Mechanism of combustion instability in a lean premixed dump combustor

Results from an experimental study of the mechanism ofunstable combustion in a coaxial, optically accessible, bluff-body-stabilized dump combustor with natural gas as the fuel are reported, A parametric study was performed to investigate the effects of equivalence ratio, inlet velocity, inlet fuel distribution, inlet swirl, and centerbody recess on combustion stability. It was found that all of these parameters had an effect on the stability characteristics of this combustor. At selected unstable operating conditions, phase resolved CH chemiluminescence images were captured to study the heat-release structure during one period of pressure oscillation. The flame-flowfield interaction that is depicted in these images indicates that flame-vortex interactions,:and the resultant dame area changes, play a significant role in the instabilities that occur when there is no swirl, A simple analysis of these images, however, shelved that fluctuating flame area and equivalence ratio fluctuations both contribute to the heat release fluctuations that drive the instability. Unstable combustion with swirl appears to be fundamentally different from unstable combustion without swirl in that instabilities with swirl occur near lean blowout and appear to be associated with repeated detaching and reattaching of the flame from the centerbody.