Effect of Fuel Injection Location on Combustion Instability in a Dump Combustor

Abstract

The acoustic behavior of a model combustor with backward facing step is studied, to examine the effect fuel injection location on the combustion instability. Six different injection locations, such as far upstream of the step, near upstream of the step, injection at the step, near downstream to the step, in the recirculation zone, and far downstream to the step, are investigated. Acoustic characteristics of the combustor over a wide range of equivalence ratios and Reynolds number based on step height are carried out. Liquefied petroleum gas (LPG) is used as fuel for the acoustic characterization. The effect of injection location on the blow off limits is presented, to identify specific conditions corresponding to unstable regions of combustion. Simultaneous unsteady pressure measurement, fuel flow fluctuations (by Mie scattering) and chemiluminescence imaging are done for conditions corresponding to local maxima and local minima of acoustic amplitude as the air flow rate is varied. Methane is used as fuel for these studies. The fuel injection location dictates the effect of fuel flow fluctuations (equivalence ratio fluctuations) on heat release and the corresponding acoustic oscillations. It is shown that within an optimum distance upstream of the step, the characteristics of vortex-shedding dominated acoustic excitation are predominated by those of the equivalence ratio fluctuations. Local maxima and minima of acoustic amplitude with air flow rate when fuel is injected downstream is from constructive and destructive interference of oscillations propagating along vertical and acoustic streamlines.