The effect of inlet boundaries on combustion instability in a pressure-elevated combustor

The effect of inlet boundaries on combustion instabilities in a spray combustor is investigated experimentally in this paper. The combustor is operated at pressure-elevated conditions and fueled with aviation kerosene. Three different inlet conditions, choked diffuser, open diffuser, and no diffuser are tested. Thermoacoustic instabilities are seen only in the case with no diffuser, where significant flame heat release oscillations with periodic flame liftoff-reignition are found. Rayleigh index maps are plotted to illustrate the thermoacoustic coupling in three cases. Then, a low order network model is used to explain the observed behaviors. The predicted system stabilities and mode shapes agree well with the experimental results. Further analysis shows that, in the considered parametric range, the stability of the thermoacoustic system is very sensitive to the length of the diffuser but not sensitive to its cross-sectional area. This reveals the possibilities of using diffuser optimization to suppress combustion instabilities in real combustors.