Computational Simulations of the Effect of Backstep Height on Nonpremixed Combustion Instability

Abstract

Detailed computational simulations are used to compare the effect of backstep height on the stability characteristics of an axisymmetric dump combustor fed by separate fuel and oxidizer streams. Companion experiments demonstrate a dramatic increase in the amplitude of pressure oscillations for the smaller backstep due to combustion instability. The goal of the present simulations is to ascertain whether computations can predict combustion instability for this configuration and the degree to which they can replicate this experimental trend. Two different oxidizer-inlet boundary conditions were used: a subsonic inlet and a choked inlet. Both predicted stronger oscillations for the smaller backstep, although the disturbances in the uniform inlet case decayed to relatively low levels. The oscillations in the choked-inlet case were sustained but were still somewhat smaller than in the experiment. The simulations suggest the effect of backstep height arises because of stronger wall/vortex impingement in the smaller-step-height combustor. The instantaneous pressure and heat release are more strongly in phase for the smaller-step-height simulations, resulting in larger Rayleigh indices. Power spectral density results likewise show larger peaks for the smaller step. The sensitivity to upstream conditions suggests that care should be taken in designing both simulations and experiments.