| 1. |
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| 2. |
- Brearley, P, et al.
(author)
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Statistical behaviors of conditioned two-point second-order structure functions in turbulent premixed flames in different combustion regimes
- 2019
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In: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 31:11
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Journal article (peer-reviewed)abstract
- The second-order structure functions and their components conditioned upon various events have been analyzed for unweighted and density-weighted velocities using a Direct Numerical Simulation database. The heat release due to combustion has been shown to have significant influences on the structure functions and their components conditioned on different mixture states. The use of density-weighted velocities changes the relative magnitudes of differently conditioned structure functions but does not reduce the scatter of these magnitudes. The structure functions conditioned to constant-density unburned reactants at both points and normalized using the root-mean-square velocity conditioned to the reactants are larger at higher values of mean reaction progress variables (deeper within the flame brush), with this trend being not weakened with increasing turbulence intensity u′/SL. These results indicate that, contrary to a common belief, combustion-induced thermal expansion can significantly affect the incoming constant-density turbulent flow of unburned reactants even at u′/SL and Karlovitz number Ka as large as 10 and 18, respectively. The statistical behaviors of the structure functions reveal that the magnitude of the flame normal gradient of the velocity component tangential to the local flame can be significant, and it increases with increasing turbulence intensity. Moreover, the structure functions conditioned on both points in the heat release zone bear the signature of the anisotropic effects induced by the baroclinic torque for the flames belonging to the wrinkled flamelet and corrugated flamelet regimes. These anisotropic effects weaken with increasing turbulence intensity in the thin reaction
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| 3. |
- Chakraborty, N., et al.
(author)
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Conditional velocity statistics for high and low Damköhler number turbulent premixed combustion in the context of Reynolds Averaged Navier Stokes simulations
- 2013
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34:1, s. 1333-1345
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Journal article (peer-reviewed)abstract
- The statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes (RANS) simulations have been analysed using a Direct Numerical Simulation (DNS) database of statistically planar turbulent premixed flames for both high and low values of Damkohler number for different values of heat release parameter. It has been found that the contributions arising from chemical reaction to the conditional mean velocities and the conditional Reynolds stresses remain strong under high values of Damkohler number. The expressions for conditional mean velocity components and conditional Reynolds stresses, which are derived based on bi-modal probability density function of reaction progress variable for unity Lewis number flames, are modified in this study in such a manner that the new expressions can be used for low Damkohler number flames where bi-modal distribution is not realised. Suitable models for conditional surface-averaged velocity components and the Reynolds stresses have been identified, which are shown to work satisfactorily for all values of Damkohler number and heat release parameter considered in this analysed.
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| 4. |
- Chakraborty, N., et al.
(author)
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Effects of Lewis Number on Conditional Fluid Velocity Statistics in Low Damköhler Number Turbulent Premixed Combustion: A Direct Numerical Simulation Analysis
- 2013
-
In: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 25:4, s. 045101-
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Journal article (peer-reviewed)abstract
- The effects of global Lewis number Le on the statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes simulations have been analysed using a Direct Numerical Simulations (DNS) database of statistically planar turbulent premixed flames with a low Damkohler number and Lewis number ranging from 0.34 to 1.2. The conditional velocity statistics extracted from DNS data have been analysed with respect to the well-known Bray-Moss-Libby (BML) expressions which were derived based on bi-modal probability density function of reaction progress variable for high Damkohler number flames. It has been shown that the Lewis number substantially affects the mean velocity and the velocity fluctuation correlation conditional in products, with the effect being particularly pronounced for low Le. As far as the mean velocity and the velocity fluctuation correlation conditional in reactants are concerned, the BML expressions agree reasonably well with the DNS data reported in the present work. Based on a priori analysis of present and previously reported DNS data, the BML expressions have been empirically modified here in order to account for Lewis number effects, and the non-bimodal distribution of reaction progress variable. Moreover, it has been demonstrated for the first time that surface averaged velocity components and Reynolds stresses conditional in unburned reactants can be modelled without invoking expressions involving the Lewis number, as these surface averaged conditional quantities remain approximately equal to their conditionally averaged counterparts in the unburned mixture.
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| 5. |
- Chakraborty, N., et al.
(author)
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Effects of Lewis number on vorticity and enstrophy transport in turbulent premixed flames
- 2016
-
In: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 28:1, s. 015109-
-
Journal article (peer-reviewed)abstract
- The effects of Lewis number Le on both vorticity and enstrophy transport within the flame brush have been analysed using direct numerical simulation data of freely propagating statistically planar turbulent premixed flames, representing the thin reaction zone regime of premixed turbulent combustion. In the simulations, Le was ranged from 0.34 to 1.2 by keeping the laminar flame speed, thermal thickness, Damkohler, Karlovitz, and Reynolds numbers unchanged. The enstrophy has been shown to decay significantly from the unburned to the burned gas side of the flame brush in the Le approximate to 1.0 flames. However, a considerable amount of enstrophy generation within the flame brush has been observed for the Le = 0.34 case and a similar qualitative behaviour has been observed in a much smaller extent for the Le = 0.6 case. The vorticity components have been shown to exhibit anisotropic behaviour within the flame brush, and the extent of anisotropy increases with decreasing Le. The baroclinic torque term has been shown to be principally responsible for this anisotropic behaviour. The vortex stretching and viscous dissipation terms have been found to be the leading order contributors to the enstrophy transport for all cases, but the baroclinic torque and the sink term due to dilatation play increasingly important role for flames with decreasing Le. Furthermore, the correlation between the fluctuations of enstrophy and dilatation rate has been shown to play an important role in determining the material derivative of enstrophy based on the mean flow in the case of a low Le.
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| 6. |
- Chakraborty, N., et al.
(author)
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Statistics of conditional fluid velocity in the corrugated flamelets regime of turbulent premixed combustion: A Direct Numerical Simulation study
- 2011
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In: Journal of Combustion. - : Hindawi Limited. - 2090-1976 .- 2090-1968. ; 2011:Article ID 628208
-
Journal article (peer-reviewed)abstract
- The statistics of mean fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes (RANS) simulations have been studied using a Direct Numerical Simulation database of statistically planar turbulent premixed flame representing the corrugated flamelets regime combustion. Expressions for conditional mean velocity and conditional velocity correlations which are derived based on a presumed bimodal probability density function of reaction progress variable for unity Lewis number flames are assessed in this study with respect to the corresponding quantities extracted from DNS data. In particular, conditional surface averaged velocities ̄(u i ) Rs and the velocity correlations ̄(u i u j ) Rs in the unburned reactants are demonstrated to be effectively modelled by the unconditional velocities ̄(u i ) R and velocity correlations ̄(u i u j ) R , respectively, for the major part of turbulent flame brush with the exception of the leading edge. By contrast, conditional surface averaged velocities ̄(u i ) ps and the velocity correlations ̄(u i u j ) Ps in fully burned products are shown to be markedly different from the unconditional velocities ̄(u i ) P and velocity correlations ̄(u i u j u j ) P , respectively. © 2011 Nilanjan Chakraborty and Andrei N. Lipatnikov.
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| 7. |
- Chomiak, Jerzy, 1934, et al.
(author)
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Simple criterion of importance of laminar flame instabilities in premixed turbulent combustion of mixtures characterized by low Lewis numbers
- 2023
-
In: Physical Review E. - 2470-0045 .- 2470-0053. ; 107:1
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Journal article (peer-reviewed)abstract
- By (i) highlighting the mitigation effect of strain rates on laminar flame instabilities and (ii) comparing peak growth rates of laminar flame instabilities with strain rates generated by small-scale turbulent eddies, a simple criterion of importance of the influence of the instabilities on an increase in premixed flame surface area in turbulent flows is suggested. The criterion implies that, even in lean hydrogen-air mixtures, laminar flame instabilities can significantly affect the flame area only in weak or moderate turbulence (the Karlovitz number defined using laminar flame speed, thermal flame thickness, and Kolmogorov time scale is on the order of 10 or less under room conditions).
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| 8. |
- Elperin, T., et al.
(author)
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Turbulent diffusion of chemically reacting flows : Theory and numerical simulations
- 2017
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In: Physical review. E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 96:5
-
Journal article (peer-reviewed)abstract
- The theory of turbulent diffusion of chemically reacting gaseous admixtures developed previously [T. Elperin et al., Phys. Rev. E 90, 053001 (2014)] is generalized for large yet finite Reynolds numbers and the dependence of turbulent diffusion coefficient on two parameters, the Reynolds number and Damkohler number (which characterizes a ratio of turbulent and reaction time scales), is obtained. Three-dimensional direct numerical simulations (DNSs) of a finite-thickness reaction wave for the first-order chemical reactions propagating in forced, homogeneous, isotropic, and incompressible turbulence are performed to validate the theoretically predicted effect of chemical reactions on turbulent diffusion. It is shown that the obtained DNS results are in good agreement with the developed theory.
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| 9. |
- Hemdal, Stina, 1974, et al.
(author)
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Investigation of Charge Mixing and Stratified Fuel Distribution in a DISI Engine Using Rayleigh Scattering and Numerical Simulations
- 2022
-
In: Mechanisms and Machine Science. - Cham : Springer International Publishing. - 2211-0992 .- 2211-0984. ; , s. 87-206
-
Book chapter (other academic/artistic)abstract
- The stratified fuel distribution and early flame development in a firing spray-guided direct-injection spark-ignition (DISI) engine are characterized applying optical diagnostics. The goal is to compare effects of single and double injections on the stratified air–fuel mixing and early flame development. Vaporized in-cylinder fuel distributions resulting from both single and double injections before, during and after ignition are selectively visualized applying Rayleigh scattering. Reynolds-averaged Navier–Stokes (RANS) simulations are performed to facilitate interpretation of the obtained experimental data. Two hypotheses are tested. First, injecting the fuel as a closely coupled double injections can improve mixing. Second, the better mixing putatively associated with double injections is mainly due to either a longer mixing time or higher mixing rate (driven by turbulence generated by the injections). The optical investigation of the in-cylinder fuel distributions and early flame propagation corroborated the better mixing, showing that double injections are associated with more evenly distributed fuel, fewer local areas with high fuel concentrations, faster initial flame spread and more even flame propagation (more circular flame spreading). The results from both the experiments and the simulations support the hypothesis that delivering fuel in closely coupled double injections results in better mixing than corresponding single injections. According to the simulations, the improved mixing stems from the longer time available for mixing of the air and fuel in double injection events, which has stronger effects than the higher computed peak bulk mixing rate for single injections.
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| 10. |
- Huang, Chen, 1981, et al.
(author)
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A Chemical Model of Gasoline/Ethanol Blends
- 2009
-
In: The First Joint Meeting of the Scandinavian-Nordic and French Sections of the Combustion Institute, Copenhagen, November 9-10, 2009. ; , s. 2-
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Conference paper (other academic/artistic)
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| 11. |
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| 12. |
- Huang, Chen, 1981, et al.
(author)
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A Numerical Study on Stratified Turbulent Combustion in a Direct-Injection Spark-Ignition Gasoline Engine Using an Open-Source Code
- 2014
-
In: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 1
-
Journal article (peer-reviewed)abstract
- In recent years, a free, open source CFD software package called OpenFOAM has been attracting increasing amounts of attention as a promising, inexpensive, and efficient CFD tool for the numerical simulation of processes such as fuel injection and evaporation, turbulent mixing and burning. Here, we describe the further development of OpenFOAM to enable its use in simulating stratified turbulent combustion in DI SI engines. Advanced models of various phenomena relevant to partially premixed turbulent flames were implemented into the code, and the effects of these implementations were investigated by performing unsteady 3D RANS simulations of stratified turbulent burning in a DI SI engine. First, the Flame Speed Closure (FSC) model of premixed turbulent combustion was implemented. Second, a method for evaluating the mean density in premixed turbulent flames that is available in the standard OpenFOAM library was improved. Third, a semi-detailed chemical mechanism was introduced to describe the influence of the equivalence ratio, pressure, and temperature of the unburned gas on the burning rate and flame temperature. The flame temperature and laminar flame speed are computed, approximated and further implemented into the OpenFOAM library. Fourth, to address the influence of turbulent fluctuations in mixture composition on mean variables, a presumed Favre beta-PDF for the mixture fraction was implemented. Fifth, the implementation of the balance equation for mixture fraction variance was improved with the consideration for the evaporation source term. Finally, the mean burning rates computed with and without the aforementioned models were compared to assess the importance of the studied effects.
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| 13. |
- Huang, Chen, 1981, et al.
(author)
-
A Semi-Detailed Chemical Mechanism of Combustion of Gasoline-Ethanol Blends
- 2010
-
In: 33nd International Symposium on Combustion, Tsinghua University, Beijing, China, August 1-6, 2010. Abstracts of Work-in-Progress Poster Presentations. The Combustion Institute, Pittsburgh, 2010. File W4P014.pdf on Flash Disk..
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Conference paper (other academic/artistic)
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| 14. |
- Huang, Chen, 1981, et al.
(author)
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A Study of Two Basic Issues Relevant to RANS Simulations of Stratified Turbulent Combustion in a Spray-Guided Direct-Injection Spark-Ignition Engine
- 2014
-
In: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2014-October
-
Journal article (peer-reviewed)abstract
- A Spray-Guided (SG) Direct-Injection (DI) Spark-Ignition (SI) engine is widely recognized to be a promising technology capable for substantially reducing fuel consumption and carbon dioxide emissions. Accordingly, there is a strong need for developing models of some effects specific to stratified turbulent burning under conditions of elevated and rapidly varying pressure. Two such effects were addressed in the present work by performing unsteady three-dimensional URANS simulations of stratified turbulent combustion in a SG DISI engine. First, a simple method of evaluation equilibrium combustion temperature, implemented into the CFD code OpenFOAM ® , was improved in order to take into account the dissociation of the combustion products. Second, stratified turbulent combustion is affected by fluctuations in mixture composition. A widely used approach to modeling this effect consists of invoking a presumed Probability Density Function (PDF) for mixture fraction f . Because parameters of this PDF are determined using the first and second Favre moments of the mixture fraction field, the PDF is density-weighted. However, the canonical PDF P f is required to average certain important combustion characteristics that are straightforwardly relevant to local burning rate e.g. the laminar flame speed or the product density. In the present work, the relation between the Favre and canonical PDFs was investigated under conditions associated with burning in a SG DISI engine. Finally, the stratified turbulent combustion model which invo
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| 15. |
- Huang, Chen, et al.
(author)
-
A vented corn starch dust explosion in an 11.5 m3 vessel : Experimental and numerical study
- 2022
-
In: Journal of Loss Prevention in the Process Industries. - : Elsevier Ltd. - 0950-4230 .- 1873-3352. ; 75
-
Journal article (peer-reviewed)abstract
- A vented corn starch dust explosion in an 11.5 m3 vessel is studied using both experimental and numerical methods. The reduced explosion overpressure in the vessel is recorded using two pressure sensors mounted on the wall inside of the vessel. Unsteady three-dimensional Reynolds-Averaged Navier-Stokes simulations of the experiment are performed using the Flame Speed Closure (FSC) model of the influence of turbulence on premixed combustion. The model was thoroughly validated in previous studies and was earlier implemented into OpenFOAM CFD software. The self-acceleration of a large-scale flame kernel is associated with the influence of combustion-induced pressure perturbations on the flow of unburned reactants ahead of the kernel. Accordingly, the FSC model is extended by adapting the well-known experimental observations of the self-similarity of the kernel acceleration. Influence of different turbulence models on the simulated results is also explored. Thanks to the extension of the FSC model, the measured time-dependence of the pressure is well predicted when the k-omega-SST turbulence model is used. © 2021 The Authors
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| 16. |
- Huang, Chen, 1981, et al.
(author)
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Application of Flame Speed Closure Model to RANS Simulations of Stratified Turbulent Combustion in a Gasoline Direct-Injection Spark-Ignition Engine
- 2016
-
In: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 188:1, s. 98-131
-
Journal article (peer-reviewed)abstract
- © 2016 Taylor & Francis. The present work aims at development and validation of a tool for numerically modeling stratified turbulent combustion in a gasoline direct injection (GDI) engine. For this purpose, an open source code called OpenFOAM ® , which has been attracting growing interests from both industries and academies due to an opportunity to access the source code and to test new models without paying license fees, is further developed by implementing advanced models relevant to stratified turbulent burning. In particular, first, the Flame Speed Closure model of premixed turbulent combustion is implemented in order to simulate flame propagation through inhomogeneously premixed reactants. Second, a newly calculated approximation of the laminar flame speed of gasoline-air mixtures as a function of the equivalence ratio, pressure, and temperature is implemented in order to simulate dependence of burning rate on the local mixture composition. Third, a newly calculated approximation of the combustion temperature of gasoline-air mixtures as a function of the equivalence ratio, pressure, and product enthalpy is implemented in order to allow for dissociation of combustion products and heat losses. Fourth, a presumed mixture-fraction probability density function (PDF) approach is implemented in order to simulate the influence of turbulent fluctuations in the mixture fraction on the local burning rate. In addition to commonly used mass-weighted mixture-fraction PDF, a more consistent model that deals also with the canonical mixture-fraction PDF is developed and the two approaches are compared. Numerical results that show the influence of the aforementioned implementations on computed global characteristics of stratified combustion in a research GDI engine are discussed. The developed numerical tool is quantitatively validated by comparing computed pressure traces in the GDI engine with experimental data obtained in three different cases associated with two different loads, late injection timings, and short time intervals between the injection and spark ignition.
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| 17. |
- Huang, Chen, 1981, et al.
(author)
-
Chemical model of gasoline-ethanol blends for internal combustion engine applications
- 2010
-
In: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627.
-
Journal article (peer-reviewed)abstract
- A semi-detailed chemical mechanism for combustion of gasoline-ethanol blends, which is based on sub-mechanisms of gasoline surrogate and for ethanol is developed and validated aiming at CFD engine modeling. The gasoline surrogate is composed of iso-octane, toluene, and n-heptane in volumetric proportions of 55%:35%:10%, respectively. In this way, the hydrogen-carbon atomic ratio H/C, which is around 1.87 for real gasoline, is accurately reproduced as well as a mixture equivalence ratio that is important for Gasoline Direct Injection engine applications. The integrated mechanism for gasoline-ethanol blends includes 120 species participating in 677 reactions. The mechanism is tested against experimental data on ignition delay times and laminar flame speeds, obtained for various n-heptane/iso-octane/toluene/ethanol-air mixtures under various equivalence ratios, initial temperatures, and pressures. Chemical, thermodynamic and transport properties used in the calculations are discussed.
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| 18. |
- Huang, Chen, 1981, et al.
(author)
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Comparison of presumed PDF models of turbulent flames
- 2012
-
In: Journal of Combustion. - : Hindawi Limited. - 2090-1976 .- 2090-1968. ; 2012:564621, s. 15-
-
Journal article (peer-reviewed)abstract
- Over the past years, the use of a presumed probability density function (PDF) for combustion progress variable or/and mixture fraction has been becoming more and more popular approach to average reaction rates in premixed and partially premixed turbulent flames. Commonly invoked for this purpose is a beta-function PDF or a combination of Dirac delta functions, with the parameters of the two PDFs being determined based on the values of their first and second moments computed by integrating proper balance equations. Because the choice of any of the above PDFs appears to be totally arbitrary as far as underlying physics of turbulent combustion is concerned, the use of such PDFs implies weak sensitivity of the key averaged quantities to the PDF shape. The present work is aimed at testing this implicit assumption by comparing mean heat release rates, burning velocities, and so forth, averaged by invoking the aforementioned PDFs, with all other things being equal. Results calculated in the premixed case show substantial sensitivity of the mean heat release rate to the shape of presumed combustion-progress-variable PDF, thus, putting the approach into question. To the contrary, the use of a presumed mixture-fraction PDF appears to be a sufficiently reasonable simplification for modeling the influence of fluctuations in the mixture fraction on the mean burning velocity provided that the mixture composition varies within flammability limits.
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| 19. |
- Huang, Chen, 1981, et al.
(author)
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Gasoline Direct Injection - Simulations and Experiments
- 2011
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In: ILASS2011. The 24th European Conference on Liquid Atomization and Spray Systems, Estoril, Portugal, September, 5-7, 2011. ; , s. 4 pagews-
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Conference paper (other academic/artistic)
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| 20. |
- Huang, Chen, 1981, et al.
(author)
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Modelling of Gasoline and Ethanol Hollow-Cone Sprays Using OpenFOAM
- 2011
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In: 2011 JSAE/SAE International Powertrains, Fuels & Lubricants, Aug. 30 - Sep. 2, 2011, Kyoto, Japan. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
-
Conference paper (peer-reviewed)abstract
- Over the past few years, an open-source code calledOpenFOAM has been becoming a promising CFD toolfor multi-dimensional numerical simulations of internalcombustion engines. The primary goal of the presentstudy is to assess the feasibility of the code forcomputations of hollow-cone sprays discharged by anoutward-opening pintle-type injector by simulating theexperiments performed recently by Hemdal et al.(SAE 2009-01-1496) with gasoline and ethanolsprays under the following conditions: air temperatureTair=295 or 350 K, air pressure pair=6 bar, fueltemperature Tfuel=243, or 295, or 320 K, and fuelinjection pressure pinj=50, or 125, or 200 bar. Tosimulate the experiments, a pintle injector model andthe physical properties of gasoline were implementedin OpenFOAM. The flow field calculated using thepintle injector model is more realistic than that yieldedby the default unit injector model normally used inOpenFOAM. Moreover, a number of modificationswere made to the standard implementation of severalspray models in OpenFOAM, with modifications in theimplementation of the KHRT model having noticeableeffects on the accuracy of the simulated liquidpenetration and Sauter mean diameter (SMD).Results of numerous simulations performed byrunning OpenFOAM and activating various spraymodels indicate that (i) a combination of theRosin-Rammler distribution with Reitz-Diwakarsecondary breakup model and (ii) the KHRT modelyield the best agreement between the measured andcomputed spray penetration length, with the lattermodel showing the best performance as far as theSMD obtained from high-pressure (200 bar) sprays isconcerned.
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| 21. |
- Huang, Chen, 1981, et al.
(author)
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Numerical and Experimental Study of Stratified Turbulent Combustion in a Spray-Guided Gasoline Direct Injection Engine
- 2015
-
In: Lecture Notes in Mobility. - Cham : Springer International Publishing. - 2196-5544 .- 2196-5552. - 9783319179988 ; , s. 77-84
-
Conference paper (other academic/artistic)abstract
- Direct Injection (DI) of gasoline into cylinder of a Spark Ignition (SI) engine is widely recognized to be a promising technology capable for significantly reducing fuel consumption and carbon dioxide emissions as compared to a port-fuel injection SI engine. In particular, spray-guided (SG) GDI combustion systems allow for further improvement in fuel efficiency. Moreover, efficient CFD tools fornumerical simulations of spray and combustion processes have been becoming increasingly important in engine development. In previous papers, a so-called FlameSpeed Closure (FSC) model was implemented into an open source code OpenFOAM® with the capability of addressing important phenomena in SG GDI engines, e.g. fluctuations in mixture composition and the proper evaluation of combustion temperature for the products. In this paper, the aforementioned FSC model is applied to investigate the stratified turbulent combustion in a SG GDI enginein the frame work of unsteady 3D Reynolds-Averaged Navier–Stokes (RANS) simulations. The computed results are compared with the measured pressure traces obtained in the same research group for both low and medium load conditions.Further on, the calculated Reynolds-averaged progress variable is compared to the experimentally observed images.
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| 22. |
- Huang, Chen, et al.
(author)
-
Unsteady 3-D RANS simulations of dust explosion in a fan stirred explosion vessel using an open source code
- 2020
-
In: Journal of Loss Prevention in the Process Industries. - : Elsevier Ltd. - 0950-4230 .- 1873-3352. ; 67
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Journal article (peer-reviewed)abstract
- Dust explosion is a constant threat to industries which deal with combustible powders such as woodworking, metal processing, food and feed, pharmaceuticals and additive industries. The current standards regarding dust explosion venting protecting systems, such as EN 14491 (2012) and NFPA 68 (2018), are based on empirical correlations and neglect effects due to complex geometry. Such a simplification may lead to failure in estimating explosion overpressure, thus, increasing risk for injuries and even fatalities at workplaces. Therefore, there is a strong need for a numerical tool for designing explosion protecting systems. This work aims at contributing to the development of such a tool by (i) implementing a premixed turbulent combustion model into OpenFOAM, (ii) verifying the implementation using benchmark analytical solutions, and (iii) validating the numerical platform against experimental data on cornflour dust explosion in a fan-stirred explosion vessel, obtained by Bradley et al. (1989a) under well-controlled laboratory conditions. For this purpose, the so-called Flame Speed Closure model of the influence of turbulence on premixed combustion is adapted and implemented into OpenFOAM. The implementation of the model is verified using exact and approximate analytical solutions for statistically one-dimensional planar and spherical turbulent flames, respectively. The developed numerical platform is applied to unsteady three-dimensional Reynolds Averaged Navier-Stokes simulations of the aforementioned experiments. The results show that the major trends, i.e. (i) a linear increase in an apparent turbulent flame speed St,b with an increase in the root mean square (rms) turbulent velocity u' and (ii) and an increase in St,b with an increase in the mean flame radius, are qualitatively predicted. Furthermore, the measured and computed dependencies of St,b(u') agree quantitatively under conditions of weak and moderate turbulence. © 2020
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| 23. |
- Huang, Chen, 1981, et al.
(author)
-
Vented dust explosions: comparing experiments, simulations and standards
- 2022
-
In: 10th International Seminar on Fire and Explosion Hazards. - 9788272067211 ; , s. 1-9
-
Conference paper (peer-reviewed)abstract
- A vented corn starch dust explosion in an 11.5 m3 vessel is studied by comparing experiments, simulations and the standards. The reduced explosion overpressure inside the vessel is recorded using two pressure sensors installed on the inner wall of the vessel. 3D Unsteady Reynolds-Averaged Navier-Stokes simulations of the experiment are performed using the Flame Speed Closure (FSC) model and its extended version. The FSC model predicts the influence of turbulence on premixed combustion, and the extended version allows for self-acceleration of a large-scale flame kernel, which is associated with the combustion-induced thermal expansion effect. Such an extension is highly relevant to large-scale industrial application. The explosion overpressure-time trace computed using the extended FSC model agrees reasonably well with the experimental data. Furthermore, the effect of vent size and ignition location on the explosion overpressure is studied by comparing the simulation results and the standards. The developed numerical tool and model is especially useful for scenarios, which are not addressed in the standards, and it deserves further study in simulations of other large-scales dust or gaseous explosions together with comparison with experiments.
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| 24. |
- Kaltashov, Igor A., et al.
(author)
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LC/MS at the whole protein level: Studies of biomolecular structure and interactions using native LC/MS and cross-path reactive chromatography (XP-RC) MS
- 2018
-
In: Methods. - : Elsevier BV. - 1095-9130 .- 1046-2023. ; 144, s. 14-26
-
Research review (peer-reviewed)abstract
- Interfacing liquid chromatography (LC) with electrospray ionization (ESI) to enable on-line MS detection had been initially implemented using reversed phase LC, which in the past three decades remained the default type of chromatography used for LC/MS and LC/MS/MS studies of protein structure. In contrast, the advantages of other types of LC as front-ends for ESI MS, particularly those that allow biopolymer higher order structure to be preserved throughout the separation process, enjoyed relatively little appreciation until recently. However, the past few years witnessed a dramatic surge of interest in the so-called “native” (with “non-denaturing” being perhaps a more appropriate adjective) LC/MS and LC/MS/MS analyses within the bioanalytical and biophysical communities. This review focuses on recent advances in this field, with an emphasis on size exclusion and ion exchange chromatography as front-end platforms for protein characterization by LC/MS. Also discussed are the benefits provided by the integration of chemical reactions in the native LC/MS analyses, including both ion chemistry in the gas phase (e.g., limited charge reduction for characterization of highly heterogeneous biopolymers) and solution-phase reactions (using the recently introduced technique cross-path reactive chromatography).
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| 25. |
- Lai, J., et al.
(author)
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Statistical behaviour of vorticity and enstrophy transport in head-on quenching of turbulent premixed flames
- 2017
-
In: European Journal of Mechanics, B/Fluids. - : Elsevier BV. - 0997-7546. ; 65:Sept.-Oct., s. 384-397
-
Journal article (peer-reviewed)abstract
- The transport of vorticity and enstrophy in the near-wall region for head-on quenching of turbulent combustion by an isothermal inert wall has been analysed using three-dimensional Direct Numerical Simulation (DNS) data of statistically planar turbulent premixed flames characterised by various global Lewis numbers Le (ranging from 0.8 to 1.2) and turbulence intensities. In all cases the vorticity magnitude shows its maximum value at the wall and the vorticity magnitude drops significantly from the unburned to the burned gas side of the flame-brush. Moreover, the vorticity magnitude shows an increasing trend with decreasing Le, and increasing turbulence intensity. A significant amount of anisotropy has been observed between the vorticity components within the flame-brush and this anisotropy increases as the wall is approached. The baroclinic torque term has been found to be principally responsible for this anisotropic behaviour. The vortex-stretching and viscous dissipation terms remain the leading order contributors to the vorticity and enstrophy transport for all cases when the flame is away from the wall, but as flame approach the wall, the baroclinic torque begins to play an increasingly important role. The combined molecular diffusion and dissipation contribution to the enstrophy transport remains negative away from the wall but it changes its sign near the wall due to the torque arising from dilatation rate gradient. Detailed physical explanations have been provided for the observed influences of flame and wall on the statistical behaviours of vorticity and enstrophy and the various terms of their transport equations.
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| 26. |
- Lee, Hsu Chew, et al.
(author)
-
A DNS study of extreme and leading points in lean hydrogen-air turbulent flames – Part I: Local thermochemical structure and reaction rates
- 2022
-
In: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 235
-
Journal article (peer-reviewed)abstract
- A Direct Numerical Simulation (DNS) study of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by a low Lewis number Le and three different Karlovitz numbersKa ranging from 3 to 33 is performed, with the same complex-chemistry flames being also simulated by setting molecular diffusivities of all species equal to the heat diffusivity of the mixture. The simulations predict a significant increase in a ratio of turbulent burning velocity to the laminar flame speed in the former (Le<1) flames when compared to the latter (equidiffusive) flames. Extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) are found at each instant. In the equidiffusive flames, such extreme FCR and HRR are close to their peak values in the unperturbed laminar flame. If Le is low, the former rates are significantly higher than the latter ones due to an increase in the local temperature, equivalence ratio, and radical mass fractions, caused by diffusive-thermal effects. While the studied extreme points may appear sufficiently far from the leading edge of the instantaneous flame brush, leading points characterized by a lower, but still high (Le<1) FCR or HRR are observed close to the leading edge at each instant. Various local characteristics (temperature, equivalence ratio, species mass fractions and their gradients, reaction rates, etc.) of the extreme and leading points are explored and significant differences between zones characterized by high FCR or HRR are revealed. For instance, in the latter zones, major chemical pathways are changed. Moreover, while the extreme HRRs strongly fluctuate in time, with their mean and rms values being significantly increased by Ka, the extreme FCRs fluctuate weakly and are close at different Ka, thus, implying that almost the same extreme FCR can be reached in substantially different local burning structures.
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| 27. |
- Lee, Hsu Chew, et al.
(author)
-
A DNS study of extreme and leading points in lean hydrogen-air turbulent flames - part II: Local velocity field and flame topology
- 2022
-
In: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 235
-
Journal article (peer-reviewed)abstract
- Data obtained in recent direct numerical simulations (Lee et al.) of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by three different Karlovitz numbers Ka ranging from 3 to 33 are further analyzed in order to explore local characteristics and structure of (i) extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) and (ii) leading points that are also characterized by a high FCR or HRR, but advance furthest into unburned reactants. Results show that, on the one hand, common characteristics of flame perturbations (curvature, strain and stretch rates, displacement speed) fluctuate significantly in the extreme or leading, FCR or HRR points and are different in different flames. Moreover, other two-point local quantities such as the local gradients of combustion progress variables or species (e.g., the radical H) mass fractions are different in different flames. Therefore, a common simple configuration of a perturbed laminar flame cannot be used as a catchall model of the entire local structure of zones surrounding the discussed points at various Ka. On the other hand, single-point local characteristics (temperature, species mass fractions, rates of their production) of the FCR extreme points are comparable in all three turbulent flames and in the critically strained planar laminar flame. In particular, the FCRs in the extreme points fluctuate weakly and are approximately equal to each other and to the peak FCR in the critically strained laminar flame. The latter finding implies that (i) the maximum FCR evaluated in the critically strained laminar flame could be used to characterize, in a first approximation, the local FCR in the extreme or leading points in turbulent flames, thus, supporting the leading point concept, and (ii) almost the same extreme FCR can be reached in substantially different local burning structures.
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| 28. |
- Lee, Hsu Chew, et al.
(author)
-
A numerical support of leading point concept
- 2022
-
In: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 47:55, s. 23444-23461
-
Journal article (peer-reviewed)abstract
- Unsteady three-dimensional Direct Numerical Simulation (DNS) data obtained from 16 statistically planar and one-dimensional, complex-chemistry, lean (equivalence ratio is equal to 0.50 or 0.35) hydrogen-air flames propagating in forced, intense, small-scale turbulence (Karlovitz number up to 565) are reported. The data are analyzed to compare roles played by leading and trailing edges of a premixed turbulent flame brush in its propagation. The comparison is based on the following considerations: (i) positively (negatively) curved reaction zones predominate at the leading (trailing, respectively) edge of a premixed turbulent flame brush and (ii) preferential diffusion of molecular or atomic hydrogen results in increasing the local fuel consumption and heat release rates in positively or negatively, respectively, curved reaction zones. Therefore, turbulent burning velocities computed by deactivating differential diffusion effects for all species with the exception of either H2 or H are compared for assessing roles played by leading and trailing edges of a premixed turbulent flame brush in its propagation. By analyzing the DNS data, a significant increase in the local fuel consumption and heat release rates due to preferential diffusion of H2 or H is documented close to the leading or trailing, respectively, edges of the studied flame brushes. Nevertheless, turbulent burning velocities computed by activating preferential diffusion solely for H2 are significantly higher than turbulent burning velocities computed by activating preferential diffusion solely for H. This result indicates an important role played by the leading edge in the propagation of the explored turbulent flame brushes
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| 29. |
- Lee, HsuChew, et al.
(author)
-
Displacement speed, flame surface density, and burning rate in highly turbulent premixed flames characterized by low Lewis numbers
- 2023
-
In: Journal of Fluid Mechanics. - 0022-1120 .- 1469-7645. ; 961
-
Journal article (peer-reviewed)abstract
- Direct Numerical Simulation data obtained earlier from four pairs of moderately or highly turbulent, lean hydrogen-air, complex-chemistry flames are analyzed to explore influence of molecular diffusion on differently averaged Flame Surface Densities (FSD), displacement speeds Sd, and various terms in the FSD transport equation. For this purpose, each pair involves (i) a flame where mixture averaged molecular diffusivities are adopted and Lewis number Le is significantly less than unity and (ii) an equidiffusive flame where all molecular diffusivities are set equal to molecular heat diffusivity of the mixture and Le=1, with other things being equal in the two flames. Reported results show, in particular, that significantly higher turbulent burning rates simulated in the former flames result mainly from an increase in the local fuel consumption rate, whereas an increase in flame surface area plays a secondary role, especially in more intense turbulence. The rate increase stems from (i) an increase in the peak local fuel consumption rate and (ii) an increase in a width of a zone where the rate is significant. The latter phenomenon is of more importance in richer flames and both phenomena are most pronounced in the vicinity of the flame leading edges, thus, further supporting a crucial role played by the leading edge of a premixed turbulent flame in its propagation. Moreover, mean displacement speed differs significantly from the laminar flame speed even in the equidiffusive flames, vary substantially across flame brush, and may be negative at the leading edges of highly turbulent flames.
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| 30. |
- Lee, HsuChew, et al.
(author)
-
Influence of equivalence ratio on turbulent burning velocity and extreme fuel consumption rate in lean hydrogen-air turbulent flames
- 2022
-
In: Fuel. - : Elsevier BV. - 0016-2361. ; 327
-
Journal article (peer-reviewed)abstract
- Unsteady three-dimensional Direct Numerical Simulations of seven statistically one-dimensional, planar, highly turbulent, complex-chemistry, lean H2-air flames are performed using either mixture averaged or equidiffusive model of molecular transport. The equivalence ratio is varied from 0.35 to 0.70 and the Karlovitz number Ka is varied from 3 to 565. Normalized turbulent burning velocities UT/SL are strongly increased when using the mixture-averaged model, with an increase by a factor of 4.1 being documented even at Ka as high as 565. Here, SL is the laminar flame speed. Moreover, the increase in UT/SL is significantly more pronounced in leaner flames, which are characterized by a thinner reaction zone and a larger Zel’dovich number. Furthermore, UT/SL is increased by the turbulence length scale. The extreme (maximum over the computational domain at a single instant) local values of fuel consumption rate (FCR) exhibit a high degree of universality, i.e., in all studied cases and at all instants, these rates are close to the peak values of FCR obtained from the counterpart critically strained, twin, counter-flow laminar premixed flames. This finding appears to directly support a corner-stone hypothesis of the leading point concept of premixed turbulent burning, thus, suggesting the use of characteris tics of the critically strained laminar premixed flames as input parameters for models of turbulent combustion of lean H2/air mixtures.
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| 31. |
- Lee, HsuChew, et al.
(author)
-
Influence of molecular transport on burning rate and conditioned species concentrations in highly turbulent premixed flames
- 2021
-
In: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 928
-
Journal article (peer-reviewed)abstract
- Apparent inconsistency between (i) experimental and Direct Numerical Simulation (DNS) data that show the significant influence of differential diffusion on turbulent burning rate and (ii) recent complex-chemistry DNS data that indicate mitigation of the influence of differential diffusion on conditioned profiles of various local flame characteristics at high Karlovitz numbers is explored by analyzing new DNS data obtained from lean hydrogen-air turbulent flames. Both aforementioned effects are observed by analyzing the same DNS data provided that the conditioned profiles are sampled from the entire computational domain. On the contrary, the conditioned profiles sampled at the leading edge of the mean flame brush do not indicate the mitigation, but are significantly affected by differential diffusion phenomena,e.g., because reaction zones are highly curved at the leading edge. This observation is consistent with a significant increase in the computed turbulent burning velocity with decreasing Lewis number, with all the results considered jointly being consonant with the leading point concept of premixed turbulent combustion. The concept is further supported by comparing DNS data obtained by allowing for preferential diffusion solely for a single species, either atomic or molecular hydrogen.
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| 32. |
- Lee, HsuChew, et al.
(author)
-
Lewis number and preferential diffusion effects in lean hydrogen–air highly turbulent flames
- 2022
-
In: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:3
-
Journal article (peer-reviewed)abstract
- Unsteady three-dimensional direct numerical simulations of highly turbulent, complex-chemistry, lean hydrogen-air flames were performed by changing the equivalence ratio F, root mean square velocity u' , and turbulence length scale L. For each set of {F, u', L}, to explore the influence of molecular transport coefficients on the turbulent burning velocity UT , four cases were designed: (i) mixture-averaged diffusivities; (ii) diffusivities equal to the heat diffusivity a of the mixture for all species; (iii) mixture-averaged diffusivities for all species with the exception of O2 , whose diffusivity was equal to the diffusivity DH2 of H2 to suppress preferential diffusion effects; and (iv) mixture-averaged diffusivities multiplied with a/DH2 to suppress Lewis number effects but retain preferential diffusion effects. The computed results show a significant increase in UT due to differences in molecular transport coefficients even at Karlovitz number Ka as large as 565. The increase is documented in cases (i) and (iii) but is not observed in case (iv) —indicating that this phenomenon is controlled by Lewis number effects, whereas preferential diffusion effects play a minor role. The phenomenon is more pronounced in leaner flames, with all other things being equal. While the temperature profiles (c) conditionally averaged at the local value of the combustion progress variable c and sampled from the entire flame brushes are not sensitive to variations in molecular transport coefficients at high Ka, the (c)-profiles sampled from the leading edges of the same flame brushes show significant increase in the local temperature in cases (i) and (iii) characterized by a low Lewis number.
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| 33. |
- Lee, HsuChew, et al.
(author)
-
Turbulent burning velocity and thermodiffusive instability of premixed flames
- 2023
-
In: Physical Review E. - 2470-0045 .- 2470-0053. ; 108:3
-
Journal article (peer-reviewed)abstract
- Reported in the paper are results of unsteady three-dimensional direct numerical simulations of laminar and turbulent, lean hydrogen-air, complex-chemistry flames propagating in forced turbulence in a box. To explore the eventual influence of thermodiffusive instability of laminar flames on turbulent burning velocity, (i) a critical length scale $\Lambda_{n}$ that bounds regimes of unstable and stable laminar combustion is numerically determined by gradually decreasing the width $\Lambda$ of computational domain until a stable laminar flame is obtained and (ii) simulations of turbulent flames are performed by varying the width from $\Lambda<\Lambda_{n}$ (in this case, the instability is suppressed) to $\Lambda>\Lambda_{n}$ (in this case, the instability may grow). Moreover, simulations are performed either using mixture-averaged transport properties (low Lewis number flames) or setting diffusivities of all species equal to heat diffusivity of the mixture (equidiffusive flames), with all other things being equal. Obtained results show a significant increase in turbulent burning velocity $U_T$ when the boundary $\Lambda=\Lambda_{n}$ is crossed in weak turbulence, but almost equal values of $U_T$ are computed at $\Lambda<\Lambda_{n}$ and $\Lambda>\Lambda_{n}$ in moderately turbulent flames characterized by Karlovitz number equal to 3.4 or larger. These results imply that thermo-diffusive instability of laminar premixed flames substantially affects burning velocity in weak turbulence only, in line with a simple criterion proposed by Chomiak and Lipatnikov (Phys. Rev. E 107, 015102, 2023).
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| 34. |
- Lipatnikov, Andrei, 1961
(author)
-
A balance equation for modeling conditioned enthalpies in premixed turbulent flames
- 2015
-
In: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 162:10, s. 3691-3703
-
Journal article (peer-reviewed)abstract
- In order to model the difference between enthalpies conditioned to unburned and burned gases, caused by compression of the mixture in the combustion chamber of a spark ignition engine, a new balance equation for the correlation between fluctuations in the enthalpy and combustion progress variable is proposed to be used, derived and closed invoking a few assumptions that appear to be plausible at low Mach and unity Lewis numbers. Subsequently, the difference in the conditioned enthalpies is determined within the framework of the well-known BML paradigm of premixed turbulent burning in the flamelet regime. In the paper, this new approach is analytically compared with commonly used balance equations for enthalpy conditioned to unburned gas. Moreover, in order to investigate basic features of the newly proposed balance equation, it is applied to numerical simulations of a statistically stationary, planar, one-dimensional premixed turbulent flame subject to increasing pressure. Instead of modeling the mean flame structure, it is approximated using complementary error function, with the flame speed and thickness being input parameters in the present study. Computed results show that the difference in the conditioned enthalpies is increased when the density ratio or the pressure growth rate is increased and when the turbulent flame speed or mean flame brush thickness is decreased. Moreover, computed enthalpy difference is sensitive to a submodel of turbulent flux of the combustion progress variable, with the use of gradient diffusion approximation appearing to be an improper solution. Furthermore, computed enthalpy difference is sensitive to a closure relation for enthalpy conditioned to flamelets.
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| 35. |
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| 36. |
- Lipatnikov, Andrei, 1961, et al.
(author)
-
A direct numerical simulation study of the influence of flame-generated vorticity on reaction-zone-surface area in weakly turbulent premixed combustion
- 2019
-
In: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 31:5
-
Journal article (peer-reviewed)abstract
- Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios sigma= 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of sigma= 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large magnitude of the baroclinic torque term in the enstrophy transport equation, with the effect being more pronounced at larger values of the mean combustion progress variable. If the density ratio is low, e.g., sigma= 2.5, the baroclinic torque weakly affects the vorticity field within the mean flame brush and the aforementioned effect is not pronounced.
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| 37. |
- Lipatnikov, Andrei, 1961, et al.
(author)
-
A direct numerical simulation study of vorticity transformation in weakly turbulent premixed flames
- 2014
-
In: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 26:10
-
Journal article (peer-reviewed)abstract
- Database obtained earlier in 3D Direct Numerical Simulations (DNS) of statistically stationary, 1D, planar turbulent flames characterized by three different density ratios σ is processed in order to investigate vorticity transformation in premixed combustion under conditions of moderately weak turbulence (rms turbulent velocity and laminar flame speed are roughly equal to one another). In cases H and M characterized by σ = 7.53 and 5.0, respectively, anisotropic generation of vorticity within the flame brush is reported. In order to study physical mechanisms that control this phenomenon, various terms in vorticity and enstrophy balance equations are analyzed, with both mean terms and terms conditioned on a particular value c of the combustion progress variable being addressed. Results indicate an important role played by baroclinic torque and dilatation in transformation of average vorticity and enstrophy within both flamelets and flame brush. Besides these widely recognized physical mechanisms, two other effects are documented. First, viscous stresses redistribute enstrophy within flamelets, but play a minor role in the balance of the mean enstrophy Ω ¯ ¯ ¯ within turbulent flame brush. Second, negative correlation u ′ ⋅∇Ω ′ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ between fluctuations in velocity u and enstrophy gradient contributes substantially to an increase in the mean Ω ¯ ¯ ¯ within turbulent flame brush. This negative correlation is mainly controlled by the positive correlation between fluctuations in the enstrophy and dilatation and, therefore, dilatation fluctuations substantially reduce the damping effect of the mean dilatation on the vorticity and enstrophy fields. In case L characterized by σ = 2.5, these effects are weakly pronounced and Ω ¯ ¯ ¯ is reduced mainly due to viscosity. Under conditions of the present DNS, vortex stretching plays a minor role in the balance of vorticity and enstrophy within turbulent flame brush in all three cases.
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| 38. |
- Lipatnikov, Andrei, 1961, et al.
(author)
-
A DNS assessment of linear relations between filtered reaction rate, flame surface density, and scalar dissipation rate in a weakly turbulent premixed flame
- 2019
-
In: Combustion Theory and Modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 23:2, s. 245-260
-
Journal article (peer-reviewed)abstract
- Linear relations between (i) filtered reaction rate and filtered flame surface density (FSD) and (ii) filtered reaction rate and filtered scalar dissipation rate (SDR), which are widely used in Large Eddy Simulation (LES) research into premixed turbulent combustion, are examined by processing DNS data obtained from a statistically 1D planar flame under weakly turbulent conditions that are most favourable for the two approaches (flamelet combustion regime, single-step chemistry, equidiffusive mixture, adiabatic burner, and low Mach number). The analysis well supports the former approach provided that the filtered reaction rate is combined with filtered molecular transport term. In such a case, both the RANS and LES FSD approaches are based on local relations valid within weakly perturbed flamelets. Accordingly, simply recasting RANS expressions to a filtered form works well. On the contrary, while the FSD and SDR approaches appear to be basically similar at first glance, the analysis does not support the latter one, but shows that a ratio of the filtered reaction rate to the filtered SDR is strongly scattered within the studied flame brush, with its conditionally mean value varying significantly with Favre-filtered combustion progress variable. As argued in the paper, these limitations of the LES SDR approach stem from the fact that it is based on a relation valid after integration over weakly perturbed flamelets, but this relation does not hold locally within such flamelets. Consequently, when a sufficiently small filter is applied to instantaneous fields, the filter may contain only a part of the local flamelet, whereas the linear relation holds solely for the entire flamelet and may not hold within the filtered flamelet volume. Thus, the present study implies that straightforwardly recasting well established RANS equations to a filtered form is a flawed approach if the equations are based on integral features of local burning.
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| 39. |
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| 40. |
- Lipatnikov, Andrei, 1961, et al.
(author)
-
A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames
- 2018
-
In: Combustion Theory and Modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 22:1, s. 131-155
-
Journal article (peer-reviewed)abstract
- Data obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterised by different density ratios σ are analysed to study the influence of thermal expansion on flame surface area and burning rate. Results show that, on the one hand, the pressure gradient induced within a flame brush owing to heat release in flamelets significantly accelerates the unburned gas that deeply intrudes into the combustion products in the form of an unburned mixture finger, thus causing largescale oscillations of the burning rate and flame brush thickness. Under the conditions of the present simulations, the contribution of this mechanism to the creation of the flame surface area is substantial and is increased by σ, thus implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal. The apparent inconsistency between these results implies the existence of another thermal expansion effect that reduces the influence of σ on the flame surface area and burning rate. Investigation of the issue shows that the flow acceleration by the combustion-induced pressure gradient not only creates the flame surface area by pushing the finger tip into the products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to the high-speed (at σ = 7.53) axial flow of the unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces the residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened owing to a shorter residence time.
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| 41. |
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| 42. |
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| 43. |
- Lipatnikov, Andrei, 1961, et al.
(author)
-
A priori DNS study of applicability of flamelet concept to predicting mean concentrations of species in turbulent premixed flames at various Karlovitz numbers
- 2020
-
In: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 222, s. 370-382
-
Journal article (peer-reviewed)abstract
- Complex-chemistry direct numerical simulation (DNS) data obtained earlier from lean hydrogen-air flames associated with corrugated flame (case A), thin reaction zone (case B), and broken reaction zone (case C) regimes of turbulent burning are analysed to directly assess capabilities of the flamelet approach to predict mean concentrations of species in a premixed turbulent flame. The approach consists in averaging dependencies of mole fractions, reaction rates, temperature, and density on a single combustion progress variable c, which are all obtained from the unperturbed laminar flame. For this purpose, four alternative definitions of c are probed and two probability density functions (PDFs) are adopted, i.e. either an actual PDF extracted directly from the DNS data or a presumed β-function PDF obtained using the DNS data on the first two moments of the c(x,t)-field. Results show that the mean density and mean mole fractions of H2, O2, and H2O are well predicted using both PDFs for each c, although the predictive capabilities are little worse in case C. In cases A and B, the use of the actual PDF and the fuel-based c also offers an opportunity to well predict mean mole fractions of O and H, whereas the mean mole fraction of OH is slightly underestimated. In the highly turbulent case C, the same approach performs worse, but still appears to be acceptable for evaluating the mean radical concentrations. The use of the β-function PDFs or another combustion progress variable yields substantially worse results for these radicals. When compared to the mean mole fractions, the mean rate of product creation, i.e. the source term in the transport equation for the mean combustion progress variable, is worse predicted even for a quantity (species concentration or temperature) adopted to define c and using the actual PDF. Consequently, turbulent burning velocity is not predicted either.
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| 44. |
- Lipatnikov, Andrei, 1961
(author)
-
A priori test of perfectly stirred reactor approach to evaluating mean fuel consumption and heat release rates in highly turbulent premixed f lames
- 2023
-
In: International Journal of Engine Research. - 1468-0874 .- 2041-3149. ; In Press
-
Journal article (peer-reviewed)abstract
- Unsteady three-dimensional Direct Numerical Simulation (DNS) data obtained earlier by Dave et al. (J Fluid Mech 2020; 884: A46) from a statistically planar and one-dimensional, highly turbulent, moderately lean hydrogen-air flame propagating in a box are processed to perform a priori test of perfectly stirred reactor model. The test aims in particularly at estimating mesh resolution (or filter width within large eddy simulation framework) required to neglect variations in the temperature and mixture composition within a computational cell when evaluating mean (or filtered) fuel consumption and heat release rates. For this purpose, fuel consumption and heat release rates sampled directly from the DNS data and averaged over a cube of width ∆ are compared with fuel consumption and heat release rates calculated using the temperature and species concentrations averaged over the same cube. Moreover, turbulent burning velocities computed by integrating the former and latter rates are compared with one another. A ratio of ∆ to a laminar flame thickness δL is varied from 0.44 to 1.8. The obtained results indicate that the tested simple approach performs reasonably well (poor) if ∆< 0.5δL (∆> δL , respectively). This result is further supported by directly filtering fuel consumption rate in a laminar premixed flame. The values of the thickness δL , calculated using detail chemical mechanisms for different fuels under elevated temperatures and pressures associated with combustion in piston engines, indicate that it is difficult to satisfy the constraint of ∆< 0.5δL in contemporary unsteady multidimensional numerical simulations of turbulent burning in such engines.
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| 45. |
- Lipatnikov, Andrei, 1961
(author)
-
A priori test of perfectly stirred reactor approach to evaluating mean fuel consumption and heat release rates in highly turbulent premixed flames
- 2023
-
In: International Journal of Engine Research. - 1468-0874 .- 2041-3149. ; 24:9, s. 4034-4043
-
Journal article (peer-reviewed)abstract
- Unsteady three-dimensional Direct Numerical Simulation (DNS) data obtained earlier by Dave et al. (J Fluid Mech 2020; 884: A46) from a statistically planar and one-dimensional, highly turbulent, moderately lean hydrogen-air flame propagating in a box are processed to perform a priori test of perfectly stirred reactor model. The test aims in particularly at estimating mesh resolution (or filter width within large eddy simulation framework) required to neglect variations in the temperature and mixture composition within a computational cell when evaluating mean (or filtered) fuel consumption and heat release rates. For this purpose, fuel consumption and heat release rates sampled directly from the DNS data and averaged over a cube of width (Formula presented.) are compared with fuel consumption and heat release rates calculated using the temperature and species concentrations averaged over the same cube. Moreover, turbulent burning velocities computed by integrating the former and latter rates are compared with one another. A ratio of (Formula presented.) to a laminar flame thickness (Formula presented.) is varied from 0.44 to 1.8. The obtained results indicate that the tested simple approach performs reasonably well (poor) if (Formula presented.) ((Formula presented.), respectively). This result is further supported by directly filtering fuel consumption rate in a laminar premixed flame. The values of the thickness (Formula presented.), calculated using detail chemical mechanisms for different fuels under elevated temperatures and pressures associated with combustion in piston engines, indicate that it is difficult to satisfy the constraint of (Formula presented.) in contemporary unsteady multidimensional numerical simulations of turbulent burning in such engines.
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| 46. |
- Lipatnikov, Andrei, 1961, et al.
(author)
-
A Semi-Detailed Chemical Mechanism for Gasoline: Further Validation Against Experimental Data on Laminar flame Speeds of Gasoline/O2/N2 and n-Heptane/iso-Octane/Air Mixtures
- 2012
-
In: 34th International Symposium on Combustion, Warsawe University of Technology, Warsaw, Poland, 29 Jule - 3 August 2012. Abstracts of Work-in-Progress Poster Presentations. The Combustion Institute, Pittsburgh, 2012. File W5P079.pdf on Flash Disk. ; , s. 1-
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Conference paper (other academic/artistic)
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| 47. |
- Lipatnikov, Andrei, 1961, et al.
(author)
-
A Simple Test of Presumed PDF Approach to Modeling Premixed Turbulent Flames
- 2012
-
In: 34th International Symposium on Combustion, Warsawe University of Technology, Warsaw, Poland, 29 Jule - 3 August 2012. Abstracts of Work-in-Progress Poster Presentations. The Combustion Institute, Pittsburgh, 2012. File W4P082.pdf on Flash Disk..
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Conference paper (other academic/artistic)
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| 48. |
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| 49. |
- Lipatnikov, Andrei, 1961, et al.
(author)
-
A study of the effects of pressure-driven transport on developing turbulent flame structure and propagation
- 2004
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In: Combustion Theory and Modelling. ; 8(2), s. 211-225
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Journal article (peer-reviewed)abstract
- Properties of solutions of a generalized normalized balance equation for the Favre-averaged combustion progress variable are numerically studied in the simplest case of a statistically planar, one-dimensional, stationary and uniform flow of unburned mixture. The focus is placed on the dependence of the solutions on pressure-driven transport for several closures of the mean rate of product creation. The results show that:(1) The flame structure is self-similar if the pressure-driven transport is sufficiently strong, but the self-similarity can be also obtained even for zero pressure-driven transport by using a particular closure of the mean rate of product creation;(2) Both burning velocity and flame thickness decrease if the pressure-driven transport increases and this effect can be reduced to a decrease in the asymptotically fully-developed quantities. An analysis of a more general progress variable balance equation, performed by invoking the sole assumption of the self-similarity of the flame structure, quantitatively confirms many numerical results, in particular,(1) the profile of the progress variable,(2) the scaling of the asymptotically fully-developed flame brush thickness and burning velocity, and(3) the development of the flame brush thickness and burning velocity in the cases of weak and strong pressure-driven transport. The analysis straightforwardly shows that the above general balance equation may be reduced to the Zimont equation with modified diffusivity provided that the flame structure is self-similar.
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- Lipatnikov, Andrei, 1961
(author)
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A test of conditioned balance equation approach
- 2011
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33:1, s. 1497-1504
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Journal article (peer-reviewed)abstract
- To model the effect of a premixed flame on turbulent scalar transport, new balance equations for mass and velocities conditioned either on unburned or burned mixture were recently derived by the author. The goal of this work is to test the equations by simulating various measurements of countergradient scalar transport in premixed turbulent flames stabilized in impinging jets. To do so, standard balance equations for (i) mass, (ii) the Favre-averaged combustion progress variable, and (iii) the Favre-averaged axial velocity and new balance equations for (iv) the difference between axial velocities conditioned on unburned and burned mixture and for radial velocities conditioned on (v) unburned and (vi) burned mixture were numerically solved. To place the focus of testing on the conditioned equations, some model parameters were adjusted in order for the computed axial profiles of the mean combustion progress variable and the mean axial velocity to agree well with experimental data. In contrast, the model parameters directly relevant to the conditioned equations (iv)-(vi) were neither varied nor tuned. The computed axial profiles of the axial conditioned velocities and axial scalar flux agree well with experimental data, thus indicating that the conditioned balance equation approach is a promising tool. The numerical results also show that: (i) in the front zone of the flames studied here, countergradient scalar transport is mainly caused by the pressure drop across flamelets, while (ii) the mean pressure gradient plays a more important role in the middle of the flame brush, and (iii) the magnitude of countergradient scalar flux is limited by chemical reactions in flamelets. © 2010 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.
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