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Combustion and Flame
Journal Prestige (SJR): 2.427
Citation Impact (citeScore): 5
Number of Followers: 144  
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ISSN (Print) 0010-2180
Published by Elsevier Homepage  [3206 journals]
  • Modified multipurpose reduced chemistry for ethanol combustion
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Alejandro Millán-Merino, Eduardo Fernández-Tarrazo, Mario Sánchez-Sanz, Forman A. Williams
  • Experimental and numerical analysis of the autoignition behavior of NH3
           and NH3/H2 mixtures at high pressure
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Liming Dai, Sander Gersen, Peter Glarborg, Howard Levinsky, Anatoli Mokhov
  • Single-head detonation propagation in a partially obstructed channel
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Mark Kellenberger, Gaby CiccarelliAbstractRecent experiments have revealed the propagation of a sustained single-head detonation propagation through an obstructed channel. In this study, new experiments and analysis of the complex three-dimensional behaviour of such a single-headed wave in stoichiometric hydrogen-oxygen mixtures is presented. An optically accessible channel of square cross-section containing obstacles with 50% area blockage was used in conjunction with soot-foils and dual-axis high-speed schlieren imaging to study the phenomenon over an initial pressure range of 17 kPa to 24 kPa. Detailed analysis of the wave reveals a transverse asymmetry generated by detonation diffraction whereby a transverse detonation propagates in a zig-zag fashion down the channel reflecting off opposite channel sidewalls. Analysis of the single triple-point trajectory reveals a detonation propagation velocity between 1.4 and 0.5VCJ, with the transverse detonation propagating at a constant 0.9VCJ until reflection on the channel sidewall. The trajectory of the triple-point is undisturbed by the presence of obstacles. In the vicinity of the triple-point, flow analysis indicates prompt ignition (
  • Scaling and prediction of transfer functions in lean premixed
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Eirik Æsøy, José G. Aguilar, Samuel Wiseman, Mirko R. Bothien, Nicholas A. Worth, James R. DawsonAbstractFeatures of the flame transfer function (FTF) are characterized for turbulent, non-swirled, bluff body stabilized “M” flames for different hydrogen and methane blends including pure hydrogen flames. An increase in the cut-off frequency of the FTF is observed for increasing hydrogen concentration. Modulations in the form of peaks and troughs in the gain and the phase were also observed and are shown to be caused by the interaction of two different flow disturbances, acoustic and convective, originating upstream of the flame. The first mechanism is due to the acoustic velocity fluctuations imposed at the base of the flame. A Strouhal number scaling based on the flame height and bulk velocity is shown to collapse the phase slopes and the cut-off frequencies. The second mechanism is shown to be due to vortex shedding from the grub screws used to align the bluff body in the inlet pipe. The associated convective time-delay is used to define a second Strouhal number which collapses the modulations in the gain and phase.A model is developed that separately considers the impulse response of each mechanism and is interpreted as a distribution of time lags between velocity fluctuations and the unsteady heat release rate. The distributed time lag (DTL) model consists of two distributions that are shown to capture all the features of the FTFs. The distributions show that the acoustic and convective mechanisms behave as a low pass filter and band pass filter, respectively. This results in a band of frequencies where they interact through superposition driving fluctuations of heat release rate. Similar interactions are shown to exist in the forced cold flow revealing that they are of hydrodynamic origin. Further, the band of frequencies are shown to be centered around the natural shedding frequency of the grub screws appearing as peaks in the unforced energy spectra of the velocity at the dump plane.Finally, a generalized model which takes as an input the bulk velocity, flame height and a geometric parameter is derived assuming a linear dependency of the DTL parameters. The model is shown to predict the behavior of the FTFs relatively well and can potentially be used to analyse regions in the operating conditions map which have not been experimentally tested.
  • Deflagration-to-detonation transition in hot HMX and HMX-based
           polymer-bonded explosives
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Gary R. Parker, Eric M. Heatwole, Matthew D. Holmes, Blaine W. Asay, Peter M. Dickson, John M. McAfeeAbstractThe deflagration-to-detonation transition (DDT) in hot, thermally damaged HMX (δ-phase) and HMX-based polymer-bonded explosives (PBX 9501, LX-14, LX-10 and PBX 9012) differs in some respects from what has been observed in similar tests (DDT tube experiments) with room temperature granular explosives. We provide streak images with other observations and demonstrate the behavior can be binned according to the degree of porosity evolved from physical and chemical damage to the compositions. In each bin, the DDT behavior eventually organizes to resemble Type I DDT, but differs in the early-stage burn phenomena and how a propulsive thermal explosion event arises. We argue that the hot explosive properties, such as permeability and compressibility, and the morphological characteristics of the thermal damage, control the physical mechanism for establishment of the thermal explosion. In some cases, these observations may need to be carefully implemented in numerical models to increase predictive value for these materials at elevated temperature. With high-porosity (ϕ  ≈  20–50%), the PBXs behaved like granular beds. At intermediate levels of porosity (ϕ  ≈  4–20%), the transition occurred over longer distances and the process is characterized by a weak, slow convective burn precursor that sets up thermal runaway to explosion behind the flame infiltration front. In the low-porosity bin (ϕ  ≈  0–4%), where run lengths were short, the thermal explosion is the result of compressive, and possibly also, deconsolidative burning. It is clear that the high-temperature conditions imposed in these experiments caused sensitization towards DDT and this effect was most apparent in tests where the explosive was heated until runaway and auto-ignition where transition distances were among the shortest measured. Practical findings include there being some safety benefit for having binders in HMX formulations, especially when the binder is thermally stable.
  • An experimental investigation on oscillating length scale of gas pipeline
           leakage flame restricted by parallel sidewalls
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Qiang Wang, Jin Yan, Liming Shi, Fei TangAbstractThis work focuses on studying the oscillating length scale of gas pipeline leakage flame restricted by parallel sidewalls with various separation distances. A series of experiments are conducted with 3, 6 and 10 mm nozzles and propane is used as fuel. Results show that reduction of the sidewall separation distance hinders the air entrainment, and provokes significant enlargement of the oscillation amplitude. Besides, the oscillating length scale of propane jet fire is also found to increase with the fuel flow rate. Finally, a new physical model, based on the scaling analysis of the fuel flow field and previous work, is proposed to characterize the variation of oscillating length for the restricted propane jet fire. The results obtained in this work and in previous studies, correlate with a better agreement using the present model than a previous suggested model. This work is not only a significant supplement to the flame oscillation instability physics from previous results for the flame restricted by parallel sidewalls, but also can provide some scientific basis to the design and management on the gas fuel energy storage and transportation systems in the cities to reduce the possible fire thread to the surrounding buildings.
  • Analysis of combustion modes in a cavity based scramjet
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Jiangheng L. Ruan, Pascale Domingo, Guillaume RibertAbstractLarge eddy simulations (LES) of non reactive and reactive flows in a cavity-based scramjet combustor configuration from the U.S Air Force Research Laboratory (AFRL) are performed. These simulations feature a 22 species and 206 reactions chemical scheme for ethylene/air. The ability of LES to reproduce the main features found in the experiment is first emphasised such as the average velocity field and the stability of the combustion for the case studied. The influence of the mesh resolution and of the thermal wall condition on the simulation results is also investigated along with the soundness of the use of a laminar model for the filtered source terms. The results of the simulations with the finest grid (resolution of 100 micrometers in the flame region) are then employed to gain understanding in the flame dynamics. This reactive simulation shows the persistence of the two recirculation zones already present in the non reactive flow. The globally high temperature into the cavity helps to sustain a reactive zone located in the mixing layer above the cavity. Combustion first occurs in a diffusion dominated regime followed by the efficient burning of a well stirred mixture (rich then lean). A significant diffusion dominated burning is also found inside the cavity. The links between the residence time inside the cavity and the efficiency of the combustion are explored along with the velocity/heat release correlation.
  • Pressure gradient tailoring effects on the mechanisms of bluff-body flame
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Anthony J. Morales, Jonathan Reyes, Peter H. Joo, Isaac Boxx, Kareem A. AhmedAbstractThe mechanisms of flame blowout under pressure gradient effects are explored for a bluff-body stabilized flame. The blowout process is induced through equivalence ratio reduction from a lean stabilized flame to complete blowout. Simultaneous high-speed particle image velocimetry (PIV) and C2*/CH* chemiluminescence imaging diagnostics are used to obtain the instantaneous flame structure, vorticity field, equivalence ratio, and local strain rate during the extinction process. The goal is to elucidate the effect of flame-generated vorticity on lean flame extinction. Three test-sections configured as a nozzle, a rectangular duct, and a diffuser, are used to alter the downstream pressure gradient yielding high, nominal, and low magnitudes of flame-generated baroclinic torque. For all three configurations, the flame brush narrows and the shear layer vorticity expands in the transverse direction resulting in flame-shear interactions and extinction. The flame-shear layer interaction increases the strain rate along the flame; however, the strong flame-generated vorticity for the nozzle case delayed the strain rate increase by keeping the flame away from the shear layer the longest. The sharp increase in the Karlovitz number above unity caused by the sudden increase in the strain rate corresponds to the time of flame brush contraction and shear layer width expansion. It is shown that the downstream pressure gradient can either augment or attenuate the time required for the Karlovitz number to reach a critical value of unity, which is associated with local extinctions along the flame. In all of the test-section configurations, the flame-generated vorticity has a weak influence on the Bénard von Kármán (BVK) instability mode and its harmonics. The Strouhal number during blow-out remained relatively constant in all of the cases showing greater sensitivity to the shear layer length than to the BVK frequency.
  • On the interaction between turbulence and ethanol spray combustion using a
           dynamic wrinkling model coupled with tabulated chemistry
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Fernando Luiz Sacomano Filho, Arash Hosseinzadeh, Amsini Sadiki, Johannes JanickaAbstractInvestigations of the turbulence-flame interaction are conducted for ethanol spray combustion with dynamic flame surface wrinkling (DFSW) models in the context of the Artificially Thickened Flame (ATF) approach. DFSW modeling strategies derived for single-phase flows are extended to improve the characterization of the mixture stratification encountered in dilute spray flames. The two-phase flow is described following an Eulerian–Lagrangian approach within the Large Eddy Simulation (LES) framework under consideration of two-way coupling between both gaseous and liquid phases. In particular, combustion reactions are captured by means of the Flamelet Generated Manifold (FGM) method. An evaporation model accounting for the inter-phase non-equilibrium is applied to address droplet’s heat and mass transfer. Main aspects of the suggested DFSW modeling strategy are systematically analyzed. The obtained results demonstrate fair improvements not only in the estimation of the flame structure, but also in the prediction of the spray properties in comparison to a reference model and the available experimental data. This outcome reveals the strong link between turbulence, flame and dispersed phase, as well as the necessity of a suitable computation of each one of these phenomena.
  • Non-intrusive laser-induced breakdown spectroscopy in flammable mixtures
           via limiting inverse-bremsstrahlung photon absorption
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Sungkyun Oh, Campbell D. Carter, Youchan Park, Sangeun Bae, Hyungrok DoAbstractFor the purpose of employing laser-induced breakdown spectroscopy (LIBS), nanosecond laser pulses (6 ns FWHM) from a standard Q-switched Nd:YAG laser (2nd harmonic) are modulated or chopped—using a novel method utilizing a variable air-pressure optical cell—to limit the inverse-Bremsstrahlung photon absorption process occurring in the breakdown plasma. The resulting plasma does not ignite flammable mixtures, but plasma emission is sufficiently strong to enable the characterization of reactants without strong perturbation (i.e., no ignition or shock wave results from the breakdown plasma). Two strong atomic emission lines, H (656 nm) and N II (568 nm), are chosen to find correlations between the plasma emission spectrum and the fuel concentration; plasma emission is captured after a delay of 20 ns with a gate width of 60 ns, the time over which there are well defined emission peaks. The chopped laser pulse is created within the pressurized optical cell with an initial breakdown plasma, by focusing the beam in high-pressure air. The pulse width of the laser beam transmitted through the cell is dependent on the cell pressure, and for this work, a pulse duration of approximately 600 ps was derived from the cell operated at a pressure of 10 bar. The chopped pulses were used for LIBS, and a 2D concentration distribution of a stoichiometric methane-air flow in ambient air were recorded without combustion reactions initiated by the laser-induced plasma.
  • Theoretical kinetics of the
           C2H4 + NH2 reaction
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Tam V.-T. Mai, Minh v. Duong, Lam K. HuynhThe detailed kinetic mechanism of the C2H4 + NH2 reaction, an important reaction in both combustion and atmospheric chemistry, is first theoretically reported for a wide range of conditions (T = 250 – 2000 K & P = 1 – 76,000 Torr). The accurate composite electronic structure method W1U was used to explore the potential energy surface (PES) on which the temperature-and pressure-dependent kinetic behaviors of the title reaction were characterized using the complementary deterministic and stochastic Master Equation/Rice–Ramsperger–Kassel–Marcus (ME/RRKM) rate models. Corrections of the hindered internal rotation (HIR) treatment and quantum tunneling effect were included in the calculations. It is revealed that the title reaction can proceed via addition and H-abstraction channels leading to four different products (i.e., three addition products: CH2=CH-NH2 + H (P1), CH3-CH=NH + H (P2), CH2=NH + CH3 (P3) and one H-abstraction product: C2H3 + NH3 (P4)). The addition pathway is found favorable at low temperatures and high pressures: e.g., it dominates at temperatures lower than 650 K and 1250 K at P = 1 and 76 000 Torr, respectively. The computed rate constants are in good accordance with literature values; thus, the kinetic parameters, together with the thermodynamic data of the species involved, can be used confidently for modeling/simulation of nitrogen-related applications in atmospheric and combustion conditions.
  • The effects of naphthalene-addition to alkylbenzenes on soot formation
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Carson Chu, Murray J. ThomsonAbstractNaphthalene and alkylbenzenes are present in practical transportation fuels. This study investigates the impact of naphthalene addition to alkylbenzenes on soot formation. Naphthalene was added to two kinds of alkylbenzenes, namely, 1,2,4-trimethylbenzene and n-propylbenzene. Because they are isomers, the effect of molecular structure is isolated. The sooting characteristics of naphthalene-added alkylbenzenes are compared to pure alkylbenzenes in laminar coflow flames. The fuel and carbon mass flow rates were kept constant for all cases. The soot volume fraction measurements show that n-propylbenzene is sensitive to naphthalene addition. In contrast, no significant changes in soot volume fraction were observed for the 1,2,4-trimethylbenzene flames. A slight increase in primary particle diameter was observed for both naphthalene-added n-propylbenzene and 1,2,4-trimethylbenzene, suggesting that naphthalene promotes soot surface growth. The calculated number densities show that naphthalene addition promotes soot nucleation for n-propylbenzene but not for 1,2,4-trimethylbenzene. The flames were simulated with the CoFlame code with the CRECK mechanism. The model partially agrees with the experimental results, as the model agrees with the case of 1,2,4-trimethylbenzene but underestimates the effect of naphthalene addition to n-propylbenzene. More understanding of the PAH formation beyond naphthalene is required. In conclusion, the study suggests that the effect of naphthalene addition on soot formation is fuel-type dependent.
  • Impact of spray-wall interaction on the in-cylinder spatial unburned
           hydrocarbon distribution of a gasoline partially premixed combustion
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Vallinayagam Raman, Qinglong Tang, Yanzhao An, Hao Shi, Priybrat Sharma, Gaetano Magnotti, Junseok Chang, Bengt JohanssonAbstractPartially premixed combustion (PPC) often adopts the early fuel-injection strategy that could result in spray-wall interaction involved with piston top-land crevice. This interaction may produce a significant impact on engine combustion and unburned hydrocarbons (UHC) emission, which is still not well understood. In this study, we investigated the detailed spray-wall interaction and its effects on the two-stage ignition, i.e. low- and high-temperature heat release (LTHR and HTHR), and the in-cylinder spatial UHC distribution of PPC in a full-view optical engine at low engine load. The PRF 70 fuel was used as the gasoline surrogate. The high-speed imaging of the natural flame luminosity was acquired to quantify the flame probability distribution. The qualitative fuel-tracer, formaldehyde, and UHC planar laser-induced fluorescence (PLIF) imaging techniques were employed to reveal the fuel, LTHR and UHC distribution characteristics, respectively. The LTHR, HTHR and UHC distribution formed by the fuel trapped in the piston top-land crevice were visualized by PLIF imaging techniques for the first time. The PLIF results indicate that the main UHC formed in the PPC engine comes from the central part of the cylinder close to the injector nozzle, where the overall equivalence ratio is low and the injector dribbling is an important source of UHC. The UHC formed in the piston crevice of the PPC engine depends on the local equivalence ratio of the fuel trapped in the crevice. When the overall equivalence ratio of the charge in the crevice is relatively high, the trapped fuel undergoes both LTHR and HTHR and produces negligible UHC. However, the UHC from the piston crevice becomes considerable when the fuel injection timing is too early so that an overly lean mixture is generated. Based on the above findings, three implications of the PPC operation at low engine load for low UHC emission and high engine efficiency are proposed.
  • Three-dimensional behaviour of quasi-detonations
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Mark Kellenberger, Gaby CiccarelliAbstractBuilding on previous experiments conducted in an obstructed narrow rectangular channel, new details of the three-dimensional propagation behaviour of supersonic combustion waves have been revealed. In this study, a square channel equipped with 50% blockage ratio obstacles was used. Average velocity measurements coupled with high-speed schlieren photography and sooted glass sheets were used to simultaneously capture wave propagation and triple-point trajectories from multiple fields-of-view. Experiments were carried out in mixtures of stoichiometric hydrogen-oxygen at initial pressures between 9 kPa and 60 kPa in a 3.66 m long, by 7.62 cm square cross-section channel with optical access. Results show that the increased channel width results in a lower maximum pressure for which fast-flame propagation occurs. At higher initial pressures, detonation kernels were initiated at the obstacle face-sidewall interface in either a symmetrical (both sides) or an asymmetrical (single side) formation across the channel width. Wall reflection generated detonations evolve to form transverse detonations propagating diagonally across the channel width in the shock-compressed region following the obstacle. The single wall ignition was found to lead to a stable single-head “zig-zag” detonation (diagonal propagation driven by sidewall reflection) at initial pressures from 17 kPa to 24 kPa where transverse detonation reflection leads to the generation of a reactive Mach stem that survives diffraction at the next obstacle pair. Soot foils displayed a unique narrow vertical band of cells where the transverse wave collides with the channel sidewall in this propagation mode, which is the only mode to not involve obstacle reflection re-initiation. The channel width w, being larger than the obstacle opening d, makes it possible for the transverse modes seen in an obstacle-free channel to lock in, like the single-head detonation propagation observed. Continuous detonation propagation through the channel core was seen at high CJ velocity deficits beginning at d/λ = 6.3, where λ is the detonation cell width, with higher initial pressures having cellular structure reach the channel walls between obstacles. Thus, continuous detonation propagation is governed by the diffraction process around the obstacles and d is the governing length scale.
  • Effects of ignition disturbance on flame propagation of methane and
           propane in a narrow-gap-disk-burner
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Hye Jin Jang, Sang Min Lee, Nam Il KimAbstractUnsteady flame propagation within a narrow channel, namely a Hele-Shaw burner, exhibits complicated phenomena. Recently, a new narrow-gap-disk-burner (NGDB) was developed, of which the disk-gap could be varied continuously and precisely. Although various complicated flame structures have been observed successfully, their dependency on the initial ignition has not been clarified. In this study, the volume of the ignition part was varied to introduce disturbance at the ignition stage, and the propagation characteristics of premixed methane and propane flames were investigated. Conclusively, quenching distance was not significantly affected by the ignition volume, especially in propane-rich conditions. In contrast, the flame structure and propagation velocity were sensitive to the ignition volume if it was larger than a critical volume, and when the disk-gap was approximately 1.5 times the quenching distance. A strong initial disturbance could generate complicated cellular structures coupled not only with shear stress but also with heat transfer. These cellular structures could increase the flame propagation velocity when the Lewis number was sufficiently smaller than unity. In contrast, the flame shape became smoother when the disk-gap was sufficiently larger than the quenching distance. Thus, the flame propagation velocity was comparable to the laminar burning velocity when it was less affected by the initial disturbance.
  • Liftoff and blowout of the Emmons flame: Analysis of the triple flame
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Shangpeng Li, Qiang Yao, Chung K. Law, Wenkai Liang, Jiankun ZhuoAbstractThe steady burning and stabilization of the boundary layer diffusion flame over a gasifying condensed fuel surface, commonly called the Emmons flame, is an important problem in the study of boundary layer combustion. We investigate herein, theoretically and numerically, the liftoff distance and blowout limit of the Emmons flame, through the corresponding response of the controlling triple flame in the leading edge of the bulk flame. An explicit solution of the flame liftoff distance and the critical blowout limit is derived, with the theoretical results agreeing well with the numerical simulation for an extensive range of the system parameters. In particular, it is shown that the transversal velocity gradient (TVG) ahead of the triple flame renders the flame harder to liftoff and blowout, with this effect increasing for increasing TVG and decreasing triple flame curvature, which is related to the mixture fraction gradient. Furthermore, the Spalding mass transfer number, Bv, for the surface segment ahead of the flame front affects the flame stabilization and blowout limit by modifying the similarity structure of the flow and the location of stoichiometry. Thermal expansion of the flow around the triple flame together with the surface viscous drag also significantly promotes flame stabilization.
  • Energy analysis of unsteady negative edge flames in a periodic flow
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Stephen W. Grib, Michael W. RenfroAbstractNon-premixed laminar or turbulent flames can undergo extinction when scalar dissipation rates exceed a critical value. When local extinction occurs, negative edge flames that act to expand the region of extinction are initially present. Steady negative edge flames have been previously studied in a combustor designed to support the negative propagation velocity of the flame edge, but unsteady negative edge flames have not received significant attention. The current study uses numerical simulations in a simple flowfield to produce unsteady negative edge flames which oscillate in response to imposed strain rate fluctuations in order to understand the dynamic response of extinction edge propagation. An energy balance through the edge shows that the unsteady behavior can be well described by a simple extension of the energy balance from steady flames. Furthermore, the edge velocity was calculated, and a physical interpretation of the extinction velocity was derived using the energy budget. The time scale of the imposed strain rate fluctuations was varied to show a limited response time for the extinction edge flame dynamics.
  • Experimental and numerical studies on electric field distribution of a
           premixed stagnation flame under DC power supply
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Yihua Ren, Wei Cui, Heinz Pitsch, Shuiqing LiAbstractIn this work, we achieve a sub-breakdown electric field measurement in a premixed stagnation flame by electric-field-induced second-harmonic generation (ESHG) using a nanosecond laser. Under the application of a DC voltage, the premixed flame can transit from a flat substrate-stabilized mode to a conical nozzle-stabilized mode due to the two-way interaction between the electric and hydrodynamic responses of the flame. The average electric fields along the laser pathway for these two flame modes are measured at different heights above the burner. Combining the measurement and the numerical simulation of a charge transport model, we further elucidate that the flame affects the electric field distribution in two different ways. For the flat flame mode, the electric field strength is shielded at the flame front and then quickly increases both upstream and downstream, reaching the maximum at the electrodes. The charge transport model reveals that the electrostatic shielding effect of the flame front can be attributed to the charge redistribution in the chemi-ionized conductive layer. For the conical flame mode, the electric field strength has a large gradient near the conical tip and remains almost zero downstream. The conical flame front then serves as a conductive layer to guide the charge transport under the application of the electric field. The positive and negative charges are separated at different radial positions because of the inclined asymmetric flame front. Thus, the largest electric field gradient is generated upstream of the flame conical tip, where the net charge and the electric body force maximize and create a virtual flame stabilization mode.
  • Energetic ionic liquid hydroxyethylhydrazinium nitrate as an alternative
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Umakant Swami, Krishnamachary Senapathi, Krishna M. Srinivasulu, Jayaraman Desingu, Arindrajit ChowdhuryAbstractThe propellant synthesis community is constantly looking for green alternative monopropellants. Energetic ionic liquids have several attractive properties such as high energy content, high bulk density, low vapor pressure, high thermal stability, wide liquidus range, low corrosiveness, low toxicity, and ease of handling. The combustion characteristics of an energetic ionic liquid hydroxyethylhydrazinium nitrate (HEHN) was conducted in a pressurized chamber. The performance of HEHN was compared to that of the monopropellant Otto fuel II (OF-II) typically used for torpedo-propulsion. A liquid strand combustion study was performed in an atmosphere of air and nitrogen with chamber pressures varying from 10 to 90 bar. Regressing surface profiles and subsequent burning rates were obtained at different chamber pressures. A B-type thermocouple of 46 µm wire diameter was used to measure the monopropellant flame temperature of HEHN. Thermogravimetric analysis was performed to study the thermal decomposition of HEHN to understand its thermal stability and Fourier transform infrared spectroscopy (FTIR) was utilized to determine the possible reasons behind the high burning rates of HEHN. The gains in the specific impulse and density specific impulse coupled with enhanced burning rates and reasonable thermal stability are expected to establish HEHN as a frontrunner for propulsion and power-generation in oxygen-deficient scenarios.
  • Effect of boron content in B·BiF3 and B·Bi composites on their ignition
           and combustion
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Siva Kumar Valluri, Karthick Kumarasen Ravi, Mirko Schoenitz, Edward L. DreizinAbstractComposite powders combining boron with BiF3 and Bi in different amounts were prepared by high energy milling. Thermal analysis in an argon-oxygen mixture showed significant oxidation starting about 200 K lower than for pure boron. Selective oxidation of metallic Bi at low temperatures was observed. Composites containing either Bi or BiF3 ignited more readily than pure boron when heated by a CO2 laser beam. The composites containing BiF3 ignited more readily than boron when in contact with a hot wire. Burn times of particles aerosolized in air and ignited using the CO2 laser were measured as durations of the recorded emission pulses produced by burning particles. Statistical distributions of the measured burn times were correlated with the respective powder's particle size distributions. Compared to elemental boron, burn times of all prepared composites were shorter, including those containing only 10 wt.% of BiF3 or ca. 8 wt.% of Bi, and for most composites combustion temperatures were higher.
  • Ignition delay time and laminar flame speed measurements of mixtures
           containing diisopropyl-methylphosphonate (DIMP)
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Olivier Mathieu, Travis Sikes, Waruna D. Kulatilaka, Eric L. PetersenAbstractDiisopropyl-methylphosphonate (DIMP) is a common surrogate of Sarin for which a detailed kinetics mechanism was developed in 2002 (Glaude et al.— [12]). In the present paper, ignition delay times and laminar flame speeds of DIMP-based mixtures were studied around atmospheric pressure for the first time. Methane and hydrogen were used as baseline fuels, and mixtures of these fuels were doped with DIMP. The DIMP was added at 10% mol. of the fuel concentration for the shock-tube experiments (three equivalence ratios (0.5, 1.0, and 2.0) were investigated with methane). A stoichiometric DIMP/O2 mixture was also studied. For the laminar flame speed experiments, an equivalence ratio sweep was performed, and DIMP was added at 0.1% vol. of the total mixture, although DIMP was added to up to 0.5% vol. for the maximum flame speed condition with H2. Results showed that adding DIMP promotes the ignition of methane but also largely inhibits its laminar flame speed. A decrease in the flame speed with DIMP addition was observed with H2 as well, and the maximum flame speed was found to decrease linearly with the amount of DIMP added. The new results with DIMP were compared to data obtained recently for other Sarin surrogates. The influence of the structure of these surrogates on combustion properties was discussed. The kinetics model for DIMP does not capture the peculiar OH* profiles observed in the shock tube for the DIMP/O2 mixture, and the ignition delay times for the H2-DIMP mixture were very poorly predicted. On the other hand, ignition delay times for the CH4-DIMP mixture were predicted with good-to-fair agreement for most cases. A chemical analysis showed that a sub-mechanism for iso-propanol needs to be added to the mechanism. An updated detailed kinetics model, with a state-of-the-art hydrocarbon chemistry (including iso-propanol chemistry) and updates in the phosphorous chemistry and thermodynamic database was tested. This updated mechanism provides predictions that are less accurate than the original model in most cases, indicating a strong need to revisit the reactions describing the thermal decomposition of DIMP.
  • Large-eddy simulation of dual-fuel spray ignition at different ambient
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Bulut Tekgül, Heikki Kahila, Ossi Kaario, Ville VuorinenAbstractHere, a finite-rate chemistry large-eddy simulation (LES) solver is utilized to investigate dual-fuel (DF) ignition process of n-dodecane spray injection into a methane–air mixture at engine-relevant ambient temperatures. The investigated configurations correspond to single-fuel (SF) ϕCH4= 0 and DF ϕCH4= 0.5 conditions for a range of temperatures. The simulation setup is a continuation of the work by Kahila et al. (2019, Combustion and Flame) with the baseline SF spray setup corresponding to the Engine Combustion Network (ECN) Spray A configuration. First, ignition is investigated at different ambient temperatures in 0D and 1D studies in order to isolate the effect of chemistry and chemical mechanism selection to ignition delay time (IDT). Second, 3D LES of SF and DF sprays at three different ambient temperatures is carried out. Third, a reaction sensitivity analysis is performed to investigate the effect of ambient temperature on the most sensitive reactions. The main findings of the paper are as follows: (1) DF ignition characteristics depend on the choice of chemical mechanism, particularly at lower temperatures. (2) Addition of methane to the ambient mixture delays ignition, and this effect is the strongest at lower temperatures. (3) While the inhibiting effect of methane on low- and high-temperature IDT’s is evident, the time difference between these two stages is shown to be only slightly dependent on temperature. (4) Reaction sensitivity analysis indicates that reactions related to methane oxidation are more pronounced at lower temperatures. The provided quantitative results indicate the strong ambient temperature sensitivity of the DF ignition process.
  • Cookoff modeling of a melt cast explosive (Comp-B)
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Michael L. Hobbs, Michael J. Kaneshige, William W. Erikson, Judith A. Brown, Mark U. Anderson, Steven N. Todd, David G. MooreAbstractA universal cookoff model (UCM) is applied to the melt cast explosive Comp-B composed of RDX and TNT. The UCM uses a simple kinetic mechanism with rates and thermophysical properties determined specifically for Comp-B. The success of the UCM is primarily attributed to the flexible form of the rate expression as well as accurate thermophysical properties obtained from small-scale experiments. The rate expressions use distributed activation energies in conjunction with rate multipliers to account for accelerations caused by 1) dissolved RDX, 2) liquid RDX, and 3) pressure. Our finite element model addresses Comp-B cookoff from the pristine state, through melting of the TNT binder, partial dissolution of RDX in the hot TNT, and melting of the remaining RDX as the Comp-B thermally ignites. Typically, the UCM is used for explosives that do not flow. However, we have included a buoyancy-driven flow model to account for multiphase fluid movement. Predicted temperature fields were sensitive to flow, which caused the hotter material to rise. Our predictions of ignition times were also sensitive to RDX dissolving in hot TNT causing an acceleration of the RDX decomposition.
  • Combustion of 3D printed 90 wt% loading reinforced nanothermite
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Jinpeng Shen, Haiyang Wang, Dylan J. Kline, Yong Yang, Xizheng Wang, Miles Rehwoldt, Tao Wu, Scott Holdren, Michael R. ZachariahAbstractThe use of Al-based nano-energetic materials has been limited in part due to difficulties in fabrication of high-density composites. In this paper, free-standing energetic composites with loading of up to 90 wt% Al-CuO were fabricated by 3D printing. A polymer hybrid of 3 wt% hydroxy propyl methyl cellulose (HPMC), 3.5 wt% nitrocellulose (NC) and 3.5 wt% polystyrene (PS), enables fabrication of mechanically strong and highly reactive composites. The energy flux can be readily tuned through the combustion speed and flame temperature by changing equivalence ratio. The highest energy flux was found to occur under fuel rich conditions (equivalence ratio = 2.4) which also corresponds to the maximum combustion speed (25 cm/s) despite the fact that the flame temperatures was lower. The Young's modulus of free-standing burn sticks was found to be as high as ~1 GPa, which is comparable to pure polypropylene. PS polymer flakes created during the high shear direct write process is believed to be critical to the enhanced mechanical properties we observed. The burning behavior using other oxidizers corresponds closely with that observed with mixed powders but with the added strength offered in a printed structure. This study offers an attractive route for safe, reliable and scalable additive manufacturing of Al-based nano-energetic materials at high energy densities.
  • Simulated production of OH, HO2, CH2O, and CO2 during dilute fuel
           oxidation can predict 1st-stage ignition delays
    • Abstract: Publication date: Available online 5 February 2020Source: Combustion and FlameAuthor(s): Zachary J. Buras, Cosmin Safta, Judit Zádor, Leonid ShepsAbstractChemical kinetics simulations are used to explore whether detailed measurements of relevant chemical species during the oxidation of very dilute fuels (less than 1 Torr partial pressure) in a high-pressure plug flow reactor (PFR) can predict autoignition propensity. We find that for many fuels the timescale for the onset of spontaneous oxidation in dilute fuel/air mixtures in a simple PFR is similar to the 1st-stage ignition delay time (IDT) at stoichiometric engine-relevant conditions. For those fuels that deviate from this simple trend, the deviation is closely related to the peak rate of production of OH, HO2, CH2O, and CO2 formed during oxidation. We use these insights to show that an accurate correlation between simulated profiles of these species in a PFR and 1st-stage IDT can be developed using convolutional neural networks. Our simulations suggest that the accuracy of such a correlation is 10–50%, which is appropriate for rapid fuel screening and may be sufficient for predictive fuel performance modeling.
  • Effects of CO2 on soot formation in ethylene pyrolysis
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Junyu Mei, Xiaoqing You, Chung K. LawAbstractEffects of CO2 on soot formation during ethylene pyrolysis were investigated in a laminar flow reactor with the addition of various amounts of CO2 (0–99.5% in mole fraction). Based on a quantitative dilution sampling technique and a scanning mobility particle sizer, soot particle size distributions and the associated global properties, including soot volume fraction, number density, and soot induction delay time, were examined. Results show that while addition of a small amount of the CO2 (0 - 10%) tends to promote soot formation as the total number and volume of soot particles increase and the soot induction delay time decreases, its further increase, from 10% to 99.5%, leads to an obvious reduction of the soot nucleation and mass growth rates. Subsequent kinetic modeling of the gas-phase chemistry showed that with increasing CO2 concentration, the corresponding concentrations of the soot precursors, namely benzene and pyrene, first increase and then decrease, which is consistent with the observed trend in soot formation. Further sensitivity and reaction path analyses of benzene formation indicate that CO2 addition produces more hydroxyl radicals, such that while the presence of a small amount of hydroxyl radicals increases the propargyl concentration and thereby promotes the formation of soot precursors, excessive hydroxyl radicals lead to more oxidation and hence inhibit soot formation.
  • Comprehensive mechanism of initial stage for lignin pyrolysis
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Jian Li, Xiaowei Bai, Yang Fang, Yingquan Chen, Xianhua Wang, Hanping Chen, Haiping YangAbstractIn this study, the initial reaction of lignin pyrolysis was explored in depth with light gas emission combined with liquid bio-oil composition and physical-chemical structure of solid bio-char. The result reveals that lignin pyrolysis undergoes a complex initial reaction between 160 and 330 °C with significant mass loss (about 20%). Lignin molecular network begins to crack down with lignin monomer linkage breaking and light gas molecular (CO2 and CO) formation as temperature reaching 160 °C. Also, polymerization reaction involving forming larger molecular weight lignin pieces took place between 160 and 200 °C. After that (200–330 °C), lignin cracking reaction is the main reaction with lignin molecular accelerating to form heavy (dimers and trimers) and light units (monomers) in bio-oil and causing more mass loss. At the same temperature range, lignin soften reaction happens and results in lignin surface adhesion, microscopic surface structure changing and functional group evolution. As temperature over 330 °C, lignin initial pyrolysis stage finished and entered intense pyrolysis stage, with more oxygen contained functional group cracking and light lignin pieces formed.
  • Prechamber ignition: An exploratory 2-D DNS study of the effects of
           initial temperature and main chamber composition
    • Abstract: Publication date: May 2020Source: Combustion and Flame, Volume 215Author(s): Sotirios Benekos, Christos E. Frouzakis, George K. Giannakopoulos, Michele Bolla, Yuri M. Wright, Konstantinos BoulouchosAbstractA numerical investigation was conducted to study ignition and the initial phases of methane combustion in a two-dimensional setup consisting of a pre- (PC) and main (MC) chamber. The effect of the wall thermal boundary condition (isothermal Tw=500 K or adiabatic), initial mixture temperature (Tu=300 and 800 K) and equivalence ratio in the main chamber (ϕMC=0.5 and 1.0) was studied. A stoichiometric mixture was used in the PC and the mixtures were initially quiescent in all cases. Flame propagation in the prechamber is affected mainly by the initial temperature, while the thermal state of the wall plays a minor role. The transient jet that is generated at the exit of the orifice connecting the two chambers creates an intense flow field with vortical structures that, depending on the initial temperature, persist for a long time or dissipate quickly affecting combustion in the main chamber. Depending on the local flow and mixing conditions close to the orifice exit, the hot jet can be broken into small kernels at low Tu or forms quickly a flame torch at hot conditions, strongly affecting ignition of the MC mixture and flame propagation in the main chamber. In the lean MC cases, the intense mixing with the stoichiometric PC mixture creates local compositions that are more favorable for ignition by the hot turbulent reactive jet that subsequently exits from the PC at a temperature that is significantly lower than the adiabatic flame temperature of the corresponding mixture. Despite the short residence time, the reactive state of the mixture is affected as it flows through the nozzle. The flame structures in the MC are described in terms of the progress variable and mixture fraction and compared to flamelet-type calculations. The local flame structure differs strongly from that of the 1-D unstrained premixed flame, particularly for the low Tu cases. The flamelet-type calculations show that ignition of the most reactive mixture is enhanced by the radicals in the hot reactive jet, while scalar dissipation rate accelerates the ignition of the whole mixture. The 2-D simulations show that ignition is significantly longer than what is predicted by the flamelet calculations.
  • Laser pulse initiation of RDX-Al and PETN-Al composites explosion
    • Abstract: Publication date: Available online 29 January 2020Source: Combustion and FlameAuthor(s): Boris P. Aduev, Denis R. Nurmukhametov, Igor Y. Liskov, Alexandr V. Tupitsyn, Gennadiy M. BelokurovAbstractThe explosive thresholds Hcr of the composites cyclotrimethylenetrinitramine (RDX)-Al and pentaerythritol tetranitrate (PETN)-Al under the action of neodymium laser (wavelength 532 nm, pulse duration 14 ns) were experimentally determined for the first time as a function of the aluminum content. The experiments were performed using aluminum nanopowder with average diameter 100 nm. The gas-dynamic energy losses were prevented. The minimal thresholds achieved in the experimental conditions were determined that are Hcr=1 J/cm2 for the composites RDX-Al at the mass fraction of Al 0.2% and Hcr=0.33 J/cm2 for PETN-Al composites at aluminum mass fraction 0.1%.
  • A comprehensive study of flamelet tabulation methods for pulverized coal
           combustion in a turbulent mixing layer—Part II: Strong heat losses and
           multi-mode combustion
    • Abstract: Publication date: Available online 6 January 2020Source: Combustion and FlameAuthor(s): Xu Wen, Martin Rieth, Arne Scholtissek, Oliver T. Stein, Haiou Wang, Kun Luo, Andreas Kronenburg, Jianren Fan, Christian HasseAbstractThis paper is a continuation of our work done in Part I, in which the a priori and budget analyses were conducted, Wen et al. (2019). In this work, we focus on addressing specific and recurring issues in flamelet modeling for pulverized coal combustion, including strong heat losses, multi-mode combustion and reaction progress variable definition. First, extended flamelet formulations are developed that can take into account strong heat loss effects in pulverized coal combustion systems. Then, to characterize multi-mode combustion in pulverized coal flames, a coupled premixed and non-premixed flamelet model is developed using the combustion mode index. Finally, the effects of reaction progress variable definition on the flamelet predictions are quantified. A state-of-the-art direct numerical simulation database is employed to challenge the newly developed flamelet models. The tabulated thermo-chemical quantities are compared with the reference direct numerical simulation results through a priori analyses. Comparisons show that the newly developed flamelet models which take into account strong heat loss effects can predict the gas temperature and species mass fractions correctly. The adiabatic flamelet models over-predict the corresponding thermo-chemical quantities in regions where the interphase heat transfer is significant. Coupled with a linear extrapolation method, the prediction of the gas temperature with the adiabatic flamelet models can be improved. The performance of the multi-mode flamelet model depends on whether the local combustion mode can be correctly identified. The conventional combustion mode index based on the gradients of fuel and oxidizer species mass fractions cannot correctly identify the combustion mode in the entire combustion field.
  • Effect of premilling Al and CuO in acetonitrile on properties of Al·CuO
           thermites prepared by arrested reactive milling
    • Abstract: Publication date: April 2020Source: Combustion and Flame, Volume 214Author(s): Mehnaz Mursalat, Mirko Schoenitz, Edward L. DreizinAbstractThermite powders with molar composition 8Al·3CuO were prepared in two stages by Arrested Reactive Milling (ARM). In the first stage, the starting materials Al and CuO were milled separately in acetonitrile. Composite powders were then prepared in the second milling stage with hexane as process control agent and in the four possible combinations of one, both, or neither starting material being premilled in acetonitrile. Composites were characterized for morphology, size distribution, surface area, and reactive properties at low heating rates (thermal analysis) and high heating rates (ignition). Whether or not CuO was premilled, dense composites formed without premilling of Al. If Al was premilled in acetonitrile, however, loose agglomerates of refined Al and CuO particles formed in the second milling stage. Premilling changed the low-temperature reactions leading to ignition in the 8Al·3CuO thermites. These changes are attributed to increased porosity of the formed composites if aluminum is premilled with acetonitrile. It is shown that greater refinement and lower ignition temperatures are achievable using two-stage milling.
  • Coupled reaction mechanism reduction for the hetero-/homogeneous
           combustion of syngas over platinum
    • Abstract: Publication date: April 2020Source: Combustion and Flame, Volume 214Author(s): Ran Sui, Wenkai Liang, John Mantzaras, Chung K. LawAbstractCoupled reduced mechanisms were developed for the hetero-/homogeneous combustion of fuel-lean and fuel-rich H2/CO/O2/N2 mixtures in a Pt-coated planar channel, using a method based on the Directed Relation Graph (DRG) and for a wide range of operating conditions for which detailed measurements are available. It is demonstrated that catalytic and gas-phase reaction mechanisms can be reduced together for all the fuel-lean cases. On the other hand, when joint reduction is performed for low-pressure fuel-rich cases (using a strict threshold value to capture the relatively weak, yet still important coupling between the catalytic and gas-phase reaction pathways) the result is a less efficient reduction process. For the high-pressure fuel-rich cases the catalytic-gaseous chemical coupling is weak enough to be neglected such that the reduction can be conducted separately for higher reduction efficiency. The reduced mechanisms reproduced well the major gaseous species concentrations, gas temperatures and homogeneous ignition distances obtained with the detailed mechanisms, thus demonstrating the capacity of the applied method in reducing catalytic/gas-phase reaction mechanisms. In addition, it is shown that for fuel-lean stoichiometries the reduction could provide a rapid indication if gas-phase combustion is ignited, without the need of full simulations. The reduced mechanisms are expected to facilitate large-scale simulations, with fidelity, for the design and thermal management of practical catalytic combustion systems.
  • Morphology and internal structure of soot particles under the influence of
    • Abstract: Publication date: April 2020Source: Combustion and Flame, Volume 214Author(s): Lingzhe Rao, Yilong Zhang, Sanghoon Kook, Kenneth S. Kim, Chol-Bum KweonAbstractA new multi-location soot sampling method is used to enhance the knowledge about the structural evolution of in-flame particles in a light-duty optical diesel engine. Through thermophoresis-based particle sampling performed at multiple in-bowl locations, the soot structures are shown for both early formation stage and later stage from the same combustion event. Three different jet-spacing angles of 45°, 90° and 180° were studied to analyse how different levels of jet–jet interaction impact the soot particle morphology and internal structure. One selected jet–jet interaction condition was further analysed to show differences in soot structures between the up-swirl side and down-swirl side of the wall jets. From transmission electron microscopes (TEM) images of the sampled soot particles and their statistical size analysis, it was found soot particles initially formed within 45∘ separated jet–jet interaction region have un-solidified premature aggregates due to limited carbonisation in the locally fuel-rich mixtures. When these soot particles travelled on the down-swirl side of the jets, they became solidified and carbonised while the oxidation was evident from the smaller soot primary particle and longer carbon-layer fringe and lower tortuosity. The higher mixing on the up-swirl side of the jets further enhanced the soot oxidation, resulting in even smaller soot primary particle, fragmentation of large soot aggregates, and even longer and less curved carbon-layer fringes. Regarding jet–jet interaction, the 180° jet spacing angle created no jet–jet interaction condition on the soot sampler locations. For smaller jet-spacing angles, the increase in jet–jet interaction promoted the soot formation as evidenced by larger and more complex soot aggregates formed due to more active soot aggregation and agglomeration. The soot oxidation became limited at higher jet–jet interaction conditions, which led to more amorphous soot internal structures.
  • DMMP pyrolysis and oxidation studies at high temperature inside a shock
           tube using laser absorption measurements of CO
    • Abstract: Publication date: April 2020Source: Combustion and Flame, Volume 214Author(s): Sneha Neupane, Ramees K. Rahman, Jessica Baker, Farhan Arafin, Erik Ninnemann, Kyle Thurmond, Chun-Hung Wang, Artëm E. Masunov, Subith S. VasuAbstractDimethyl methyl phosphonate (DMMP) is an organo-phosphorous compound (OPC) used as a fire suppressant and a simulant for sarin, a chemical warfare agent. There exists a critical need to gather combustion data at high heating rate and high temperatures conditions, similar to what exists during destruction process of chemical weapons. In the present work, DMMP pyrolysis and oxidation were carried out behind reflected shock waves at temperatures of 1300–1700 K and pressures of 1.5–1.8 atm. Lean, stoichiometric, and rich DMMP mixtures (Φ = 0.23, 0.5, 1, 2) were investigated for oxidation experiments. Laser absorption spectroscopy utilizing a quantum cascade laser near 4.9 µm was used to measure intermediate CO concentration formed during the pyrolysis and oxidation processes. To the best of our knowledge, we present the first intermediate concentration data at the reported conditions for DMMP. A tentative kinetic model, based on the AramcoMech2.0 mechanism with Lawrence Livermore National Lab (LLNL)’s OPC incineration chemistry, was utilized in Chemkin-Pro to predict CO yield during the processes. The model provided fair prediction of CO yield during DMMP pyrolysis, however, overpredicted the CO yield for oxidation. Sensitivity and rate of production analyses were carried out to understand important reactions leading to CO formation.
  • Influence of heat-loss on compressibility-driven flames propagating from
           the closed end of a long narrow duct
    • Abstract: Publication date: April 2020Source: Combustion and Flame, Volume 214Author(s): Vadim N. Kurdyumov, Moshe MatalonAbstractThis study focuses on the dynamics of compressibility-driven flames that emerge in narrow tubes, closed at their ignition end, when conductive heat losses through the walls are appreciable. A narrow gap approximation is used to reduce the governing equations to an effectively one-dimensional problem. In long channels this problem admits traveling-wave solutions which we have investigated numerically for finite values of the Zel’dovich number, and asymptotically in the limit of large Zel’dovich numbers. In particular, we describe the flame structure and the dependence of the propagation speed on the physico-chemical parameters, including the heat loss and compressibility parameters, and examine the transition from compressibility-driven to isobaric flames when systematically reducing the representative Mach number.
  • Implications of real-gas behavior on refractive index calculations for
           optical diagnostics of fuel–air mixing at high pressures
    • Abstract: Publication date: April 2020Source: Combustion and Flame, Volume 214Author(s): Christopher T. Wanstall, Ajay K. Agrawal, Joshua A. BittleAbstractThree models to compute the refractive index of gaseous mixtures at real gas conditions are presented with the purpose to improve the accuracy of state relationships between refractive index and thermodynamic properties. Models are compared with experimental data to determine one that is applicable to high pressure mixtures with non-ideal thermodynamic behavior near or above a fluids’ critical point. The optimal model is applied to analyze adiabatic thermal mixing of fuel and air at typical diesel engine conditions. Results show that the ideal gas mixture law is appropriate in the vapor region, assuming it is known, for example, from experimental measurements. Finally, the model is applied for binary fuel-air mixing at supercritical conditions to demonstrate its full potential.
  • A comprehensive study of flamelet tabulation methods for pulverized coal
           combustion in a turbulent mixing layer — Part I: A priori and budget
    • Abstract: Publication date: Available online 15 June 2019Source: Combustion and FlameAuthor(s): Xu Wen, Martin Rieth, Arne Scholtissek, Oliver T. Stein, Haiou Wang, Kun Luo, Andreas M. Kempf, Andreas Kronenburg, Jianren Fan, Christian HasseAbstractIn this work, a comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer is conducted. At first, a priori analyses are conducted to evaluate the suitability of the premixed and non-premixed flamelet models for the studied pulverized coal flame with multiple combustion modes. Then, to clarify why a certain flamelet model does work or not work in certain regions, a more in-depth investigation of the premixed and non-premixed flamelet models is conducted through a budget analysis. The results show that the first and second derivatives in physical space can be well reproduced by the tabulated manifolds in trajectory variable space for both the premixed and non-premixed flamelet models, between which the non-premixed flamelet model performs slightly better. For the time derivative, large discrepancies can be observed, although the predicted variation trend overall follows the reference results. Through the analysis of the individual budget terms in the trajectory variable space, the individual trajectory variable’s contributions to the convection and diffusion of thermo-chemical variables are quantified. Through the analysis of the individual budget terms for the sensible enthalpy and the CO mass fraction governing equations, the influences of the space transformation on the individual transport process (e.g., convection, diffusion, etc.) are clarified. Overall, the findings obtained from the budget analyses are consistent with those obtained from the a priori analyses.
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