Combustion-induced pressure effects in supersonic diffusion flames

Abstract

A turbulent diffusion flame at a convective Mach number 1.2 was investigated using direct numerical simulation (DNS). The DNS employed the full time-dependent compressible Navier-Stokes equations coupled with a one-step chemical reaction governed by the Arrhenius kinetics. Detailed study of combustion-induced pressure effects on turbulence generation, conserved scalar, and stagnation enthalpy transport was conducted. Local countergradient diffusion (CGD) of a conserved scalar flux was observed for the first time in a diffusion flame where heat release was strong enough while gradient diffusion prevailed when heat release was zero or weak. The CGD occurred in spite of the absence of an externally imposed mean pressure gradient and was attributed to combustion-induced pressure fluctuations. The balance of the turbulent kinetic energy budget was strongly influenced by the pressure dilatation and the (combustion-induced) mean pressure work when heat release was strong. Both terms can be a source or a sink of turbulence, depending on the intricate interactions between turbulence and combustion. However, the temporal change in pressure ϱp/ϱt had an insignificant influence on the stagnation enthalpy transport. A linear relation between the stagnation enthalpy and the mixture fraction was confirmed, which could lead to considerable simplification in modeling high-speed turbulent combustion.