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Nilanjan Chakraborty - One of the best experts on this subject based on the ideXlab platform.
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Vorticity budgets in premixed combusting turbulent flows at different Lewis Numbers
Physics of Fluids, 2017Co-Authors: César Dopazo, Luis Cifuentes, Nilanjan ChakrabortyAbstract:A direct numerical simulations database of statistically planar turbulent premixed flames using a simple Arrhenius type irreversible chemistry for different values of global Lewis Numbers, Le, (0.34, 0.60, 0.80, 1.00, 1.20) has been examined to analyze the effects of Le on vorticity transport within the flame. To meet this objective, a general enstrophy conservation equation has been considered, which distinctly describes contributions from vortex-stretching, destruction by volumetric dilatation rates, baroclinic and viscous force torques, viscous transport, and dissipation. The average statistical behavior of the various contributions conditioned upon the value of the reaction progress variable, c, has been analyzed in the preheat and reacting regions of the flame. The mean values of enstrophy monotonically decays with c from fresh reactants toward hot products for Le equal to 0.8, 1.0, and 1.2; vortex-stretching and viscous dissipation are the leading contributors, while the remaining contributions are ...
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Streamline segment statistics of premixed flames with nonunity Lewis Numbers.
Physical review. E Statistical nonlinear and soft matter physics, 2014Co-Authors: Nilanjan Chakraborty, Lipo Wang, Markus KleinAbstract:The interaction of flame and surrounding fluid motion is of central importance in the fundamental understanding of turbulent combustion. It is demonstrated here that this interaction can be represented using streamline segment analysis, which was previously applied in nonreactive turbulence. The present work focuses on the effects of the global Lewis number (Le) on streamline segment statistics in premixed flames in the thin-reaction-zones regime. A direct numerical simulation database of freely propagating thin-reaction-zones regime flames with Le ranging from $0.34$ to $1.2$ is used to demonstrate that Le has significant influences on the characteristic features of the streamline segment, such as the curve length, the difference in the velocity magnitude at two extremal points, and their correlations with the local flame curvature. The strengthenings of the dilatation rate, flame normal acceleration, and flame-generated turbulence with decreasing Le are principally responsible for these observed effects. An expression for the probability density function (pdf) of the streamline segment length, originally developed for nonreacting turbulent flows, captures the qualitative behavior for turbulent premixed flames in the thin-reaction-zones regime for a wide range of Le values. The joint pdfs between the streamline length and the difference in the velocity magnitude at two extremal points for both unweighted and density-weighted velocity vectors are analyzed and compared. Detailed explanations are provided for the observed differences in the topological behaviors of the streamline segment in response to the global Le.
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effects of Lewis number on flame surface density transport in turbulent premixed combustion
Combustion and Flame, 2011Co-Authors: Nilanjan Chakraborty, R S CantAbstract:Abstract The transport of flame surface density (FSD) in turbulent premixed flames has been studied using a database obtained from Direct Numerical Simulation (DNS). Three-dimensional freely propagating developing statistically planar turbulent premixed flames have been examined over a range of global Lewis Numbers from 0.6 to 1.2. Simplified chemistry has been used and the emphasis is on the effects of Lewis number on FSD transport in the context of Reynolds-averaged closure modelling. Under the same initial conditions of turbulence, flames with low Lewis Numbers are found to exhibit counter-gradient transport of FSD, whereas flames with higher Lewis Numbers tend to exhibit gradient transport of FSD. Stronger heat release effects for lower Lewis number flames are found to lead to an increase in the positive (negative) value of the dilatation rate (normal strain rate) term in the FSD transport equation with decreasing Lewis number. The contribution of flame curvature to FSD transport is found to be influenced significantly by the effects of Lewis number on the curvature dependence of the magnitude of the reaction progress variable gradient, and on the combined reaction and normal diffusion components of displacement speed. The modelling of the various terms of the FSD transport equation has been analysed in detail and the performance of existing models is assessed with respect to the terms assembled from corresponding quantities extracted from DNS data. Based on this assessment, suitable models are identified which are able to address the effects of non-unity Lewis number on FSD transport, and new or modified models are suggested wherever necessary.
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effects of Lewis number on turbulent scalar transport and its modelling in turbulent premixed flames
Combustion and Flame, 2009Co-Authors: Nilanjan Chakraborty, R S CantAbstract:Abstract The behaviour of the turbulent scalar flux in premixed flames has been studied using Direct Numerical Simulation (DNS) with emphasis on the effects of Lewis number in the context of Reynolds-averaged closure modelling. A database was obtained from DNS of three-dimensional freely propagating statistically planar turbulent premixed flames with simplified chemistry and a range of global Lewis Numbers from 0.34 to 1.2. Under the same initial conditions of turbulence, flames with low Lewis Numbers are found to exhibit counter-gradient transport, whereas flames with higher Lewis Numbers tend to exhibit gradient transport. The Reynolds-averaged transport equation for the turbulent scalar flux is analysed in detail and the performance of existing models for the unclosed terms is assessed with respect to corresponding quantities extracted from DNS data. Based on this assessment, existing models which are able to address the effects of non-unity Lewis number on turbulent scalar flux transport are identified, and new or modified models are suggested wherever necessary. In this way, a complete set of closure models for the scalar flux transport equation is prescribed for use in Reynolds-Averaged Navier–Stokes simulations.
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effects of Lewis number on scalar transport in turbulent premixed flames
Physics of Fluids, 2009Co-Authors: Nilanjan Chakraborty, R S CantAbstract:The effects of global Lewis number on scalar transport in turbulent premixed flames are studied using direct numerical simulation. Under the same initial conditions of turbulent flow field, it is observed that flames with global Lewis Numbers significantly smaller than unity tend to exhibit countergradient transport, whereas the extent of gradient transport is shown to increase with increasing global Lewis Numbers. The velocity difference between reactants and products in the flame normal direction is shown to be significantly affected by the global Lewis number. The flame normal acceleration is shown to increase with decreasing Lewis number, leading to an increase in the magnitude of the mean pressure gradient in the mean direction of flame propagation. This effect is shown to be is responsible for promoting countergradient transport in low Lewis number flames. It is also shown that turbulent transport of flame surface density tends to exhibit countergradient behavior for the flames with Lewis number sig...
R S Cant - One of the best experts on this subject based on the ideXlab platform.
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effects of Lewis number on flame surface density transport in turbulent premixed combustion
Combustion and Flame, 2011Co-Authors: Nilanjan Chakraborty, R S CantAbstract:Abstract The transport of flame surface density (FSD) in turbulent premixed flames has been studied using a database obtained from Direct Numerical Simulation (DNS). Three-dimensional freely propagating developing statistically planar turbulent premixed flames have been examined over a range of global Lewis Numbers from 0.6 to 1.2. Simplified chemistry has been used and the emphasis is on the effects of Lewis number on FSD transport in the context of Reynolds-averaged closure modelling. Under the same initial conditions of turbulence, flames with low Lewis Numbers are found to exhibit counter-gradient transport of FSD, whereas flames with higher Lewis Numbers tend to exhibit gradient transport of FSD. Stronger heat release effects for lower Lewis number flames are found to lead to an increase in the positive (negative) value of the dilatation rate (normal strain rate) term in the FSD transport equation with decreasing Lewis number. The contribution of flame curvature to FSD transport is found to be influenced significantly by the effects of Lewis number on the curvature dependence of the magnitude of the reaction progress variable gradient, and on the combined reaction and normal diffusion components of displacement speed. The modelling of the various terms of the FSD transport equation has been analysed in detail and the performance of existing models is assessed with respect to the terms assembled from corresponding quantities extracted from DNS data. Based on this assessment, suitable models are identified which are able to address the effects of non-unity Lewis number on FSD transport, and new or modified models are suggested wherever necessary.
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effects of Lewis number on turbulent scalar transport and its modelling in turbulent premixed flames
Combustion and Flame, 2009Co-Authors: Nilanjan Chakraborty, R S CantAbstract:Abstract The behaviour of the turbulent scalar flux in premixed flames has been studied using Direct Numerical Simulation (DNS) with emphasis on the effects of Lewis number in the context of Reynolds-averaged closure modelling. A database was obtained from DNS of three-dimensional freely propagating statistically planar turbulent premixed flames with simplified chemistry and a range of global Lewis Numbers from 0.34 to 1.2. Under the same initial conditions of turbulence, flames with low Lewis Numbers are found to exhibit counter-gradient transport, whereas flames with higher Lewis Numbers tend to exhibit gradient transport. The Reynolds-averaged transport equation for the turbulent scalar flux is analysed in detail and the performance of existing models for the unclosed terms is assessed with respect to corresponding quantities extracted from DNS data. Based on this assessment, existing models which are able to address the effects of non-unity Lewis number on turbulent scalar flux transport are identified, and new or modified models are suggested wherever necessary. In this way, a complete set of closure models for the scalar flux transport equation is prescribed for use in Reynolds-Averaged Navier–Stokes simulations.
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effects of Lewis number on scalar transport in turbulent premixed flames
Physics of Fluids, 2009Co-Authors: Nilanjan Chakraborty, R S CantAbstract:The effects of global Lewis number on scalar transport in turbulent premixed flames are studied using direct numerical simulation. Under the same initial conditions of turbulent flow field, it is observed that flames with global Lewis Numbers significantly smaller than unity tend to exhibit countergradient transport, whereas the extent of gradient transport is shown to increase with increasing global Lewis Numbers. The velocity difference between reactants and products in the flame normal direction is shown to be significantly affected by the global Lewis number. The flame normal acceleration is shown to increase with decreasing Lewis number, leading to an increase in the magnitude of the mean pressure gradient in the mean direction of flame propagation. This effect is shown to be is responsible for promoting countergradient transport in low Lewis number flames. It is also shown that turbulent transport of flame surface density tends to exhibit countergradient behavior for the flames with Lewis number sig...
D. A. Santavicca - One of the best experts on this subject based on the ideXlab platform.
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surface properties of turbulent premixed propane air flames at various Lewis Numbers
Combustion and Flame, 1993Co-Authors: T.-w. Lee, G.l. North, D. A. SantaviccaAbstract:Abstract Surface properties of turbulent premixed flames including the wrinkled flame perimeter, fraction of the flame pocket perimeter, flame curvature, and orientation distributions have been measured for propane-air flames at Lewis Numbers ranging from 0.98 to 1.86 and u′ S L = 1.42–5.71 . The wrinkled flame perimeter is found to be greater for the thermodiffusively unstable Lewis number (Le u′ S L tested, and show a much stronger dependence on the turbulence condition than on the Lewis number. Similarly, the flame orientation distributions show a trend from anisotropy toward a more uniform distribution with increasing u′ S L at a similar rate for all Lewis Numbers. Thus, for turbulent premixed propane/air flames for a practical range of Lewis number from 0.98 to 1.86, the effect of Lewis number is primarily to affect the flame structures and thereby flame surface areas and flame pocket areas, while the flame curvature and orientation statistics are essentially determined by the turbulence properties.
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Surface properties of turbulent premixed propane/air flames at various Lewis Numbers☆
Combustion and Flame, 1993Co-Authors: T.-w. Lee, G.l. North, D. A. SantaviccaAbstract:Abstract Surface properties of turbulent premixed flames including the wrinkled flame perimeter, fraction of the flame pocket perimeter, flame curvature, and orientation distributions have been measured for propane-air flames at Lewis Numbers ranging from 0.98 to 1.86 and u′ S L = 1.42–5.71 . The wrinkled flame perimeter is found to be greater for the thermodiffusively unstable Lewis number (Le u′ S L tested, and show a much stronger dependence on the turbulence condition than on the Lewis number. Similarly, the flame orientation distributions show a trend from anisotropy toward a more uniform distribution with increasing u′ S L at a similar rate for all Lewis Numbers. Thus, for turbulent premixed propane/air flames for a practical range of Lewis number from 0.98 to 1.86, the effect of Lewis number is primarily to affect the flame structures and thereby flame surface areas and flame pocket areas, while the flame curvature and orientation statistics are essentially determined by the turbulence properties.
Satoshi Kadowaki - One of the best experts on this subject based on the ideXlab platform.
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The effects of radiation on the dynamic behavior of cellular premixed flames generated by intrinsic instability
Proceedings of the Combustion Institute, 2011Co-Authors: Satoshi Kadowaki, Hidekazu Takahashi, Hideaki KobayashiAbstract:Abstract Two-dimensional unsteady calculations of reactive flow on the basis of the compressible Navier–Stokes equation were performed to elucidate the effects of radiation on the dynamic behavior of cellular premixed flames generated by intrinsic instability. The disturbance superimposed on a planar flame evolved owing to hydrodynamic and diffusive-thermal effects, and then the cellular flame front formed. The dynamic behavior of cellular flames appeared at low Lewis Numbers, and it became stronger as radiative heat loss increased. The island of the unburned gas surrounded by the burned gas was observed in non-adiabatic flames with low Lewis Numbers. The average cell size of the non-adiabatic flame was slightly small compared with the adiabatic flame, even though the critical wavelength of the former flame was larger than that of the latter flame. This indicates that the radiation has a pronounced influence on the dynamics of premixed flames with low Lewis Numbers. Owing to the dynamic behavior, the burning velocity of cellular flames changed drastically with time, which was due principally to the combination and division of cells. The average burning velocity of the non-adiabatic cellular flame was somewhat small compared with the adiabatic cellular flame. This is because that the burning velocity of planar flames decreases owing to radiation and that the dynamic behavior of cellular flames becomes stronger. In addition, the average burning velocity became larger monotonically as the length of computational domain increased. The reason is that the long wavelength components of disturbances play a significant role in the front shape of cellular flames.
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Numerical simulation on the instability of cylindrically expanding premixed flames with radiative heat loss at low Lewis Numbers
ASME JSME 2011 8th Thermal Engineering Joint Conference, 2011Co-Authors: Takafumi Kusakai, Satoshi KadowakiAbstract:The instability of cylindrically expanding premixed flames with radiative heat loss was studied by two-dimensional unsteady calculations of reactive gases, based on the diffusive-thermal model equation. When the Lewis number was unity, instability phenomena were not observed. When the Lewis number was sufficiently low, on the other hand, cellular-shaped fronts on adiabatic and non-adiabatic cylindrical flames were observed, which was due to diffusive-thermal instability. As radiative heat loss increased, the behavior of cellular fronts became more unstable. This indicated that the radiation promoted the unstable behavior of flame fronts at low Lewis Numbers. When radiative heat loss was much large compared with the quenching condition of a planar flame, cylindrical flames were broken up and several small flames appeared. This was in qualitative agreement with the experimental results on the dynamic behavior of lean hydrogen-air premixed flames with radiative heat loss under the low gravity condition. Several small flames appeared on the grounds that large curvature of flame fronts was necessary to keep high temperature against radiative heat loss.Copyright © 2011 by ASME
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Body-force effect on the lateral movement of cellular flames at low Lewis Numbers.
Physical review. E Statistical nonlinear and soft matter physics, 2001Co-Authors: Satoshi KadowakiAbstract:The body-force effect on the lateral movement of cellular flames is studied by unsteady calculations of reactive flows at low Lewis Numbers. We employ the compressible Navier-Stokes equation including chemical reaction to take account of the hydrodynamic effect caused by thermal expansion. A sinusoidal disturbance with the linearly most unstable wavelength is superimposed on a plane flame to simulate the formation of a cellular flame. The superimposed disturbance grows initially with time, and then the flame front changes from a sinusoidal to a cellular shape. After the cell formation, the cellular flame moves laterally at Lewis Numbers lower than unity. The reason is that the diffusive-thermal effect, and the nonlinear effect of the flame front, play a primary role in the appearance of the lateral movement of cells. The body-force effect has a great influence on the lateral velocity of cells. When flames are propagated upward, the lateral velocity decreases as the acceleration increases, even though the body-force effect has a destabilizing influence. When flames are propagated downward, on the other hand, the lateral velocity takes a maximum value at the specific acceleration and decreases with an increase in acceleration. The dependence of lateral velocity on the acceleration is due to the augmentation and diminution in maximum flame temperature and to the broadness and narrowness of a high-temperature region behind a convex flame front.
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Flame velocity of cellular flames at low Lewis Numbers
Combustion Science and Technology, 2001Co-Authors: Satoshi KadowakiAbstract:The flame velocity of cellular flames at low Lewis Numbers is numerically studied, based on the compressible Navier-Stokes equation including a one-step chemical reaction. The flame velocity of a cellular flame is always larger than that of a plane flame and increases as the Lewis number becomes lower. When the Lewis number is unity, the flame velocity is proportional to the surface area. When the Lewis number is lower than unity, on the other hand, the increment of the flame velocity is greater than that of the surface area. The local flame velocity increases (decreases) at a convex (concave) flame front with respect to the unburned gas. The increase in the flame velocity at a convex flame front exceeds the decrease at a concave one, which is due to the Arrhenius nonlinearity. Thus, the flame-velocity increment is greater than the surface-area increment at Lewis Numbers lower than unity.
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numerical study on the formation of cellular premixed flames at high Lewis Numbers
Physics of Fluids, 2000Co-Authors: Satoshi KadowakiAbstract:Intrinsic instability and cell formation of premixed flames at high Lewis Numbers (Le=1.0–3.0) are studied by two-dimensional, unsteady calculations of reactive flows. The relation between the growth rate and the wave number, i.e., the dispersion relation, is obtained to study intrinsic instability due to hydrodynamic and diffusive-thermal effects. The growth rate is positive at small wave Numbers, and the marginal wave number separating stable and unstable ranges is found. The growth rate decreases and the unstable range narrows as the Lewis number becomes higher, since the diffusive-thermal effect has a stabilizing influence at Lewis Numbers higher than unity. Positive growth rates caused by the hydrodynamic effect form a cellular flame. To study the formation of cellular flames, the disturbance with the linearly most unstable wave number is superimposed on a plane flame. The superimposed disturbance evolves, and eventually a cellular flame front is formed. The higher the Lewis number, the greater the cell size and cell depth. In addition, a stationary cellular flame is obtained when the inlet velocity of the unburned gas is set to the flame velocity, since cells on flame do not move laterally.
T.-w. Lee - One of the best experts on this subject based on the ideXlab platform.
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surface properties of turbulent premixed propane air flames at various Lewis Numbers
Combustion and Flame, 1993Co-Authors: T.-w. Lee, G.l. North, D. A. SantaviccaAbstract:Abstract Surface properties of turbulent premixed flames including the wrinkled flame perimeter, fraction of the flame pocket perimeter, flame curvature, and orientation distributions have been measured for propane-air flames at Lewis Numbers ranging from 0.98 to 1.86 and u′ S L = 1.42–5.71 . The wrinkled flame perimeter is found to be greater for the thermodiffusively unstable Lewis number (Le u′ S L tested, and show a much stronger dependence on the turbulence condition than on the Lewis number. Similarly, the flame orientation distributions show a trend from anisotropy toward a more uniform distribution with increasing u′ S L at a similar rate for all Lewis Numbers. Thus, for turbulent premixed propane/air flames for a practical range of Lewis number from 0.98 to 1.86, the effect of Lewis number is primarily to affect the flame structures and thereby flame surface areas and flame pocket areas, while the flame curvature and orientation statistics are essentially determined by the turbulence properties.
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Surface properties of turbulent premixed propane/air flames at various Lewis Numbers☆
Combustion and Flame, 1993Co-Authors: T.-w. Lee, G.l. North, D. A. SantaviccaAbstract:Abstract Surface properties of turbulent premixed flames including the wrinkled flame perimeter, fraction of the flame pocket perimeter, flame curvature, and orientation distributions have been measured for propane-air flames at Lewis Numbers ranging from 0.98 to 1.86 and u′ S L = 1.42–5.71 . The wrinkled flame perimeter is found to be greater for the thermodiffusively unstable Lewis number (Le u′ S L tested, and show a much stronger dependence on the turbulence condition than on the Lewis number. Similarly, the flame orientation distributions show a trend from anisotropy toward a more uniform distribution with increasing u′ S L at a similar rate for all Lewis Numbers. Thus, for turbulent premixed propane/air flames for a practical range of Lewis number from 0.98 to 1.86, the effect of Lewis number is primarily to affect the flame structures and thereby flame surface areas and flame pocket areas, while the flame curvature and orientation statistics are essentially determined by the turbulence properties.
