Longitudinal Mode Combustion Instabilities in a Dump Combustor

The mechanism by which longitudinal modes of a dump combustor are excited has been investigated. The unsteady combustion is a result of the shedding of large scale vortical structures from the flameholder. Driving and damping as determined by Rayleigh's criterion were investigated by using the cross-spectrum and phase of the fluctuating pressure and radiation intensity signals at various locations in the combustor. Thus, the excitation of a particular mode was found to depend on the pressure mode shape and the magnitude and phase of the velocity fluctuation at the flameholder. Fluid mechanical mixing and the chemical reaction rate of the fuel also effect the distribution of heat release and hence the locations of driving and damping. Finally, a mechanism for existence of the limit cycle is discussed.