A computational study of combustion instabilities due to vortex shedding

The influence of unsteady vortex shedding on combustion instabilities is examined using simulation databases For two configurations: a dump combustor and a model afterburner equipped with a bluff-body flame holder for different operating conditions. These databases have been generated using a recently developed flame-wrinkling large eddy simulation (LES) model, described elsewhere. As a first necessary step, a detailed comparison is made between predicted and experimentally obtained statistics to quantify the accuracy of the LES and determine whether LES reproduces the observed statistical trends in the experiments. The objective of the present study is to examine these databases using visualization techniques in order to investigate the mechanisms responsible for combustion instabilities in relation to unsteady vortex phenomena. Combustion instabilities are found, experimentally as well as computationally. in both configurations and under different operating conditions. Based on the LES databases, the origin of these instabilities can be identified and attributed to key events such as vortex shedding, excitation of pressure fluctuations due to exothermicity, extinction by strain, and operating conditions. The unsteady behavior of the dump combustor is dominated by extinction due to the high strain rate, while the dynamics of the model afterburner was associated mainly with excitation of pressure fluctuations due to exothermicity and operating conditions.