Small Scale Features of Velocity and Scalar Fields in Turbulent Premixed Flames

Abstract

Because the ultimate stages of turbulent mixing take place at the unresolved scales in either Reynolds Average Navier Stokes (RANS) or Large Eddy Simulation (LES) approaches of turbulent reactive flows, the closure of molecular dissipation rates still remains an essential problem in the field of turbulent combustion. In the present study, turbulent flames with premixed reactants are considered in the flamelet regime of turbulent premixed combustion: Damköhler and Karlovitz number's values are such that Da > 1 and Ka < 1. In this situation, the mixing rate clearly depends upon the turbulence characteristics but is also strongly influenced by the laminar flamelet structures that drive the instantaneous gradients of the reactive species. In the present work, the analysis is focused on some among the different unclosed terms that arise in the transport equation of the mean reactive scalar dissipation rate. New closures relying on the fast chemistry assumptions are proposed for these production terms and similarities and differences with existing models are discussed. Previous works devoted to the modeling of the scalar dissipation rate have recently shown that both reactive features and expansion phenomena strongly influence the behavior of some terms with respect to the passive scalar situation of reference, and the present study confirms the relevance of these findings. Finally, Direct Numerical Simulation (DNS) databases are considered to assess the validity of the proposed models. These DNS correspond to the calculations of three-dimensional statistically steady planar turbulent premixed flames obtained for three distinct values of the gas expansion factor τ =(T b − T u )/T u namely 1.5, 4 and 6.5, where subscripts u and b refer to unburned reactants and burned products A. Mura (B) · V. Robin · M. Champion LCD UPR 9028 du CNRS Flow Turbulence Combust (2009) 82:339-358 respectively. With respect to previous studies, these data allow a detailed check of the validity of the different modeling proposals when the gas expansion factor is varied.

Dynamic Closure of Scalar Dissipation Rate for Large Eddy Simulations of Turbulent Premixed Combustion: A Direct Numerical Simulations Analysis

“…The scalar gradient ∇c aligns with the most compressive principal strain rate when turbulent straining dominates over the strain rate arising from flame normal acceleration and vice versa [16,23,40]. The strengthening of turbulent straining and weakening of the strain rate induced by flame normal acceleration with increasing Karlovitz number are mimicked by the empirical Ka dependences in C * 3 and C * 4 respectively [14][15][16][19][20][21]. Any other functions, which account for the asymptotic behaviour of C * 3 and C * 4 in relation to Karlovitz number variation, can also be used in principle to parameterise these model parameters.…”

“…A number of analyses [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] concentrated on closures of SDR in turbulent premixed flames in the context of RANS but relatively limited effort [6][7][8] has been directed to the modelling of SDR for LES of turbulent premixed combustion. The combustion process takes place mostly at the sub-grid level in premixed flames, as the flame thickness often remains smaller than the filter size for most practical LES.…”

Influence of Turbulent Scalar Mixing Physics on Premixed Flame Propagation

“…While this is true for turbulent flows with passive scalars, recent studies [15][16][17][18][19][20] have shown that the scalar dissipation rate is strongly influenced by chemical reactions in premixed flames. The heat release directly affects the local density, which induces alterations in the local dynamics of turbulence, scalar fields, and their interaction and thus, one can envisage a two-way coupling between heat release and the turbulence.…”

Investigation of Flame Stretch in Turbulent Lifted Jet Flame

“…Many DNS (Swaminathan and Grout, 2006;Chakraborty and Swaminathan, 2007a;Kim and Pitsch, 2007;Mura et al, 2008Mura et al, , 2009Chakraborty et al, 2009;Richardson et al, 2010;Chakraborty et al, 2010) and experimental (Hartung et al, 2008;Steinberg et al, 2012) studies of premixed combustion and stratified combustion (Malkeson and Chakraborty, 2011) have demonstrated that the reactive scalar gradient vector aligns with the most extensive principal component of the turbulent strain rate when the local heat release is strong. This is in contrast to the passive scalar physics where the scalar gradient is known to align with the most compressive component.…”