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Nilanjan Chakraborty - One of the best experts on this subject based on the ideXlab platform.

  • surface density function evolution and the influence of Strain rates during turbulent boundary layer flashback of hydrogen rich premixed combustion
    Physics of Fluids, 2020
    Co-Authors: Umair Ahmed, Nilanjan Chakraborty, Abhishek L Pillai, Ryoichi Kurose
    Abstract:

    The statistical behavior of the magnitude of the reaction progress variable gradient [alternatively known as the surface density function (SDF)] and the Strain rates, which govern the evolution of the SDF, have been analyzed for boundary layer flashback of a premixed hydrogen-air flame with an equivalence ratio of 1.5 in a fully developed turbulent channel flow. The non-reacting part of the channel flow is representative of the friction velocity based Reynolds number Reτ = 120. A skeletal chemical mechanism with nine chemical species and twenty reactions is employed to represent hydrogen-air combustion. Three definitions of the reaction progress variable (RPV) based on the mass fractions of H2, O2, and H2O have been considered to analyze the SDF statistics. It is found that the mean variations of the SDF and the displacement speed Sd depend on the choice of the RPV and the distance away from the wall. The preferential alignment of the RPV gradient with the most extensive principal Strain rate strengthens with an increase in distance from the cold wall, which leads to changes in the behaviors of normal and Tangential Strain rates from the vicinity of the wall toward the middle of the channel. The differences in displacement speed statistics for different choices of the RPV and the wall distance affect the behaviors of the normal Strain rate due to flame propagation and curvature stretch. The relative thickening/thinning of the reaction layers of the major species has been explained in terms of the statistics of the effective normal Strain rate experienced by the progress variable isosurfaces for different wall distances and choices of RPVs.

  • influence of the lewis number on effective Strain rates in weakly turbulent premixed combustion
    Combustion Science and Technology, 2018
    Co-Authors: Cesar Dopazo, Luis Cifuentes, D Alwazzan, Nilanjan Chakraborty
    Abstract:

    ABSTRACTThe influence of the global Lewis number, Le, on the statistical behavior of the “effective” normal and Tangential Strain rates have been analyzed based on three-dimensional direct numerical simulation data of freely propagating statistically planar turbulent premixed flames with Le = 0.34, 0.60, 0.80, 1.00, and 1.20. The volumetric dilatation rate is found to be mostly positive and its magnitude increases with decreasing Le. The flow normal Strain rate predominantly assumes positive values and thus tends to pull adjacent iso-scalar surfaces apart, which reduces scalar gradients. By contrast, the “added” normal Strain rate due to derivatives of the displacement speed normal to iso-surfaces has the propensity to push them closer together, and therefore increase the magnitude of scalar gradients. The balance between flow and added normal Strain rates along with the advective transport determines whether scalar gradients are enhanced or destroyed. Iso-surface elementary area stretching by the fluid f...

  • modelling of the Tangential Strain rate term in the flame surface density transport equation in the context of reynolds averaged navier stokes simulations a direct numerical simulation analysis
    Mathematical Problems in Engineering, 2014
    Co-Authors: Mohit Katragadda, Sean P Malkeson, Nilanjan Chakraborty
    Abstract:

    A direct numerical simulation (DNS) database of freely propagating statistically planar turbulent premixed flames with a range of different values of Karlovitz number Ka, turbulent Reynolds number , heat release parameter , and global Lewis number Le has been used to assess the models of the Tangential Strain rate term in the generalised flame surface density (FSD) transport equation in the context of Reynolds averaged Navier Stokes (RANS) simulations. The Tangential Strain rate term has been split into contributions arising due to dilatation rate and flame normal Strain rate (). Subsequently, and () were split into their resolved (i.e., and ()) and unresolved ( and ()) components. Detailed physical explanations have been provided for the observed behaviours of the components of the Tangential Strain rate term. This analysis gave way to the modelling of the unresolved dilatation rate and flame normal Strain rate contributions. Models have been identified for and () for RANS simulations, which are shown to perform satisfactorily in all cases considered, accounting for the variations in Ka, , and Le. The performance of the newly proposed models for the FSD Strain rate term have been found to be either comparable to or better than the existing models.

  • turbulent reynolds number dependence of flame surface density transport in the context of reynolds averaged navier stokes simulations
    Proceedings of the Combustion Institute, 2013
    Co-Authors: Nilanjan Chakraborty, R S Cant
    Abstract:

    Abstract The influence of turbulent Reynolds number Re t on the statistical behaviour and modelling of flame surface density (FSD) transport has been analysed using a direct numerical simulation (DNS) database of freely propagating turbulent premixed flames with values of Re t ranging from 20 to 100. The variation of Re t is brought about by modifying the Damkohler and Karlovitz numbers independently of each other. The findings indicate that the qualitative behaviour of the various terms of the FSD transport equation remains unaffected by the changes in turbulent Reynolds number, but their relative contributions to the transport of FSD are affected to some extent. The effects of turbulent Reynolds number on the modelling of the turbulent transport term, the Tangential Strain rate term and the combined propagation and curvature terms are addressed in detail. It is demonstrated that model parameters for the turbulent transport and Tangential Strain rate term exhibit turbulent Reynolds number dependence for small values of Re t and assume asymptotic values for Re t ⩾ 50 . By contrast, the model parameters for the combined propagation and curvature term are found to be insensitive to the variation of turbulent Reynolds number. Existing models for the turbulent transport term and the Tangential Strain rate term are modified to account for the observed turbulent Reynolds number dependence.

  • modelling of the Tangential Strain rate term of the flame surface density transport equation in the context of reynolds averaged navier stokes simulation
    Proceedings of the Combustion Institute, 2011
    Co-Authors: Mohit Katragadda, Sean P Malkeson, Nilanjan Chakraborty
    Abstract:

    Abstract The modelling of the Tangential Strain rate term in the Flame Surface Density (FSD) transport equation in the context of Reynolds Averaged Navier–Stokes (RANS) simulations of turbulent premixed combustion has been addressed by a priori analysis of a Direct Numerical Simulation (DNS) database of statistically planar freely propagating flames with wide variations of Damkohler number Da , heat release parameter τ and global Lewis number Le . It has been found that the dilatation rate contribution to the FSD transport strengthens with increasing value of τ and decreasing value of Le . The behaviour of the normal Strain rate term shows significant differences in response to Da and Le . It has been found that the normal Strain rate contribution to the FSD transport remains a sink term for the flames with high and small values of Da and Le , respectively, where the effects of Strain rate induced by heat release a chem overcome the effects of turbulent Straining a turb . By contrast, the effects of a turb overcomes the effects of a chem for low Da flames with Le ⩾ 1 , which leads to a positive value of the normal Strain rate term towards the unburned gas side, but this term becomes negative towards the burned gas side due to strong a chem overcoming a turb in the regions of intense heat release. The strengthening of the dilatation rate and a chem at small and large values of Le and Da , respectively, is explicitly taken into account to propose new models for the Strain rate contributions to the FSD transport. The new model is shown to satisfactorily capture the effects of Damkohler number Da , heat release parameter τ , and global Lewis number Le , on the Tangential Strain rate term of the FSD transport equation for all the cases considered in this study.

Fabrizio Bisetti - One of the best experts on this subject based on the ideXlab platform.

  • on the statistics of flame stretch in turbulent premixed jet flames in the thin reaction zone regime at varying reynolds number
    Proceedings of the Combustion Institute, 2019
    Co-Authors: Stefano Luca, Antonio Attili, Ermanno Lo Schiavo, Francesco Creta, Fabrizio Bisetti
    Abstract:

    Abstract Direct Numerical Simulations (DNS) are conducted to study the statistics of flame surface stretch in turbulent jet premixed flames. Emphasis is placed on the rates of surface production and destruction and their scaling with the Reynolds number. Four lean methane/air turbulent slot jet flames are simulated at increasing Reynolds number and up to Re ≈ 22 × 103, based on the bulk velocity, slot width, and the reactants’ properties. The Karlovitz number is held approximately constant and the flames fall in the thin reaction zone regime. The simulations feature finite rate chemistry and mixture-average transport. Our data indicate that the area of the flame surface increases up to the streamwise position corresponding to 80% of the average flame length and decreases afterwards as surface destruction overtakes production. It is observed that the Tangential rate of Strain is responsible for the production of flame surface in the mean and surface destruction is due to the curvature term. In addition, it is found that these two terms are both significantly larger than their difference, i.e., the net surface stretch.The statistics of the Tangential Strain rate are in good agreement with those for infinitesimal material surfaces in homogeneous isotropic turbulence. Once scaled by the Kolmogorov time scale, the means of both contributions to stretch are largely independent of location and equal across flames with different values of the Reynolds number. Surface destruction is due mostly to propagation into the reactants where the surface is folded into a cylindrical shape with the center of curvature on the side of the reactants. The joint statistics of the displacement speed and curvature of the reactive surface are nuanced, with the most probable occurrence being that of a negative displacement speed of a flat surface, while the surface averaged displacement speed is positive as expected.

Jacqueline H Chen - One of the best experts on this subject based on the ideXlab platform.

  • preferential diffusion effects on the burning rate of interacting turbulent premixed hydrogen air flames
    Combustion and Flame, 2002
    Co-Authors: Jacqueline H Chen
    Abstract:

    Abstract The upstream interaction of twin premixed hydrogen-air flames in 2-D turbulence is studied using direct numerical simulations with detailed chemistry. The primary objective is to determine the effect of flame stretch on the overall burning rate during various stages of the interaction. Preferential diffusion effects are accounted for by varying the equivalence ratio from symmetric rich-rich to lean-lean interactions. The results show that the local flame front response to turbulence is consistent with previous understanding of laminar premixed flames, in that rich premixed flames become intensified in regions of negative Strain or curvature, while the opposite response is found for lean premixed flames. The overall burning rate history with respect to the surface density variation is found to depend on the mixture condition; the consumption rate enhancement advances (follows) the surface enhancement for the rich-rich (lean-lean) case. For the lean-lean case, a self-turbulization mechanism results in a large positive skewness in the area-weighted mean Tangential Strain statistics. Because of the statistical dominance of positive stretch on the flame surface, the lean-lean case results in a significantly larger burning enhancement (over a twofold increase) in addition to the surface density production. For the case of rich-rich interaction, the abundance in hydrogen species results in an instantaneous overshoot of the radical pool in the post-flame region, resulting in an additional “burst” in the reactant consumption rate history, suggesting its potential impact on the pollutant formation process.

  • correlation of flame speed with stretch in turbulent premixed methane air flames
    27. international symposium on combustion Boulder CO (United States) 2-7 Aug 1998, 1998
    Co-Authors: Jacqueline H Chen
    Abstract:

    Direct numerical simulations of two-dimensional unsteady premixed methane/air flames are performed to determine the correlation of flame speed with stretch over a wide range of curvatures and Strain rates generated by intense two-dimensional turbulence. Lean and stoichiometric premixtures are considered with a detailed C{sub 1}-mechanism for methane oxidation. The computed correlation shows the existence of two distinct stable branches. It further shows that exceedingly large negative values of stretch can be obtained solely through curvature effects which give rise to an overall nonlinear correlation of the flame speed with stretch. Over a narrower stretch range, {minus}1 {le} Ka {le} 1, which includes 90% of the sample, the correlation is approximately linear, and hence, the asymptotic theory for stretch is practically applicable. Overall, one-third of the sample has negative stretch. In this linear range, the Markstein number associated with the positive branch is determined and is consistent with values obtained from comparable steady counterflow computations. In addition to this conventional positive branch, a negative branch is identified. This negative branch occurs when a flame cusp, with a center of curvature in the burnt gases, is subjected to intense compressive Strain, resulting in a negative displacement speed. Negative flame speeds are also encountered for extensive Tangential Strain rates exceeding a Karlovitz number of unity, a value consistent with steady counterflow computations.

  • correlation of flame speed with stretch in turbulent premixed methane air flames
    1997 fall technical meeting of the Eastern State Section of the Combustion Institute: chemical physical processes in combustion Hartford CT (United St, 1997
    Co-Authors: Jacqueline H Chen
    Abstract:

    In the flamelet approach of turbulent premixed combustion, the flames are modeled as a wrinkled surface whose propagation speed, termed the {open_quotes}displacement speed,{close_quotes} is prescribed in terms of the local flow field and flame geometry. Theoretical studies suggest a linear relation between the flame speed and stretch for small values of stretch, S{sub L}/S{sub L}{sup 0} = 1 - MaKa, where S{sub L}{sup 0} is the laminar flame speed, Ka = {kappa}{delta}{sub F}/S{sub L}{sup 0} is the nondimensional stretch or the Karlovitz number, and Ma = L/{delta}{sub F} is the Markstein number. The nominal flame thickness, {delta}{sub F}, is determined as the ratio of the mass diffusivity of the unburnt mixture to the laminar flame speed. Thus, the turbulent flame model relies on an accurate estimate of the Markstein number in specific flame configurations. Experimental measurement of flame speed and stretch in turbulent flames, however, is extremely difficult. As a result, measurement of flame speeds under Strained flow fields has been made in simpler geometries, in which the effect of flame curvature is often omitted. In this study we present results of direct numerical simulations of unsteady turbulent flames with detailed methane/air chemistry, thereby providing an alternative method of obtaining flame structure and propagation statistics. The objective is to determine the correlation between the displacement speed and stretch over a broad range of Karlovitz numbers. The observed response of the displacement speed is then interpreted in terms of local Tangential Strain rate and curvature effects. 13 refs., 3 figs.

R S Cant - One of the best experts on this subject based on the ideXlab platform.

  • turbulent reynolds number dependence of flame surface density transport in the context of reynolds averaged navier stokes simulations
    Proceedings of the Combustion Institute, 2013
    Co-Authors: Nilanjan Chakraborty, R S Cant
    Abstract:

    Abstract The influence of turbulent Reynolds number Re t on the statistical behaviour and modelling of flame surface density (FSD) transport has been analysed using a direct numerical simulation (DNS) database of freely propagating turbulent premixed flames with values of Re t ranging from 20 to 100. The variation of Re t is brought about by modifying the Damkohler and Karlovitz numbers independently of each other. The findings indicate that the qualitative behaviour of the various terms of the FSD transport equation remains unaffected by the changes in turbulent Reynolds number, but their relative contributions to the transport of FSD are affected to some extent. The effects of turbulent Reynolds number on the modelling of the turbulent transport term, the Tangential Strain rate term and the combined propagation and curvature terms are addressed in detail. It is demonstrated that model parameters for the turbulent transport and Tangential Strain rate term exhibit turbulent Reynolds number dependence for small values of Re t and assume asymptotic values for Re t ⩾ 50 . By contrast, the model parameters for the combined propagation and curvature term are found to be insensitive to the variation of turbulent Reynolds number. Existing models for the turbulent transport term and the Tangential Strain rate term are modified to account for the observed turbulent Reynolds number dependence.

  • influence of lewis number on Strain rate effects in turbulent premixed flame propagation
    International Journal of Heat and Mass Transfer, 2006
    Co-Authors: Nilanjan Chakraborty, R S Cant
    Abstract:

    The effects of Tangential Strain rate on the displacement speed of turbulent premixed flames in the thin reaction zones regime are studied for three different Lewis numbers (Le = 0.8, 1.0 and 1.2) using three-dimensional compressible direct numerical simulation (DNS) of statistically planar flames. For non-unity Lewis numbers it is shown that the variations of temperature and scalar gradient in response to Tangential Strain rate on a given reaction progress variable isosurface have a profound influence on displacement speed behaviour. In the case of Le = 0.8, temperature and Tangential Strain rate are found to be positively correlated at locations of zero curvature whereas the opposite behaviour is apparent for the case of Le = 1.2. It is demonstrated that the effects of the temperature-curvature and Tangential Strain rate-curvature correlations are implicitly present in the response of the temperature to local Strain rate. The temperature-curvature correlation and Strain rate-curvature correlation are found to be in agreement with previous experimental results. Displacement speed and Strain rate are found to be weakly correlated in general, but their conditional joint pdf at zero curvature locations shows a negative correlation.

  • influence of lewis number on curvature effects in turbulent premixed flame propagation in the thin reaction zones regime
    Physics of Fluids, 2005
    Co-Authors: Nilanjan Chakraborty, R S Cant
    Abstract:

    The influence of differential diffusion on the statistical behavior of the local displacement speed (Sd) in relation to flame curvature is studied based on three-dimensional compressible direct numerical simulations (DNS) of statistically planar flames with single-step Arrhenius-type chemistry. Three different Lewis number cases (Le=0.8, 1.0, and 1.2) are considered. In order to study the influence of differential diffusion on curvature effects in flame propagation, temperature statistics are presented in terms of standard probability density functions (pdfs) and also joint pdfs with curvature for the nonunity Lewis number cases. Temperature statistics are found to be consistent with previous incompressible combustion DNS studies. It is found that both dilatation and Tangential Strain rate are negatively correlated with curvature. The relative strength of these two correlations determines the nature of the correlation between surface density function (SDF) (∣∇c∣) and curvature. It is also found that the v...

M Klein - One of the best experts on this subject based on the ideXlab platform.

  • effects of lewis number on the reactive scalar gradient alignment with local Strain rate in turbulent premixed flames
    Proceedings of the Combustion Institute, 2009
    Co-Authors: Nilanjan Chakraborty, M Klein, N Swaminathan
    Abstract:

    Abstract The effects of Lewis number Le on the reactive scalar gradient alignment with the local Strain rate have been studied using Direct Numerical Simulation data of freely propagating statistically planar turbulent premixed flames with Le ranging from 0.34 to 1.2. The alignment characteristics of the reaction progress variable gradient are explained using the statistics of dilatation rate, and flame normal and Tangential Strain rates. The strength of dilatation rate is shown to increase with decreasing Le and this effect becomes particularly strong for the flames with Le Le close to unity, contrary to the alignment of scalar gradient with the most compressive principal Strain rate in turbulent passive scalar transport. However, stronger dilatation rate effects in Le  ≪ 1 flames (e.g. Le  = 0.34) gives rise to the preferential alignment of the reactive scalar gradient with the most extensive principal Strain rate for the major portion of the flame-brush. The scalar gradient alignment with local Strain rate plays an important role in the transport of scalar dissipation rate and the flame surface density. The observed alignment with the most extensive principal Strain rate destroys the scalar gradient and the magnitude of this sink is found to increase with decreasing Lewis number for a given turbulent Reynolds number and Damkohler number.

  • influence of lewis number on the surface density function transport in the thin reaction zone regime for turbulent premixed flames
    Physics of Fluids, 2008
    Co-Authors: Nilanjan Chakraborty, M Klein
    Abstract:

    The effects of Tangential Strain rate and curvature on the surface density function (SDF) and on different terms within the SDF transport equation in the thin reaction zone regime are studied for statistically planar turbulent premixed flames with global Lewis numbers Le=0.8, 1.0, and 1.2 by using three-dimensional direct numerical simulations with simplified chemistry. A positive correlation is observed between the SDF and Tangential Strain rate, and this is explained in terms of the local statistical behaviors of Tangential Strain rate and dilatation rate. Curvature is shown to affect the SDF through the curvature response of both Tangential Strain rate and dilatation rate on a given flame isosurface. The correlation between the curvature and SDF is positive in the Le=0.8 flame and negative in the Le=1.2 flame. The curvature dependence of the SDF is weak in the case of unity Lewis number. Strain rate and curvature are found to have an appreciable effect on different terms of the SDF transport equation. ...