The impact of dilatation, scrambling, and pressure transport in turbulent premixed flames

Abstract

Premixed turbulent flames feature strong interactions between chemical reactions and turbulence that affect scalar and turbulence statistics. The focus of the present work is on clarifying the impact of pressure dilatation/flamelet scrambling effects with a comprehensive second-moment closure used for evaluation purposes. Model extensions that take into account flamelet orientation and molecular diffusion are derived. Isothermal pressure transport is included with an additional variable density contribution derived for the flamelet regime of combustion. The full closure is assessed by comparisons with direct numerical simulations (DNS) of statistically "steady" fully developed premixed turbulent planar flames at different expansion ratios. Subsequently, the prediction of lean premixed turbulent methane-air flames featuring fractal grid generated turbulence in an opposed jet geometry is considered. The overall agreement shows that "dilatation" effects contribute to counter-gradient transport and can also increase the turbulent kinetic energy significantly. Levels of anisotropy are broadly consistent with the DNS data and key aspects of opposed jet flames are well predicted. However, it is also shown that complications arise due to interactions between the imposed pressure gradient and combustion and that redistribution is affected along with the scalar flux at the leading edge. The latter is strongly affected by the reaction rate closure and, potentially, by pressure transport. Overall, the derived models offer significant improvements and can readily be applied to the modelling of premixed turbulent flames at practical rates of heat release.

Impact of molecular mixing and scalar dissipation rate closures on turbulent bluff-body flames with increasing local extinction

“…Cφ " 1.2 [57] calibrated from the use of the MC model for molecular mixing is close to the upper limit. The implied higher value of C m " 0.916 is in agreement with models for dilatation, scrambling and pressure transport in turbulent premixed flames [9]. Here, C φ " 2.3 (compared to the standard value of C φ " 2.0) is retained for consistency with the application of standard MC model and the previous studies of turbulent diffusion flames (e.g., [56]).…”

“…whereρ stands for the mean density and with β˚recently shown to be close to unity [9]. The second equality in Eq.…”

“…(5) an exception. The closure has been successfully used in a number of premixed flame studies with C R » 3.0 [9,49]. The expression leads to,…”

“…backscatter) energy transfer [8] that need to be modelled, e.g. via moment based methods [9]. The latter can also offer an efficient partial resolution of the unsteady fluid motion, particularly if closed at the second moment level, and have been used to study non-linear hydrodynamic and thermoacoustic oscillations in bluff-body stabilised turbulent premixed flames [10].…”

Combustion-induced local shear layers within premixed flamelets in weakly turbulent flows

“…The present paper aims at revealing one of the eventual causes of such a moderate progress in that area. The point is that almost all models of the influence of premixed combustion on turbulence, reviewed elsewhere, 9,10 see also a paper by Tian and Lindstedt 11 as a recent example, rely somehow on theoretical expressions that describe velocity and pressure variations in the laminar premixed flames. Such an approach is widely accepted, in particular, due to the lack of an alternative, sufficiently well developed theory of the influence of combustion on turbulence.…”