Turbulent flame propagation with large chemical-heat release

Abstract

A model is developed to describe the dependence of the turbulent-flame speed on the intensity of an isotropic excitation turbulence prescribed far upstream from the flame for arbitrarily large gas expansion within the flame. For the limit of negligible gas expansion within the flame the new prediction of the present study reduces to an established and recently verified result for isothermal front propagation. It is shown that the turbulent-flame speed varies inversely with the square of the temperature ratio across the flame when the temperature ratio is very large. For typical hydrocarbon flames the results predict generally less substantial rates of decrease of the turbulent-flame speed with increasing heat release. Variations in turbulence kinetic energies and vorticity across the flame and hydrodynamic zones upstream and downstream from the flame are determined as well, accounting for influences of gas expansion and the structure of the excitation turbulence. The results of the present work, which are valid for flame propagation in weakly turbulent flow (where the propagation speed is proportional to the square of the intensity of the excitation turbulence prescribed far upstream from the flame) extend earlier predictions that were limited to relatively small chemical-heat release. The model presented herein does not account for effects of intrinsic flame instability and is appropriate for conditions where influences of buoyancy and flame stretch on flame dynamics are not substantial.