| 1. |
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| 2. |
- Brearley, P, et al.
(författare)
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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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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 31:11
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Tidskriftsartikel (refereegranskat)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.
(författare)
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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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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 34:1, s. 1333-1345
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Tidskriftsartikel (refereegranskat)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.
(författare)
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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
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 25:4, s. 045101-
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Effects of Lewis number on vorticity and enstrophy transport in turbulent premixed flames
- 2016
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 28:1, s. 015109-
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Tidskriftsartikel (refereegranskat)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.
(författare)
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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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Ingår i: Journal of Combustion. - : Hindawi Limited. - 2090-1976 .- 2090-1968. ; 2011:Article ID 628208
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Simple criterion of importance of laminar flame instabilities in premixed turbulent combustion of mixtures characterized by low Lewis numbers
- 2023
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Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 107:1
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Turbulent diffusion of chemically reacting flows : Theory and numerical simulations
- 2017
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Ingår i: Physical review. E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 96:5
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Investigation of Charge Mixing and Stratified Fuel Distribution in a DISI Engine Using Rayleigh Scattering and Numerical Simulations
- 2022
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Ingår i: Mechanisms and Machine Science. - Cham : Springer International Publishing. - 2211-0992 .- 2211-0984. ; 114, s. 87-206
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Bokkapitel (övrigt vetenskapligt/konstnärligt)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.
(författare)
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A Chemical Model of Gasoline/Ethanol Blends
- 2009
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Ingår i: 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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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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