A computational study of direct simulation of high speed mixing layers without and with chemical heat release

Abstract

This computational study is a direct simulation calculation of high speed reacting and non-reacting flows for H₂-air system. The Navier-stokes calculations are made for convective Mach numbers of 0.38 and 0.76 with hyperbolic tangent initial profiles and a single step reaction with fine grid distribution in a time accurate mode with higher order numerical methods till a statistical steady state is achieved. After this time, about 600 time sliced values of all the variables are stored for statistical analysis. In this new study it is first shown that most of the problems of high speed combustion with air are characterised by weak heat release. The present calculations show that (i) the convective speed is reduced by heat release by 3-10% depending on convective Mach number, (ii) the variation of the mean and rms fluctuation of temperature can be explained on the basis of temperature fluctuation between the adiabatic flame temperature and the ambient, (iii) the growth rate with heat release is only 7% lower than without, (iv) the entrainment is 25% lower with heat release than without. These smaller differences in comparison to incompressible flow dynamics are argued to be due to larger enthalpy changes due to gas dynamics in comparison to heat release. It is finally suggested that problems of reduced mixing at high speeds are not severely hampered by heat, release.