swirled at atmospheric pressure was studied. Measurements were carried out at constant adiabatic temperature (1773, 1873, 1973 and 2073 K), from air to oxycombustion (enrichment OI ranging from 21 to 100%), for two inlet temperatures (T0= 373K and 473K), at constant equivalence ratio maintained at 0.91 and inlet aerodynamic conditions maintained constant: average inlet velocity at 30 m.s-1 and geometric swirl number at Sn=0.90. The experimental setup is a burner used consisting of a swirled stainless steel single injector, mounted in a combustion chamber operating at atmospheric pressure. The flames were visualized by CH* chemiluminescence. From instantaneous images, the flame contour, detected with a Matlab® processing, was determined. From this flame contour, flame height (Hf) and lift-off height (Hlo) were extracted by measuring respectively the maximum height of the flame top and the minimum height of the bottom flame. The evolution of the mean macrostructure of the flame was then studied. The results of the steam dilution were detailed and compared with those obtained with carbon dioxide. Links between the laminar flame speed and the flame structure and stability were clearly established.
       
  • Pulsed Laminar Falling Films in Vertical Tubes: Maintaining a Continuous
           Liquid Film with Reduced Film Thickness
    • Abstract: Publication date: Available online 29 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Wei Zhong, Tao He, Jon LongtinAbstractFalling-liquid-film heat transfer is important to a wide variety of industrial applications, including two-phase thermosyphons, falling-film evaporators in the food industry, heat exchangers in fuel-ethanol processing, and the chemical service industry. Particularly for laminar falling films, the heat transfer increases as the film thickness decreases, hence it is desirable to keep the films as thin as possible. This paper presents a periodically-pulsed flow concept to reduce the average mass flow inside a vertical tube. The technique provides thinner films than can be achieved with continuous flow. A drainage model is adopted to predict the film thickness as a function of flow rate and location along the tube. The concept is confirmed experimentally by measuring the film thickness in real time in a circular tube using a laser-based absorption technique. A new heat-transfer-based film continuity measurement is also integrated into the optical film measurement setup. Water at ambient temperature and pressure is used for this work. Film thickness reductions of up to 60%, and liquid-film volume-flow reductions of up to 80% compared to the minimum continuous-flow film have been obtained. The time-varying film thickness profile can also be controlled to a limited extent, which may be of interest for process industries.
       
  • An improved study of the uniformity of laminar premixed flames using laser
           absorption spectroscopy and CFD simulation
    • Abstract: Publication date: Available online 26 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Liuhao Ma, Kin-Pang Cheong, Hongbo Ning, Wei RenAbstractWe performed a combined experimental and numerical study of the flame uniformity for a standard McKenna burner by investigating the spatially-resolved temperature and species concentrations in laminar CH4/air premixed flames. In the experiment, multi-wavelength laser absorption spectroscopy (LAS) was employed for in situ, non-intrusive, and quantitative measurements of temperature, H2O and CO2. In the simulation, computational fluid dynamics (CFD) simulations were performed to model the thermochemical structures of the flames. The LAS measurements and CFD simulations were compared by varying the co-flow rate, equivalence ratio (Φ = 0.8−1.2), and the Reynolds number of the reactant flow. In general, all the simulations were in relatively good agreement with the experimental data. We observed that the flame temperature and H2O/CO2 concentrations have almost the identical radial distributions with a certain non-uniformity, which should be carefully considered when using the standard flame for combustion research. In particular, the flame uniformity could be improved by increasing the Reynolds number of reactant flow or reducing the co-flow rate.
       
  • Experimental and theoretical study of bubble coalescence and departure
           behaviors during nucleate pool boiling on uniform smooth and
           micro-pin-finned surfaces under different subcoolings and heat fluxes
    • Abstract: Publication date: Available online 26 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Jie Zhou, Baojin Qi, Yonghai Zhang, Jinjia Wei, Yang Yang, Qian CaoAbstractThe bubble behavior is significant to the boiling heat transfer process. Among which, the bubble coalescence has been widely concerned by researchers because of the complex flow and heat transfer process, which can promote the bubble departure and enhance the heat transfer coefficient. In this paper, bubble coalescence and departure were investigated in the boiling process. The variation of vapor bridge during the coalescence and the oscillation of bubble radius in the rising period were quantitatively obtained. The departure velocity of the merged bubble under different conditions is also studied, which increases with the increase of the average radius of the single bubble before coalescence. In order to reveal the mechanism of bubble coalescence and departure, the system energy balance is analyzed. Meanwhile, the influence of the net force on the kinetic energy of the merged bubble is also considered. The force-modified energy balance model is proposed and the calculated results are consistent with the experimental data. This paper focuses on the bubble departure behavior after the coalescence and analyzes the factors affecting the bubble departure velocity, which provides a new idea for the surface structure design on heat transfer enhancement from the view of promoting bubble departure.
       
  • Flow Characteristics of a Wall-Attaching Oscillating Jet over Single-wall
           and Double-Wall Geometries
    • Abstract: Publication date: Available online 25 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Shabnam Mohammadshahi, Hadi Samsam-Khayani, Omid Nematollahi, Kyung Chun KimAbstractThis paper presents the experimental results of two-dimensional time-resolved particle image velocimetry (2D-TR-PIV) measurements for two external geometries of a sweeping jet: a single-wall geometry and a double-wall geometry. The single-wall geometry has a non-confined backward-facing step, and the double-wall geometry has a confined step with a sudden expansion domain. The effect of the Reynolds number based on the hydraulic diameter (D) and mean velocity of throat (U) was also studied in the range of 2,000 to 10,000. The results show that the frequency of the oscillating jet is almost independent of the shape of the external region and varies almost linearly with the Reynolds number. In contrast, the velocity distribution of the flow is controlled by the external domain. The time-averaged velocity and vorticity data were acquired for the highest Re in the double-wall geometry, and there were three main vortices in the external domain that control the velocity distribution and a clear difference is observed. Other fields of view were also investigated in three different sections and the reattachment locations measured at a distance of 0.05D from the wall. The time-averaged results show that for both geometries, reattachment length was observed to decrease to the increase on Reynolds number.
       
  • Experimental assessment of the lean blow-off in a fully premixed annular
           combustor
    • Abstract: Publication date: Available online 23 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): R. Ciardiello, A.W. Skiba, R.L. Gordon, E. MastorakosAbstractThe behaviour of the flame in an annular combustor with multiple bluff-body injectors with swirl was investigated to provide insights into lean blow-off (LBO) mechanisms when flames interact. Two different configurations, with 12 and 18 burners, and various bulk velocities and equivalence ratios were tested. Flame shape and main features were studied by means of 5 kHz OH∗ chemiluminescence imaging and the stability limits were identified and compiled into stability regime diagrams. As the equivalence ratio of the mixture was reduced the individual flames would first exhibit a transition from a stable “W-shape” state to a stable “V-shape” state before becoming unstable close to extinction. In the 18-burner configuration LBO was characterised by random detachment and re-stabilisation of the flames over multiple burners across the chamber, until complete lift-off. In the 12-burner configuration the flame anchors on a few burners in azimuthally symmetric locations, making the overall flame less prone to global extinction. Finally, the stability curves were computed using a correlation based on the Damkhöler (Da) number and compared to single burner configurations. The beginning of the blow-off transient was found to be similar to the LBO condition for a single burner in the 12-burner setup, while the 18-burner configuration was less stable for all the conditions investigated. However, it was found that correlations based on single burner extinction data do not fully work for the extinction of interacting flames. The results provide insights into the blow-off of realistic gas turbine engines and can be used for validating models of such processes.
       
  • Experimental study of submerged gas jets in liquid cross flow
    • Abstract: Publication date: Available online 22 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Ping Dong, Bingju Lu, Shaofeng Gong, Dong ChengAbstractGas jets submerged in the liquid cross flow are prevalent in the nature and essential in numerous industrial applications. The interaction between the gas jet and liquid cross flow is not well characterized for the difficulties in experimental research. In the present work, a new experimental method was designed and the evolution of the gas jet in liquid cross flow was captured by the high speed camera. Five experimental cases with different initial liquid cross flow velocities (0.35 m/s, 0.7 m/s, 1.0 m/s, 1.5 m/s, 2.0 m/s) were performed where the momentum ratio of the initial gas jet to liquid cross flow varies to highlight the effects of the liquid cross flow velocity on the flow characteristics. The summation and statistical methods were adopted to analyze the experimental sequences, and the results from the two methods showed good agreement. The gas jet morphology and the gas liquid interface were clearly identified from the experimental results, which showed that the gas jet deflected in a less pronounced manner with less unsteadiness generated at the gas liquid interface as the liquid cross flow velocity decreases. The gas jet penetration length in the momentum region was measured and it was found to be severely influenced by the liquid cross flow, and a power relationship was found between the gas jet penetration length and the cross flow velocity. The expansion angle of the gas jet in the upstream was also shown to grow in a power function with the liquid cross flow velocity. The gas jet diameter indicates the ambient liquid entrainments and the gas liquid interface evolution. In the region where the gas jet momentum is dominant, the evolution of the gas jet diameter followed a similar pattern with the submerged gas jets without liquid cross flow. Finally, empirical correlations were developed to predict the characteristic parameters of the submerged gas jet in liquid cross flow.
       
  • Experimental Investigation on Convective Heat Transfer of Shear-thinning
           Fluids by Elastic Turbulence in a Serpentine Channel
    • Abstract: Publication date: Available online 22 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Haie Yang, Guice Yao, Dongsheng WenAbstractElastic turbulence has shown great potential to enhance heat transfer performance at the microscale. Most of the studies, however, have only considered global convective heat transfer performance along curvilinear channels, despite that the intensity of the chaotic flow varies along the streamline, leading to different local heat transfer characteristics. This work systematically investigated the local convective heat transfer performance by elastic turbulence of a shear-thinning fluid in a serpentine channel. The flow visualization along the serpentine channel was obtained and analyzed to show the existence of elastic instability and elastic turbulence. Significant enhancement of mixing was observed with the increase of polymer concentration and bulk flowrate, suggesting the occurrence of elastic instability and elastic turbulence. The variations of pressure drop, heat transfer coefficients and Nusselt numbers along the serpentine channel were analyzed to reveal local characteristics of elastic turbulence. A three-stage pressure drop profile was identified due to the variations of viscosity and elastic turbulence intensity at different flowrates and Reynolds numbers. A non-linear heat transfer performance, which increased with the increase of polymer concentrations, was observed. These are mainly attributed to the increasing intensity of elastic instability, resulting from the balance between normal stresses and streamline curvatures. A large increase of Nusselt number versus Weissenberg number was also revealed due to the coupling of shear-thinning behavior and elastic instability effects.
       
  • Enhancement of jet impingement heat transfer using surface roughness
           elements at different heat inputs
    • Abstract: Publication date: Available online 20 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): K. Nagesha, K. Srinivasan, T. SundararajanAbstractExperiments have been performed to characterise the heat transfer to a jet impinging normally on a heated surface, modified with multi-protrusions or V-grooves. The jet flow is in turbulent regime with Reynolds number lying in the range of 10,000–27,500. The dimensions of multi-protrusions and V-grooves are selected to be of the order of laminar sublayer thickness. Heat transfer enhancement for the surfaces modified by roughness elements is compared with the results for a smooth flat surface. The average Nusselt number is reported as a function of jet Reynolds number, nozzle-to-plate spacing, roughness type, and nozzle diameter for 60 W and 90 W heat inputs. The Nusselt number increase with surface roughness elements is partly due to area increase and partly due to turbulence enhancement. Well-separated multi-protrusions cause higher heat transfer enhancement than closely spaced V-grooves. For higher heat input (90 W) and longer stand-off distances (Z/d = 10), it is observed that the effects of weak impingement flow field are counter-acted by natural convection plume flow emanating from the heated surface. Hence, in such cases, impingement cooling is not effective.
       
  • Auto-ignition characteristics of high-reactivity gasoline fuel using a
           gasoline multi-hole injector
    • Abstract: Publication date: Available online 16 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Jianguo Du, Balaji Mohan, Jaeheon Sim, Tiegang Fang, William L RobertsAbstractGasoline compression ignition (GCI) engines have proven to be a highly efficient engine technology with reduced emissions. The high efficiency and reduced emissions of GCI engines heavily rely on the stratification of the fuel being injected directly into the cylinder through multi-hole injectors. Therefore, it is critical to understand the fuel stratification and auto-ignition behaviors of the fuels used in GCI engines. Thus, in this work, the auto-ignition characteristics of high-reactivity and low carbon gasoline fuel (RON 77) were studied in an optically accessible constant volume chamber. A customized high-pressure gasoline multi-hole injector was used. Reactive tests were performed at two different ambient pressures (20 and 30 bar), three different ambient temperatures (800, 900, and 1000K), three different oxygen concentrations (10, 15 and 21%) and three different injection pressures (100, 300 and 450 bar). The auto-ignition of fuel was achieved with varying ignition delay based on the experimental conditions tested. It was found that the operating conditions profoundly influences the diffusion and partially-premixed combustion mode. For high ambient pressures, temperatures, oxygen concentrations, injection pressure, and combinations, diffusion combustion mode was observed, and partially premixed combustion mode was observed at lower ambient pressures, temperatures, oxygen concentrations, injection pressures, and their combinations.
       
  • Electrical characteristics of the oxyfuel flame while cutting steel
    • Abstract: Publication date: Available online 16 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Teresa L. Pond, Christopher R. MartinAbstractWe present the first published record of electrical phenomena in the oxyfuel cutting torch flame while cutting a steel work piece. Measurements of the voltage-current characteristic between the torch and the work piece demonstrate three piecewise-linear regimes in the ±10V range. Flame resistance measaffaddlineurements in the ohmic regime are studied using three candidate excitation signals while varying fuel/oxygen ratio, feed rate, and standoff distance. Electrical characteristics are also observed during successful and unsuccessful pierce and loss-of-cut events. The flame’s electrical resistance is found to be 2.8kΩ per mm of length; a factor of 3 smaller than the same conditions without cutting oxygen. We present an argument that the drastic increase in plasma conductivity is due to an abundance of electrons produced at the work piece. We draw preliminary conclusions about the potential applicability of these measurements for the in-process detection of standoff, pierce success, and loss-of-cut events.
       
  • Experimental Validation of Unsteady Pressure-Sensitive Paint for Acoustic
           Applications
    • Abstract: Publication date: Available online 15 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Jan Göß ling, Thomas Ahlefeldt, Michael HilferAbstractFast response Pressure-Sensitive Paint (iPSP) developed at the German Aerospace Center (DLR) in Göttingen is evaluated for measurements of acoustic pressure distributions. A test facility is constructed, which allows to measure these acoustic pressure distributions with iPSP. The aim of this evaluation is to detect pressure amplitudes below 100 Pa with sinusoidal and white noise acoustic excitation between 1 and 4 kHz. The following data analysis methods are applied to increase the signal-to-noise ratio (SNR): phase averaging, proper orthogonal decomposition (POD), dynamic mode decomposition (DMD), and fast Fourier transform (FFT). DMD is identified to be very powerful in extracting acoustic pressure fluctuations and eliminating image noise, but FFT achieves comparable results in this application. The used measurement setup combined with the DMD or FFT are capable of detecting pressure levels below 11 Pa or 114 dB sound pressure level (SPL) in cases with white noise excitation with detected mode frequencies up to 4615 Hz. The minimal detectable pressure limit during this investigation is 5 Pa or 108 dB (SPL) at 1318 Hz and sinusoidal acoustic excitation. The results from iPSP are compared to conventional measurement technique, flush with the surface mounted microphones, with good agreement.
       
  • Time-resolved particle image velocimetry measurement of vortex dynamics
           behind tandem self-oscillating inverted flags in a channel flow
    • Abstract: Publication date: 1 April 2020Source: Experimental Thermal and Fluid Science, Volume 112Author(s): Yujia Chen, Zhiwen Deng, Yingzheng LiuAbstractThe coupled flapping behaviors of two tandem inverted flags and the resultant vortex dynamics in a turbulent channel flow were experimentally determined using time-resolved particle image velocimetry (TR-PIV). The highly unsteady flow fields near the tandem self-oscillating flags with dimensionless separation distances of G* = 1, 1.5, 2, 2.5 and 3 were measured using time-resolved particle image velocimetry. Furthermore, the instantaneous flag profiles were successfully identified by the structure boundary detection algorithm. The time histories of the displacement of the monitor points indicated that the front and rear flags flapped synchronously with a constant phase difference, which was linearly dependent on the separation distance. The flapping amplitude of the rear flag was inferior to that of the front flag. A vortex pair (P) and a single vortex (S) shedding from the front flag were observed in the phase-averaged velocity vector maps and vorticity strength contours. The leading edge vortex excited an anti-rotating wall-induced vortex, which had the potential to disturb the thermal boundary layer, enhancing the wall heat removal performance of the channel flow. The phase-averaged pressure fields were estimated to understand the interactions between the flapping flags and the surrounding fluid. The vortex destruction process decreased the pressure difference on both sides of the rear flag, resulting in the attenuation of the rear flag’s flapping amplitude. Finally, the cross-correlation between the flag’s displacement and the fluid’s velocity fluctuation demonstrated that the front flag’s trailing edge vortex traveled downstream and encountered the rear flag with a constant convecting velocity, which explained the linear dependency of the phase difference between the tandem flags on their separation distance.
       
  • Physical study of the 3-dimensional characteristics and free-surface
           properties of a breaking roller in bores and surges
    • Abstract: Publication date: Available online 14 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Davide Wüthrich, Rui Shi, Hubert ChansonAbstractBreaking surges and bores are observed during flood events, tidal bores and tsunamis propagating in rivers. The sudden increase in water depth generates an aerated and recirculating region, called the roller, whose turbulent behaviour is poorly understood. Based on ensemble-average analyses with multiple repetitions, this experimental work processed high-speed videos from multiple locations to characterise the spatial and temporal dynamics of the bore’s roller. The results showed different air entrainment mechanisms for increasing Froude numbers, providing adapted formulae to predict the extension of the shear layer and the air-water boundary. Seen from above, the roller toe perimeter had an indented profile rapidly evolving in time, revealing a certain level of periodicity. A statistical analysis in terms of position of the roller toe, longitudinal fluctuations and instantaneous celerities allowed for a detailed characterisation of the turbulent surface fluctuations and three-dimensional properties of the breaking bore roller, leading to a better understating of the governing process.
       
  • Liquid Jet breakup behind a Pylon in Supersonic Flow
    • Abstract: Publication date: Available online 13 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Nikhil Verma, Aravind VaidyanathanAbstractThe thin pylon has been found to be one of the most promising hypermixers, offering enhanced mixing of fuel with minimum pressure loss. In the present study, the effect of pylon height and the angled injection on the liquid jet breakup and its effect on atomization in a free stream of Mach 1.71 has been studied experimentally. Acetone is used as the liquid medium to characterize the liquid jet injection in supersonic flow. Acetone is injected into the flow in the transverse direction behind the pylon at different mass flow rates for 3 different pylon geometries and 3 angled injection cases. The height of the pylon is varied (17.32mm, 10mm, 6mm) and the injection angle is such that it impinges against the pylon wall at three different angles of 30°, 45° and 60° while maintaining the other geometrical features to be the same. The study includes Schlieren imaging to find the effect of liquid injection and Pylon on the compressible flow field, and Shadowgraphy visualization was employed to evaluate the jet disintegration and atomization; Mie Scattering visualization techniques were used to for evaluating the Lateral spread of liquid jet spray. Mathematical techniques such as temporal variance, POD and DMD are used to analyze the temporal behavior of the jet spread. It is observed that the pylon height not only increases the penetration height but also has a significant effect on the jet breakup and atomization. It is also observed that the shear layer plays a major role in jet atomization and penetration height. Angled injection case was observed to perform better than transverse injection in terms of penetration height and jet spread.
       
  • Pseudo real-time continuous measurements of particle preferential
           concentration in homogeneous isotropic turbulence
    • Abstract: Publication date: Available online 13 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Kyuho Han, Hoonsang Lee, Wontae HwangAbstractPreferential concentration is a phenomenon where particles in turbulent flow concentrate in regions of low vorticity and high strain rate, forming clusters. This phenomenon is prevalent in nature and various industrial environments. Previous measurement systems for detecting this phenomenon have been based mostly on discrete images, and is relatively slow because of the image post processing involved. We have developed a new fast-response continuous measurement system comprised of a photomultiplier tube (PMT), custom-designed lens tube, and a continuous wave Nd:YAG laser. Experiments were conducted in a chamber that creates homogeneous isotropic turbulence with no mean flow. Image-based Voronoi analysis and box counting analysis from a high-speed camera were compared against the new PMT-based method for three Stokes number cases of near 0, 2.2, and 10. As the Stokes number was varied, the new technique showed similar results compared to the conventional camera-based methods.
       
  • Flow control over a diamond-shaped cylinder using slits
    • Abstract: Publication date: Available online 11 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): İlyas KarasuAbstractIn this experimental study, flow control over a diamond-shaped cylinder using slits was investigated using Particle Image Velocimetry (PIV). The slits used to form self-generating jet flows were placed in the middle of the four sides of the diamond-shaped cylinder at a Reynolds number of 8.6×103. To investigate the effect of the slit width/diameter ratio (θ) on the flow over the diamond-shaped cylinder, different θs, such as 0.035, 0.07, 0.105, 0.14 and 0.175 were studied. Various physical parameters such as the time-averaged streamline topology, the streamwise velocity profile, the rms (root mean square) velocity components, the turbulent kinetic energy and the estimated drag coefficient have been presented. In addition to these parameters, Fast Fourier Transform (FFT) and Proper Orthogonal Decomposition (POD) analyses were performed to expand on the flow behaviors. Results showed that while values of θ up to 0.07 for slits did not significantly affect the flow, beyond θ = 0.105, significant changes were observed for the flow structure in the wake region. With increasing θ, the shear layers were smaller, thicker and the strength of the vortex shedding of the diamond-shaped cylinder was attenuated by the jet flows emanating from the slits. The slits caused a considerable decrease in the root mean square (rms) values of velocity and turbulence kinetic energy (TKE/U∞2) in the wake region, which resulted in a decrease of the estimated drag by 37%. FFT results demonstrated that as θ increased, the Strouhal number (St) of the first dominant peak increased, while the amplitudes of the first dominant peaks reduced because of vortex shrinkage in the wake. In the POD analysis, it was observed that θ was influential on the vortex shedding behavior, and higher rolling-ups formed when the control was applied. According to the results, using slits could be an effective passive control technique for diamond-shaped cylinders.
       
  • Field measurement study of wind characteristics at different measuring
           positions in a mountainous valley
    • Abstract: Publication date: Available online 11 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Jing Hongmiao, Liao Haili, Ma Cunming, Tao Qiyu, Jiang JinsongAbstractThe characteristics of wind in mountain valleys are complex; in this paper, we present a comprehensive analysis of these characteristics using data obtained from two wind masts located at hillside and at the bottom of a canyon. In view of the limited literature focusing on the effect of the location of measurement on the characteristics of wind in mountain valleys, we present a comparison of these characteristics in this paper. The wind characteristics investigated in this study include the mean wind speed, mean wind direction, turbulence intensity, gust factor, wind power spectra, and vertical coherence. The results indicate that the mean wind speed, turbulence intensity, and wind power spectra cannot be predicted accurately using specifications or standards. The mean wind direction at a high wind speed is always along the canyon. A modified exponential coherence model can depict the vertical coherence effectively. Furthermore, wind characteristics are to some extent affected by the variations in height and locations of measurement, but still valuable for engineering practice, and a 30-m wind mast can meet the needs instead of a 50-m one.
       
  • Experimentally investigating the flow characteristics of airlift pumps
           operating in gas-liquid-solid flow
    • Abstract: Publication date: Available online 9 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Zhineng Wang, Yingjian Deng, Yang Pan, Yongping Jin, Fei HuangAbstractExperiment were conducted to investigated the flow characteristics of airlift pumps operating in gas-liquid-solid three phase flow by employing a high-speed camera. The results showed that an intermittent flow consisted of film and liquid slug appeared in airlift pumps with a flow frequency of 1.5-2.6Hz. The portion of slug body directly determined the performance of airlift pumps due to a large particle concentration. In practical engineering, some methods for increasing the portion of slug body could be used for better pump performance.
       
  • Experimental study of hypersonic boundary layer transition on a flat plate
           delta wing
    • Abstract: Publication date: Available online 9 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Shiyong Yao, Yi Duan, Pan Yang, Lei Wang, Xiaoli Zhao, Changwan MinAbstractThe boundary layer transition on a flat plate delta wing was investigated in the Mach 6 quiet wind tunnel via global temperature and wall pressure measurements as well as flow visualization. The results show that the boundary layer transition on side line is earlier than that on center line, and larger angle of attack promotes the boundary layer transition. As the Reynolds number increases, the transition position of boundary layer moves upstream. The hot streaks on the center line of delta wing at 0˚ angle of attack which are symmetrically distributed with respect to the center line at 10˚ angle of attack are suspected to be arised from the vortical flow rather than the boundary layer transition. High frequency disturbances occurring nonlinear phase coupling transfer the energy to low frequency structures, and finally the low frequency large scale structures induce to boundary layer transition. The intensity of nonlinear phase coupling prior to boundary layer transition is stronger than that in turbulent boundary layer. In addition, the unstable region of disturbance waves depends on the Reynolds number.
       
  • Stability and structure of inverse swirl diffusion flames with weak to
           strong swirl
    • Abstract: Publication date: Available online 8 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): A.M. Elbaz, W.L. RobertsAbstractFlame stabilization in many practical devices is achieved primarily through swirl; however, the application of swirl to an inverse diffusion flame and its effect on both flame stability and structure have not yet been reported. Therefore in this work, flame stability and structure of inverse diffusion swirling flames are investigated. The most stable flame is achievable at a critical swirl intensity, Scr. Lifted swirl jet-like flames at swirl intensity < Scr, and compact flames at swirl intensity> Scr are observed. Simultaneous PIV/OH-PLIF measurements are conducted to investigate the flame-flow interaction for three flames. The flame at Scr is stabilized due to the mutual dependence of two flame zones, the conical flame, and the flame root. The conical flame zone is located along with the inner shear layer (ISL) between the internal recirculation zone and the jet flow, while the flame root is positioned close to the burner nozzle. The ISL is favorable for flame stabilization due to the enhancement of the mixing of burned gas and fresh-fuel/air associated with the frequently-formed vortices. The flame root is stabilized due to the instantaneous opposed flow between the hot burned gas and the fresh reactants. The flame root essentially requires the upstream flow of hot exhaust gases associated with the IRZ. It is inherently unstable at high swirl intensities due to the high strain rate coupled with the strong IRZ. At low swirl intensities, the flame root is extinguished by the high momentum of the central jet, and a lifted flame is anchored in low-velocity regions of the jet, with the flame adjusting axially and radially to meet this criterion. These results emphasize the crucial role of the flame root in stabilizing the flame and suggest that well-aimed modifications of the flow field to further enhance mixing in this region may increase the stability of lifted jet-like swirling flames.
       
  • Geometry and Orientation Effects in Non-Uniformly Heated Microchannel Heat
           Exchangers Using Supercritical Carbon Dioxide
    • Abstract: Publication date: Available online 8 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Saad A. Jajja, Jessa M. Sequeira, Brian M. FronkAbstractThere is growing interest to use supercritical carbon dioxide (sCO2) as a working fluid in thermal management applications. This study investigates the thermal-hydraulic performance of microchannel heat sinks as a function of flow channel geometry and orientation at operating conditions representative of electronics cooling applications. Three different experimental test sections, subject to non-uniform heat flux boundary conditions, were investigated. Two ostatef the test sections contained parallel arrays of rectangular microchannels with a hydraulic diameter of 750 μm and aspect ratios of 1 and 2, respectively. The third test section had a staggered array of diamond shaped micro-pins with a hydraulic diameter, based on the minimum flow area, of 525.2 μm. Data were collected for varying inlet temperature (16⩽Tin⩽50 oC), mass flux (315⩽G⩽1000 kg m-2 s-1), and heat flux (20⩽q″⩽40 W cm-2) at a fixed reduced pressure (PR) of 1.1. A data analysis method using 2-D and 3-D heat transfer models of the test sections was used to calculate the average heat transfer coefficients for each experimental condition. Additionally, a pressure drop model was developed to resolve the total measured pressure drop into its components. The results of this study indicate that the turbulent convective heat transfer was independent of orientation (top versus bottom heating) for square microchannel (aspect ratio = 1) for the conditions investigated. Increasing the aspect ratio from 1 to 2 led to an enhancement in thermal transport. Finally, the heat transfer performance of the staggered pin array flow geometry was superior to the rectangular channels, but this enhancement in heat transfer was commensurate with the increase in pressure drop. Based on these results, this paper concludes with general design recommendations for those considering the early adoption of supercritical carbon dioxide for thermal management applications.
       
  • On unsteady velocity measurements and profiling in compression waves in an
           asymmetrical trapezoidal channel
    • Abstract: Publication date: Available online 7 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Ramith Fernando, Xinqian Leng, Hubert ChansonA compression wave is a highly-unsteady rapidly-varied free-surface flow associated with a sudden rise in water surface elevation and may travel over very long distances along flat prismatic canals. New experiments were conducted to investigate the unsteady turbulent characteristics of compression waves, in a large-size channel. The focus was on the effects of the asymmetrical trapezoidal shape of the channel cross-section on the transient turbulence characteristics of the three-dimensional compression wave. The measurements showed a complicated unsteady motion down the transverse slope, underneath the leading edge of the compression wave. A marked increase in free-surface elevation was observed during the surge passage. A three-dimensional transient motion was observed. An intense transient recirculation was seen next to the invert at the base of the transverse slope and in the shallow flow zones. Visual observations and velocity measurements showed some recirculation primarily in the shallow water region. The results highlighted strong secondary currents on the transverse slope, albeit for short, transient periods.Graphical abstractInstantaneous velocity contour maps before and during a compression wave in the trapezoidal channel - Flow conditions: Q = 0.054 m3/s, h = 0.0 m, Fr1 = 1.4, d1 = 0.151 m - (a) Arrival of compression wave (b) Deceleration phase (c) Shortly after compression wave passage, (Left) Vx; (Middle) Vy; (Right) VzGraphical abstract for this article
       
  • Experimental analysis on the behavior of water drops dispersed in oil
           within a centrifugal pump impeller
    • Abstract: Publication date: Available online 6 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Rodolfo Marcilli Perissinotto, William Monte Verde, Carlos Eduardo Perles, Jorge Luiz Biazussi, Marcelo Souza de Castro, Antonio Carlos BannwartThis paper aims to investigate the behavior of water drops in a continuous oil medium inside a centrifugal pump impeller working at eight operational conditions (up to 1200 rpm and 2.8 m3/h) with two-phase liquid-liquid flows. Water-in-oil dispersions were produced with low water cuts of about 1% in volume, thus the dispersed phase became arranged as water drops. Experiments for pump performance and flow visualization were conducted using a high-speed camera and a pump prototype with a transparent shell. Flow images revealed that the large water drops usually deform, elongate, and break up into smaller ones, especially at high pump rotations and oil flow rates, while small water drops tend to keep their spherical geometry without deformations and fragmentations. A sample of drops were tracked and their equivalent diameters, residence times, and velocities were calculated. The tracking indicated that the water drops travel random trajectories in the channels, generally undergoing a deceleration along their pathway. Furthermore, the residence times and the average velocities of water drops strongly depend on the flow conditions. For the conditions tested, the water drops presented equivalent diameters between 0.1 and 5.0 mm, average velocities from 0.4 to 1.7 m/s, and residence times between 30 and 152 ms. For a more complete analysis, the results achieved in this study are constantly compared with results available in literature regarding oil drops in oil-in-water dispersions.Graphical abstractGraphical abstract for this article
       
  • Vortices evolution in the flow over a sharp leading edge at low Reynolds
           number
    • Abstract: Publication date: Available online 6 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Yanguang Long, Jiangsheng Wang, Jinjun WangAbstractParticle image velocimetry and hydrogen bubble flow visualization techniques are used to experimentally investigate the vortex shedding and their formation in a separated boundary layer induced by the leading edge of a sharp-nosed plate. The Reynolds number based on the plate thickness is 370, while the maximal mean reverse velocity is 16% of the free-stream velocity. Two kinds of shedding vortices with different scales are identified: primary vortices (PVs) shedding periodically and secondary vortices (SVs) appearing irregularly with a relatively smaller scale. PVs are formed by the Kelvin-Helmholtz instability of the free shear layer with a consideration of wall effect, which are generated in the vicinity of the crest of the bubble, downstream of their formation region. The comparisons of conditional-averaged shedding periods with and without SV indicate that the formation of SV is related to the position where the adjacent downstream PV obtains maximal circulation. An SV tends to form when this position is further upstream. Moreover, the SVs are related to the disturbance amplification of 2f0, the high order harmonic of the PV shedding frequency f0. In the first half part of the bubble, the corresponding disturbance of 2f0 shows no distinct growth until the formation of SVs.
       
  • Gas Temperature and Boundary Layer Thickness Measurements of An Inert
           Mixture Using Filtered Broadband Natural Emission of Species at Rapid
           Compression Machine Conditions (Part II)
    • Abstract: Publication date: Available online 5 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): O. Samimi-Abianeh, M. Al-Sadoon, L. BravoAbstractOur understanding of the combustion process is hindered by the boundary layer at the optical access. To be detected by the camera, the infrared emission of the mixture (or the laser) should pass the boundary layer at the window of the combustion system. Regardless of the type of experimental techniques used (either the laser or natural emission of the species), part of the emission could be absorbed by the boundary layer due to its low temperature. Hence, the measured data includes some uncertainty and errors due to the boundary-layer absorption and its thickness (it reduces the optical length), which have not yet been quantified due to the unavailability of the boundary layer thickness, especially during a transient process such as engine compression stroke.A new methodology was used to measure the gas mixture temperature and boundary layer thickness using the Rapid Compression Machine (RCM). The methodology is based on filtering and measuring the infrared natural emission of the species at two different band-pass wavelength ranges. The new methodology was applied to measure the gas temperature and boundary layer thickness by measuring the filtered infrared emission of methane, carbon monoxide and carbon dioxide at two compressed gas pressures and two compressed gas temperatures in this work. The measured and modeled gas temperatures were compared during the post compression. An excellent agreement within 1% between the measured and modeled gas temperatures were reached at all of the studied conditions.
       
  • Influence of nozzle lip geometry on the Strouhal number of self-excited
           waterjet
    • Abstract: Publication date: Available online 5 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Tengfei Cai, Boshen Liu, Fei Ma, Yan PanAbstractThe influence of nozzle lip geometry on the Strouhal number of the self-excited waterjet is studied. Jets emanating from the self-excited nozzle with various lip parameters are investigated in a high pressure cell with Reynolds number 3×105 and 4 cavitation numbers (0.04, 0.06, 0.08, 0.1). Strouhal numbers are obtained from the pressure oscillation measured by a transducer. The variables of lip geometry affecting the Strouhal number include: expansion angles (0-40°), expansion length (L2/d = 1 - 3) and straight length (L1/d = 0.15 - 0.75). The Strouhal numbers of all cases show qualitatively similar behaviour as the expansion angle increases. The general trend is that the Strouhal numbers rise rapidly at a relatively small angle. After the sudden rise, the Strouhal numbers decrease first and then increase slightly. This is due to the balance of effects from the local cavitation number changed by expansion angle and the angle itself. The Strouhal numbers monotone decrease with the increase of expansion length. Even though the straight length does not influence the value of Strouhal number during the experimental range, we can infer that there will be no self-resonating phenomenon if the straight length is too long.
       
  • Evaluation method of transit time difference for clamp-on ultrasonic
           flowmeters in two-phase flows
    • Abstract: Publication date: Available online 5 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Hideki Murakawa, Shuhei Ichimura, Katsumi Sugimoto, Hitoshi Asano, Shuichi Umezawa, Katsuhiko SugitaAbstractTo achieve efficient energy management in industrial plants, it is crucial to measure the steam flow rates at the various points of consumption. Clamp-on time-of-flight ultrasonic flowmeters are useful devices to measure the steam flow rates in existing pipes. However, it is difficult to accurately measure the steam flow rates because of the large acoustic impedance difference between the pipe material and fluid, strong signal attenuation in the fluid, and high temperature. In addition, the steam wetness increases with heat losses and the presence of liquid film and droplets that creates noise in the detected ultrasonic signals, making it difficult to distinguish the target signals from the noisy ones. Hence, a new signal processing method is proposed to determine the transit time difference, particularly at lower signal-to-noise ratios. Two ultrasonic transducers were used to measure the ultrasonic time-of-flight, which varies depending on the flow rate. The transmitted ultrasonic signals were time-dependent due to the generation of guided waves in the pipe wall. The standard deviations of the target signals increased when the flow regime transitioned from stratified to annular mist flow. The guided waves significantly influence the success ratio of determining the transit time difference between the ultrasonic signals transmitted from the upstream and downstream transducers. Based on the results, the use of the standard deviation of the target signals is proposed for estimating the transit time difference. Further, as the standard deviation varied significantly depending on the flow regime, it can be used to identify the flow regime as well.
       
  • Experimental investigation on the condensation regime and pressure
           oscillation characteristics of vertical upward steam jet condensation with
           low mass flux
    • Abstract: Publication date: Available online 4 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Qingchuan Yang, Binbin Qiu, Weixiong Chen, Daotong Chong, Jiping Liu, Junjie YanAbstractAn experimental study was conducted to investigate the condensation regime and pressure oscillation characteristics of vertical upward steam jet condensation with low mass flux. Steam mass flux and water temperature were 8.34–16.71 kg/(m2·s) and 40–85°C, respectively. Steam bubble behavior, pressure oscillation amplitude and frequency were recorded and analyzed. Four typical regimes were found, namely, chugging, detached oscillatory, detached cracked and secondary bubble impinged regimes. A bubble condensation region map was developed by considering the effects of steam mass flux and water temperature. The average amplitude of pressure oscillation initially increased, and then decreased as water temperature increased. It reached its maximum value when water temperature was approximately 60–65°C, which was the transition temperature range from the chugging regime to the detached oscillatory regime. Meanwhile, the frequency of pressure oscillation was within the range of 12–28 Hz. These values were higher than those for a vertical downward steam jet with the same steam mass flux and water temperature. A dimensionless correlation was obtained to predict the Strouhal number of pressure oscillation frequency. The predicted values corresponded well with the experimental data. The deviation was within the range of –9.12% to +8.30%.
       
  • Developing Flow Pattern Maps for Accelerated Two-Phase Capillary Flows
    • Abstract: Publication date: Available online 1 November 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Luca Pietrasanta, Mauro Mameli, Daniele Mangini, Anastasios Georgoulas, Nicholas Michè, Sauro Filippeschi, Marco MarengoAbstractThe prediction of flow pattern transitions is extremely important to understand the coupling of thermal and fluid dynamic phenomena in two phase systems and it contributes to the optimum design of heat exchangers. Two phase flow regimes have been extensively studied under controlled mass flow rate and velocity. On the other hand, less effort has been spent in the literature on the cases where the flow motion is purely thermally induced and consequently the mass flow rate or the velocity of the phases are not known a priori. In the present work, flow pattern transitions and bubble break-up and coalescence events have been investigated in a passive two phase wickless capillary loop, where the mass flow rate is intrinsically not controllable. Modified Froude, Weber and Bond numbers have been introduced, considering the actual acceleration of the fluid and the length of the bubble as merit parameters for the transitions. The proposed nondimensional investigation was developed by analysing experimental data obtained with ethanol and FC-72, as working fluids, different heat input levels (from 9 to 24 W) as well as three different gravity levels (through a parabolic flight campaign). A new empirical diabatic flow pattern map for accelerated two-phase capillary flows is presented, together with quantitative criteria for the calculation of the flow regime transitions, defining the physic limits for the bubble coalescence and break-up. This kind of new regime maps will be useful to the further development of comprehensive designing tools for passive two-phase wickless heat transfer devices.
       
  • Experimental investigation of the heat transfer characteristics and
           thermal performance inside a ribbed serpentine channel during rotational
           effects
    • Abstract: Publication date: Available online 30 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Natthaporn Kaewchoothong, Pathomporn Narato, Chayut NuntadusitAbstractThe effect of the speed of rotation on heat transfer characteristics inside a ribbed serpentine channel was studied experimentally. In this study, ribs with square cross sections were located on the leading side (LS) and trailing side (TS) of the serpentine channel wall. The serpentine channel had an aspect ratio (AR) of 1.0 and a hydraulic diameter (Dh) of 50 mm. The rib pitch-to-height ratio (p/e), the rib height-to-hydraulic diameter ratio (e/Dh), the channel length-to-hydraulic diameter ratio (L/Dh), and radius ratio of the channel (r/Dh) were fixed at 10, 0.1, 8, and 5, respectively. There were four different types of geometries for the present study, viz. a smooth wall, a 90o ribbed wall, a 60o V-ribbed wall, and a 60o V-broken ribbed wall. The Reynolds number (Re) based on the hydraulic diameter of the channel was constant at 10,000, and the rotation numbers (Ro) were varied in the range from 0.0 to 0.30. The heat transfer coefficients in a serpentine channel were measured experimentally using the steady thermochromic liquid crystals (TLC) technique. In addition, the friction factor and the thermal-hydraulic performance were measured for each case. The results showed that the ribbed wall attained better regional heat transfer performances than the smooth wall for both stationary and rotating conditions at almost all locations. The heat transfer in the first pass was the highest on the TS wall, followed by the LS wall. A similar trend also occurred in the turn region. However, in the second pass, the heat transfer was the highest for the LS wall, followed by the TS wall. Due to the rotation, the Coriolis force acted on the TS wall in the first pass and on the LS wall in the second pass, while the direction of the centrifugal force was aligned with the radius of the rotation. As a result, the average heat transfer in the first pass was larger than it was in the second pass. The maximum average heat transfer and thermal performance inside a serpentine channel occurred in the case when there was a broken V-60o rib, i.e., it was increased up to about 2.1 – 2.4 in the range of Ro = 0.0 – 0.15. The thermal performance factor for the broken V-60o rib at Ro = 0.2 – 0.3 gained about 2.4 – 2.6, which was near the case of the V-60o ribs.
       
  • Effect of subatmospheric pressures on heat transfer, vapor bubbles and dry
           spots evolution during water boiling
    • Abstract: Publication date: Available online 30 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Anton Surtaev, Vladimir Serdyukov, Ivan MalakhovAbstractThe present paper reports results of the comprehensive experimental investigation of an influence of subatmospheric pressures on multiscale heat transfer characteristics during liquid pool boiling. Experiments were carried out in the pressure range of 8.8–103 kPa at saturated water boiling using high-speed IR thermography, high-speed visualization from different sides and the specially designed transparent ITO heater. This made it possible to obtain simultaneously extensive data set on the effect of reduced pressure on main characteristics of boiling, including heat transfer coefficients, nucleation site density, growth rate and departure diameter of vapor bubbles. High-speed visualization from a bottom side of transparent heater allowed to investigate an evolution of dry spots bounded by triple contact line depending on pressure for the first time. It was demonstrated that the growth rate of dry spots is constant in time and has a non-monotonic dependence on pressure.
       
  • Phenomenological Study and Comparison of Droplet Impact Dynamics on a Dry
           Surface, Thin Liquid Film, Liquid Film and Shallow Pool
    • Abstract: Publication date: Available online 30 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Nuri Erdem Ersoy, Morteza EslamianAbstractIn the process of droplet impact on a dry surface or a liquid layer, several intriguing phenomena may occur. Due to the lack of comparative studies, this paper aims at phenomenological observation and analysis of droplet impact on a dry surface, thin liquid films, liquid films and shallow pools in the range of h* from 0 to 1.768, with Weber (We) number from 125 to 437, h* being the ratio of the liquid thickness to droplet diameter. For this purpose, a dyed water droplet was released on the substrate and a color high speed camera captured the details of the process including the interaction of the two like liquids, but with different colors. The times of reaching the maximum spreading diameters were compared for different choices of h* and We number. In addition, wave formation on the liquid film and the mixing of the like and miscible liquids were studied, briefly. A range of phenomena were observed at varying We number and h*. On a dry surface, at low We numbers, spreading occurred in the deposition mode, and the lamella at maximum spreading was pinned to the surface, whereas depinning and receding was observed at high We numbers. When the dry surface was replaced with a liquid layer, a range of impact phenomena emerged, for the same We numbers. On a liquid layer, depending on the We number and h*, crown formation and splash, surface waves, and the formation and breakup of the central Worthington jet were observed and discussed. Our experimental observations resulted in identifying three distinct regimes of thin liquid film (TLF), liquid film (LF) and shallow pool (SP), where in each regime similar impact behavior is observed, different from those observed in other regimes. Among the results, we observed that, in the range of parameters studied here, spreading is the fastest in the TLF regime, faster than spreading on a dry surface, and it generally slows down with an increase in the liquid film thickness.
       
  • Modulation of aerodynamic characteristics of a finite wall-mounted square
           cylinder through steady jet injection
    • Abstract: Publication date: Available online 29 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): M.R. Rastan, A. Sohankar, D.J. Moreau, C.J. Doolan, M. AwasthiAbstractThis paper presents the results of an experimental study that investigates the flow behavior around a finite wall-mounted square cylinder (with the aspect ratio of AR = 7) subjected to a steady jet issued from the cylinder’s front face. Two wavy and continuous jets were considered to examine the jet velocity distribution effect. The wind tunnel measurements consist of smoke flow visualization, mean drag force, mean surface pressure, and velocity data obtained from hotwire probe, mostly at Reynolds number (Red, based on the cylinder width and inlet velocity) of 1.2 × 104. The ratio of the jet flow to the freestream, Г, was changed as 0 ≤ Г < 5. Both the wavy and continuous jets reduce the downwash flow contribution with the flow from the lateral faces. An increase of Г continuously amplifies this effect, and improves the flow control performance in drag reduction. The maximum pressure drag reduction of 29% and 16% were obtained for the wavy and continuous jets, respectively. Two characteristic continuous jet flow modes, namely deflected jet (Г < 1) and deflected oscillating jet (1< Г < 2), were identified. The both continuous jet deflection mode have a leading role in the pressure reduction on the front face, and subsequently in drag reduction. Oscillation of the deflected continuous jet at high-Г also significantly increases the vortex shedding frequency in the upper half of the cylinder, and makes the mean wake flow asymmetrical. On the other hand, the base pressure recovery is the major reason for the drag reduction when the wavy jet is applied. The wavy jet shows a slight deflection at high-Г; however, the mean wake flow remains symmetrical. Finally, the influence of jet flow control on the flow topology was discussed.
       
  • Proper orthogonal decomposition analysis of near-field coherent structures
           associated with V-notched nozzle jets
    • Abstract: Publication date: Available online 28 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): H.D. Lim, Junfei Ding, Shengxian Shi, T.H. NewAbstractTime-resolved particle image velocimetry measurements and proper orthogonal decomposition (POD) analysis were conducted on freely-exhausting V-notched nozzle jets at Re=5000. Energy redistributions from low order modes to higher order modes are observed, particularly for the first two POD modes typically associated with large-scale coherent flow structures. Furthermore, analysis of the first two POD modes reveals highly cyclical large-scale coherent flow structures formed along the nozzle peak-to-peak (PP) planes, while non-cyclical incoherent flow structures are observed along the trough-to-trough (TT) planes. POD mode coefficients reveal mode pairing behaviour along the PP-planes and reduced peak frequencies in their power spectral densities. In contrast, no mode pairing behaviour is observed along the TT-planes and multiple instances of the same frequency peak transcending two adjacent POD modes are observed instead. This suggests an energy cascade process whereby large-scale flow structures are broken down into smaller-scale ones at a common frequency. Finally, a comparatively sharper nozzle leads to earlier formations of flow structures along both PP- and TT-planes but does not significantly impact upon the periodicity or coherence of the flow structures.
       
  • Microfluidics approach for determination of the miscibility gap of
           multicomponent liquid-liquid systems
    • Abstract: Publication date: Available online 28 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Martin Hübner, Mirjana MincevaAbstractThe principles of microfluidics have been used to develop a novel method to determine compositions on the binodal of a biphasic system. Within this work, a sound theoretical background of the developed method is given, and its experimental applicability is shown. The method is based on a mass balance that correlates the composition of a heterogeneous, multicomponent biphasic system with the position of the phase interface in the microchannel. The proposed method was validated using a system of water, acetone, and toluene. The calculated compositions on the binodal are in a good agreement with published literature.
       
  • Flow pattern classification in liquid-gas flows using flow-induced
           vibration
    • Abstract: Publication date: Available online 24 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Felipe de Castro Teixeira Carvalho, Maurício de Melo Freire Figueiredo, Alberto Luiz SerpaAbstractThe multiphase flow is not only the most common flow in nature but also occurs in various major industrial fields. Furthermore, in many industrial plants, the single and multiphase flows generates vibration and noise. In the context of two-phase flows, a specific case of multiphase flow, the flow pattern determination is crucial to their analysis, and despite the recent progress and developments in flow-induced vibration for two-phase flows, it is still considered an open topic. This paper develops a novel algorithm for flow pattern classification using the vibration signal from a vertical pipe conveying a liquid-gas two-phase flow to determine the flow pattern. An experimental apparatus and procedure were developed to perform this investigation. The analysis in the frequency domain showed a distinct frequency band activity for slug and churn flows. The analysis in time domain showed a significant amplitude variation for these flow patterns. Finally, by using the RMS and Pearson correlation coefficient, it was possible to classify the studied cases accurately. The results present a non-intrusive technique to identify the flow pattern in two-phase liquid-gas vertical flows.
       
  • Visualization of features of atomizing circular liquid sheet using
           simultaneous Volume Laser-Induced Fluorescence
    • Abstract: Publication date: Available online 23 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Chetankumar S. Vegad, Amit Kumar, Satyanarayanan R. ChakravarthyPresent work is an experimental investigation of primary atomization of a radially expanding circular liquid sheet produced from a cylindrical vertical water jet issuing out from the convergent nozzle and impinging orthogonally on a horizontally placed cone-disc deflector below the nozzle. The radially expanding thin circular water sheet formed from the peripheral edge of deflector disc. The atomization process of expanding liquid sheet is captured simultaneously for the first time in the top and side views using a high-speed Volume Laser-Induced Fluorescent (VLIF). The experiments were conducted at 6 kHz framing rate for various jet Weber numbers (Wejet) over a range of 996 < Wejet < 7480. The top view VLIF revealed interesting spatiotemporal features of atomizing sheet such as outer edge thickness, liquid threads, ligaments and droplets and formation of perforations punctures (perforations) on the local sites of liquid sheet. Moreover, the evolution of the perforations leading to the breakup into ligaments and droplets was also captured in top view VLIF as well. Results show the liquid sheet outer edge (rim) thickness is unaffected by change in Wejet, whereas the rim velocity is found linearly increases with increase in Wejet. The liquid threads observed at the rim are subjected to end-pinching effect (as Ohnesorge number, Oh
       
  • Void Fraction and Speed of Sound Measurement in Cavitating Flows by the
           Three Pressure Transducers (3PT) Technique
    • Abstract: Publication date: Available online 22 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Claudia Esposito, Onur Yenigun, Jean-Baptiste Gouriet, Johan Steelant, Maria Rosaria VetranoAbstractSpeed of sound and void fraction are two key parameters in the characterisation of two-phase flows. However, accurate measurements require either intrusive or complex techniques. This paper reports on the Three Pressure Transducers (3PT) technique, which derives the speed of sound by measuring pressure fluctuations and which, thanks to its robustness and simplicity, could be applicable in harsh conditions. Therefore, the aim of this paper is to study in detail the feasibility of this technique against its limits and constraints in a cavitating flow. First, a numerical assessment of the technique is proposed to determine both the optimal transducers configuration and the sampling frequency. Then, the implemented algorithm was applied to a two-phase air-water mixture with well-known properties. Finally, the 3PT algorithm was used to study the behavior of a cavitating flow induced by an orifice. This last application highlighted the possibility to use this technique to characterize the bubble flow generated by a the orifice without the use of any optical access and by using a very compact experimental arrangement. The results obtained are also qualitatively compared to the images of the flow simultaneously acquired by a high-speed camera.
       
  • Effects of surface tension and viscosity on liquid jet breakup
    • Abstract: Publication date: Available online 22 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Yi Zhan, Yusuke Kuwata, Kiyotaka Maruyama, Tomio Okawa, Koji Enoki, Mitsuhiro Aoyagi, Takashi TakataAbstractThe breakup process of liquid jet was explored through visualization using a high-speed camera to develop a prediction model for the impact frequency (the number of droplet passing per unit time). The experiments were conducted for the three jet regimes of Rayleigh, first wind-induced and second wind-induced. The five liquids (water, two ethanol aqueous solutions, and two glycerin aqueous solutions) were used as the test liquids to explore the effects of surface tension and viscosity. Since the droplets were produced due to jet breakup, the impact frequency was zero just downstream of the nozzle and increased asymptotically with an increase in the distance from the nozzle. Thus, auxiliary correlations were developed for the minimum breakup length where the jet breakup is initiated, the maximum breakup length where the jet breakup is completed, and the mean droplet size and the impact frequency in the equilibrium region downstream of the maximum breakup length. Since the correlations were dependent on the flow regime, the boundaries between the three jet regimes were also determined using the present observation results. It was demonstrated that the impact frequencies calculated by the proposed model agree with the experimental data accumulated in this work fairly well in the Rayleigh and second wind-induced regimes, while agreement was deteriorated to some extent in the transition (first wind-induced) regime. For the application to the prediction of splashing rate during coolant leakage in sodium-cooled fast reactors, a simple method to ensure conservative estimation of the impact frequency was also proposed.
       
  • Experimental Determination of the Role of Increased Surface Area in Pool
           Boiling from Nanostructured Surfaces
    • Abstract: Publication date: Available online 21 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Brendon Doran, Bin Zhang, Abigail Walker, K.C. Pratik, W.J. Meng, Arden L. MooreAbstractThe use of nanostructured surfaces to enhance pool boiling heat transfer performance has previously been demonstrated for a variety of outwardly-projecting nanostructures such as nanowires and nanotubes. Such enhancement has been attributed to a variety of factors, including greater surface area, improved wickability, and superior nucleation site density as compared to unmodified surfaces. However, since these three phenomena are inherently interlinked with the presence of the nanostructures, isolating each one for independent study to truly understand its relative importance in enhancing pool boiling heat transfer has remained a challenge. In this work, nanoporous anodized aluminum oxide (AAO) films on metallic aluminum (Al) substrates are used to serve as an inverse representation of an aligned nanowire array with similar increase in surface area but without inter-nanowire/nanotube wicking action or significant change in observed static wetting behavior relative to Al control samples with a solid native oxide film. Further, it is shown via a combination of experiment and analytical modeling that the AAO-covered Al samples studied here do not represent a significant increase in nucleation site density relative to the control samples. In this way, the influence of enhanced surface area of a nanostructured sample on pool boiling performance was isolated and quantitatively determined. Pool boiling performance for Al samples with solid native oxides and AAO films was measured using a custom-built test setup, with the commercial, low surface tension, dielectric coolant NovecTM HFE-7100 as the working fluid. Results were interpreted via a nanopore wetting model along with imaging analysis of bubble size, which collectively point to wickability and nucleation site density playing a greater role than increased surface area in nanostructure-based pool boiling enhancement.
       
  • Influences of large fillets on endwall flows in a vane cascade with
           upstream slot film-cooling
    • Abstract: Publication date: Available online 21 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Adeola S. Shote, Gazi I. Mahmood, Josua P. MeyerAbstractInvestigations in cascade employ filleted blades to influence the near endwall secondary flows and total-pressure losses. The secondary flows aggravate the aerodynamic losses and endwall thermal stresses in the gas turbine passages. Investigations of different configurations of the slot-bleed flow from the endwall of cascade upstream show significant influences on the near endwall flows. In the present paper, the near endwall flow-field in a linear cascade employing filleted vanes and bleed flow from the upstream endwall slots is measured experimentally. Two fillet profiles are tested, one is larger than the other. The objectives are to quantify the combined effects of endwall fillet, fillet profiles, and film-cooling flow on the endwall region flow-field. The fillet covers the junction of vane and endwall upstream of the cascade throat region. The discontinuous bleed-slots near the cascade entrance provide the film-cooling flow on endwall and simulate the gaps between combustor/nozzle-vane discs or stator/rotor discs in the gas turbine. The inlet Reynolds number of 2.0E+05 is based on the chord length of vane profile. The density and temperature ratios of the coolant flow to mainstream are both about 1.0. The inlet blowing ratio of the film-cooling flow varies between 1.0 and 2.8. The flow-field is measured through the distributions of flow temperature, yaw angle, axial vorticity, and total-pressure losses along the vane passage. The effects on the flow-field are then presented by comparing the cases of filleted vanes with the cases of un-filleted vanes. The results of flow yaw angle and axial vorticity in the filleted passage are smaller than those in the passage without the fillet. The yaw angles responsible for the endwall secondary flows are the smallest for the smaller fillet (Fillet-2). The temperature field indicates the pitchwise distributions of the coolant on endwall specially near the pressure side are better when the smaller fillet is employed. Also, the weakened passage vortex of the endwall secondary flows in the filleted passage reduces the total-pressure losses. Although the total-pressure losses decrease at very high coolant mass flux with and without the fillet, the losses are always smaller for the filleted passage than for the un-filleted passage. The present investigation is beneficial for improving and optimizing the endwall film-cooling in the gas turbine passage.
       
  • Dynamic Leidenfrost temperature behaviors on uniformly distributed
           micropillars
    • Abstract: Publication date: Available online 18 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Dong Eok KimAbstractIn this study, we conducted single droplet impinging experiments to measure dynamic Leidenfrost temperatures (TLF) on surfaces with the uniformly distributed circular micropillars (diameter: 10 μm, height: 20 μm), where the pillar pitches varied from 15 to 1000 μm. Interestingly, the experimental results for two cases of Weber number showed that the TLF have the maximum values at a certain pillar pitch (i.e., 120 μm). In other words, TLF increases monotonically as the pitch increases to a certain value but decreases above that pitch. To explain quantitatively the experimental results, a model for the dynamic Leidenfrost phenomenon is presented with considerations of the wetting pressure exerted by droplet kinetics and capillary force, and the nonwetting pressure due to vapor flow between the droplet and the solid surface. In the model, TLF can be determined by the competing effects of the wetting and nonwetting pressures. According to the model, the wetting pressure decreases with an increase in the pillar pitch; however, it can reduce more largely the hydraulic resistance for the vapor flow, decreasing the nonwetting pressure and thereby increasing the TLF. For resolving the peak behavior of TLF, the concepts of the droplet contact radius and related time scale are introduced. Additionally, we conjecture that the droplet contact radius at the moment that the dynamic Leidenfrost phenomenon occurs increases with the pillar pitch, and it contributes to lengthen the vapor flow path and directly affects the nonwetting pressure. The present model procedure is technically supported by simple numerical simulations and theoretical bases, and it can explain appropriately the experimental TLF behavior.
       
  • Experimental study of the liquid velocity and turbulence in a large-scale
           air-water counter-current bubble column
    • Abstract: Publication date: Available online 17 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Thomas Ziegenhein, Dirk Lucas, Giorgio Besagni, Fabio InzoliAbstractMeasuring the local liquid velocity and turbulence in large-scale bubble columns with optical methods is complex and usually limited to low gas holdups in thin geometries. Comprehensive datasets in large bubble columns are therefore seldom published. Since the importance of Computational Fluid Dynamics (CFD) is increasing for multiphase applications, such data is also important for validating models for dispersed bubbly flows. In the present work the liquid velocity and turbulence in a pilot-scale bubble column is studied and a CFD validation database is generated by completing previous measurements of the gas void fractions and bubble sizes. For this purpose, we used a Particle Shadowgraph Velocimetry (PSV) technique that was intentionally designed to study the fluid dynamics in large-scale facilities. The measurements were realized in the 5.3 m high and 0.24 m diameter counter-current bubble column at Politecnico di Milano. The superficial gas velocity ranged from 0.37 to 1.85 cm/s, the counter current superficial liquid velocity from 0 to 9.2 cm/s. All operation points are in the so-called pseudo-homogeneous flow regime, in which the integral gas holdup (ranging from 1.02 to 7.55 %) increases linear with the superficial gas velocity but an inhomogeneous flow is present. The dominant frequencies of the bubbly flow, shear rates, and turbulence levels are increasing with increasing superficial gas velocity. With increasing superficial liquid velocities, the dominant frequencies are decreasing, the averaged liquid velocities are shifted downwards, but the overall turbulence levels remain constant. In order to investigate the smaller scales at which the bubble-induced turbulence is expected, a filtering process is proposed. As a result, the filtered turbulence levels of all operation points fall on a linear trend line when plotted over the local void fraction, which is the same result obtained in other studies in small, homogenous tabletop columns. The now available database for CFD validation contains the averaged liquid velocities, basic turbulence information, local void fractions, and the bubble sizes at two different heights. The data will be in particular useful to validate the capabilities of models to upscale bubbly flows from tabletop to the pilot-scale bubble columns.
       
  • Liquid Jet Formation during a Suspended liquid Suction Process
    • Abstract: Publication date: Available online 15 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Liang Hu, Hanghang Xu, Mingbo Li, Weiting Liu, Wenyu Chen, Haibo Xie, Xin FuAbstractIn the tubular co-flowing gas-liquid jet, liquid is sucked through a nozzle suspended above the flat gas-liquid interface. The liquid hump can be induced by the sudden pressure change just beneath the nozzle entrance. At a sufficiently fast initial gas superficial velocity, the interface undergoes a transition and liquid starts to be sucked. The suction flow is in the form of an axisymmetric liquid jet surrounded by an upwards coaxial gas flow. In the present study, we discuss the evolution and characteristics of the suspended liquid suction flow. The jet dynamics include the interface deformation and development (the hump stage), the tubular co-flowing jet (the spout state) and the destabilization of the liquid jet (the jet state). Tube inner diameter, suspension height, and initial gas superficial velocity are three important parameters that influence the suction flow. Due to the inward gas flow, a relatively large amount of pressure gradients is imparted to a small mass of liquid near a free surface, which leads to interface deformation. The lateral boundary of the liquid hump circumferentially shrinks and the top liquid gradually rises following the shrinkage. There is a comparatively stable axisymmetric liquid jet surrounded by an upwards coaxial gas flow inside the tube. Surprisingly, no evidence is shown the dependence of jet spout with initial gas axisymmetric superficial velocity below the nozzle through plenty of experiment figures and statistical analysis. On the other hand, the velocity difference between the fast light fast gas flow and the slow dense liquid is critical to the destabilization of the liquid jet.
       
  • Influence of air on heat transfer of a closed-loop spray cooling system
    • Abstract: Publication date: Available online 15 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Pengfei Liu, Ranjith Kandasamy, Huicheng Feng, Teck Neng Wong, Kok Chuan TohAbstractThe influence of air must be considered when designing a direct spray cooling system for electronics because of its inevitable existence. In the present study, the influence of air on spray cooling is experimentally investigated with the air volume concentration ranging from 0.06 to 0.65 and the chamber total pressure from 0.44 bar to 1.07 bar. Two dielectric fluids (PF-5060 and FC-3284) are studied due to their compatibility with electronics. The results show that in the current experimental range the spray cooling is mainly affected by the liquid subcooling and surface superheat when air is present. The chamber total pressure appears to have a negligible influence on the spray cooling heat transfer, which differs from the spray cooling behavior without air. This argument holds true from the incidence of nucleate boiling to near the transition boiling regime as long as there is a sufficient amount of air present in the system. The air-entrainment-induced secondary nucleation can possibly leads to the insensitivity of the spray cooling heat transfer to the system pressure. A correlation is developed by incorporating the influence of the spray characteristics, the liquid properties, the chamber pressure, the degree of liquid subcooling, and the surface superheat. It provides a good prediction of the heat transfer rate for air-present spray cooling under current experimental conditions.
       
  • Experiments on pool boiling regimes and bubble departure characteristics
           of single vapor bubble under subcooled bulk conditions
    • Abstract: Publication date: Available online 14 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Surya Narayan, Tajinder Singh, Atul SrivastavaAbstractAn experimental study for understanding the effects of subcooling of bulk fluid on the phenomenon of nucleate boiling heat transfer was carried out. The whole field experimental data was recorded in a purely non-intrusive manner using rainbow schlieren deflectometry as the optical diagnostic tool. Boiling experiments were conducted over a range of heat flux conditions (1⩽q′′⩽40kW/m2) and for varying levels of subcooling (0 K ≤ ΔTsubcooling ≤ 10 K). The recorded images were first been interpreted to understand the effect of subcooling on various regimes of boiling phenomenon, which include convection, isolated nucleate boiling and vertical coalescence of the departing bubbles. Observations on convection regime showed an increasing influence of natural convection heat transfer. Based on the formation of vapor bubble and its departure characteristics, the isolated nucleate boiling regime of the classical boiling curve was further segmented into three sub-regimes namely, stable vapor bubbles, oscillating vapor bubbles and oscillating-departing vapor bubbles. A mechanism explaining the oscillations of vapor bubbles under subcooled condition was proposed that takes into account the relative dominance of condensing apex region and evaporating microlayer region. Various bubble dynamic parameters such as equivalent bubble diameter, departure frequency, aspect ratio and oscillation frequency were determined quantitatively to understand the effect of subcooling on the bubble departure characteristics. Based on the experimental results, an empirical correlation for predicting the bubble departure diameter and evaporative heat flux was developed and proposed.
       
  • Evaluation of capillary wetting performance of micro-nano hybrid
           structures for open microgrooves heat sink
    • Abstract: Publication date: Available online 11 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Jinchen Tang, Xuegong HuAbstractThermal management for various applications have promoted the development and in-depth investigations of open capillary microgrooves. Micro-nano hybrid grooves heat sink (combined of microgrooves and Ti nano-coatings) is proposed in this study. The wicking performance, which in terms of capillary wetting length, capillary wetting uniformity and capillary performance parameter determines the design and thermal management of the microgrooves heat sink. In this study, wicking performance from micro-nano hybrid grooves heat sink has been studied experimentally, with theoretical analysis. The wicking performance of the smooth and fully coated with Ti nano-coatings on borosilicate glass surfaces (3 different thicknesses), with 3 different grooves dimensions, has been studied in detail. A quantitative and automatic optical method using IR imaging is applied in this study to identify the capillary wetting length (dryout point) through the whole width of the microgrooves, even in each groove, based on the pixel differences of an IR image. Results show that the Ti nano-structured surfaces significantly improve the capillary wetting performance, compared with bare surfaces.
       
  • Experimental study on flow and heat transfer characteristics at onset of
           nucleate boiling in micro pin fin heat sinks
    • Abstract: Publication date: Available online 4 October 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Kong Lingjian, Liu Zhigang, Jia Lei, Lv Mingming, Liu YingAbstractThe micro pin fin heat sink has great application potential in the heat dissipation of high-power LED chips, phased array radars and semiconductor lasers. The onset of nucleate boiling (ONB) is a critical limit in designing heat dissipation systems. A simultaneous visualization and measurement study is conducted to investigate the ONB phenomenon of deionized water in circular, diamond, and oval micro pin fin heat sinks. Experiments are conducted under inlet subcooling of 29.6-49.5 °C, mass flux of 147.4-355.4 kg m−2s−1 and heat flux of 4.8-55.8 Wcm-2. Visualization result indicates that the ONB is always initiated at the surrounding region of a micro pin fin. The effects of additional nucleation sites caused by a micro pin fin and the temperature distribution of the surrounding region captured by an infrared camera can be used to explain the occurrence of the first bubbles. In this study, the effects of inlet subcooling, mass flux, and the cross-section shape of the micro rib on the ONB characteristics are investigated. Results show that the heat flux and wall superheat needed for the ONB increase as the mass flux and inlet subcooling increase. Moreover, the heat flux required for the ONB in the diamond and oval micro pin fin heat sinks is higher than that in the circular one. On the basis of the experimental data, the correlation of heat flux at the ONB in the micro pin fin heat sink is established, with 85.7% of the data falling within an error band of ±15%.
       
  • Optical Fibre Void Fraction Detection for Liquid Metal Fast Neutron
           Reactors
    • Abstract: Publication date: Available online 13 July 2019Source: Experimental Thermal and Fluid ScienceAuthor(s): Christophe Corazza, Kris Rosseel, Willem Leysen, Kristof Gladinez, Alessandro Marino, Jun Lim, Alexander AertsAbstractDetection and characterisation of voids in heavy liquid metals (HLM) is required for next generation nuclear reactor systems. However, its determination presents a challenge owing to their opaque nature and the high temperatures involved. Therefore, tools are needed that can be used to capture local, quantitative information at relevant nuclear operating conditions.In this work, the feasibility of using optical fibre sensors for the measurement of void fractions in liquid metals is presented. Since the functioning of optical probes is usually explained by a crude on-off model, the complexity of both the optical response and the hydrodynamic tip–interface interactions often remain hidden to new users. To clarify this point, well-controlled lab-scale experiments dealing with the response of three concept probe tip geometries are presented. Analysis of the obtained signal transients is used to provide guidelines for effective data processing. To conclusively demonstrate the principle, an optimal prototype system successfully determined the local void fraction and frequency in a pilot-scale reactor using liquid lead bismuth eutectic (LBE) as working fluid.The information obtained by the optical sensor allows for validation of computational fluid dynamics (CFD) models to aid the design of LBE-based nuclear reactors, as well as for other liquid metal–gas systems in general.
       
  • The influence of nozzle-exit geometric profile on statistical properties
           of a turbulent plane jet
    • Abstract: Publication date: Available online 31 January 2007Source: Experimental Thermal and Fluid ScienceAuthor(s): R.C. Deo, G.J. Nathan, J. Mi
       
 
 
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