Combustion effects on turbulence in a partially premixed supersonic diffusion flame

Abstract

Effects of chemical heat release on turbulence in a partially premixed diffusion flame are investigated using direct numerical simulation (DNS). The full three-dimensional time-dependent compressible Navier-Stokes equations are employed to simulate the coupling between supersonic turbulence and heat release from a one-step chemical reaction governed by the Arrhenius kinetics. Four reacting cases with increasing heat release and one nonreacting case have been studied. Combustion is found to produce strong coupling among fluctuations in velocity, pressure, density, and other thermochemical quantities. In the Reynolds stress budget, the pressure–strain term becomes dominant as the heat release increases. Despite its wave behavior and its alternate roles as a source and a sink, the pressure–strain serves to promote energy transfer from the streamwise to the transverse and spanwise directions, and from the diagonal to the off-diagonal components of the Reynolds stresses, thus reducing anisotropy. Moreover, the pressure–strain, which in the case of the turbulent kinetic energy budget can be split up into a mean pressure work and a pressure–dilatation, helps to convert chemical energy from combustion into turbulence energy, leading to the phenomenon of “combustion-generated turbulence.” Both the solenoidal dissipation and the dilatational dissipation in particular increase as the heat release increases, but their effects are secondary to those of the pressure–strain within the main combustion period. The mixing layer growth rate can be predicted directly through the integrated Reynolds stress generation, with the accuracy of prediction depending on the heat release rate and the Reynolds number. A simple mechanism for the interactions of combustion and turbulence is proposed, while the modeling difficulties involved are also outlined. Finally, the differences and similarities of heat release effects and compressibility effects are clarified.