On local countergradient diffusion in turbulent diffusion flames

Abstract

Scalar transport in turbulent diffusion flames has been studied using direct numerical simulation. The fully compressible formulation of the governing equations for turbulence and finite-rate chemistry allows the simulation of the strong coupling between turbulence and combustion at various heat release rates. The advanced numerics enables the scalar fluxes and related correlations to be computed accurately, which reveals the occurrence of local countergradient diffusion (LCGD) in non-premixed turbulent combustion of the fluxes of both conserved and nonconserved scalars, and the extent and frequency of the occurrence increases with the heat release level. A detailed scrutiny of the scalar flux transport equations identifies the dominant terms for and against LCGD. The competing factors are mainly between the combustion-generated pressure terms and the mean scalar and velocity gradient terms. However, the reaction-rate term is not directly responsible for the occurrence of LCGD, due to the nature of diffusion flames. An “effective pressure gradient” is introduced to represent the effects of both mean and fluctuating pressures induced by combustion. A new criterion for the occurrence of LCGD is formulated, which states that LCGD occurs when the combustion-generated effective pressure gradient is large than the effect of the gradient diffusion (GD) caused by the mean scalar gradient. But even before it reaches the critical level that causes actual LCGD, chemical heat release has already altered scalar transport, which may not be represented well by GD models.