Investigation of Instabilities in a Lean, Premixed Step Combustor

Abstract

A combined experimental and computational characterization of combustion instabilities in a lean, premixed backward-facing step combustor was performed. Specifically, the instabilities of interest were those encountered as the equivalence ratio was reduced to levels approaching the combustor's lean extinction limit. A quasi-one-dimensional unsteady analysis with loss and heat-addition models was developed to simulate combustion instabilities for a premixed step combustor. Experimental results indicated that the magnitude of a longitudinal acoustic disturbance grew with decreasing equivalence ratio, until it eventually triggered a lower-frequency, high-amplitude instability. Numerical results compared with experimental data demonstrated that the analysis can capture the critical frequencies observed in the combustor model. Coupling the heat release with the step flow velocities in the analysis produced an instability at the dominant resonant frequency of the combustor. Experimentally, low-frequency instabilities were visible as a flapping of the flame and increased in severity with decreasing equivalence ratio until they caused combustor blowout.