Experimental Investigation of the Acoustic Field in a Bluff-Body Combustor

Abstract

Experimental results on the longitudinal acoustic field generated by a methane-air turbulent flame stabilized downstream of a bluff body (V-gutter) confined in a rectangular duct are presented. The V-gutter runs straight across the width of the rectangular cross-section of the duct. The fuel is injected at some distance upstream of the bluff body, as in practical combustors. Varying the distance between the fuel injection and the bluff body alters the degree of fuel-air mixing and hence influences the flame structure, and consequently the nature of the generated acoustic field. Other geometrical variations considered are the length of the combustor duct, the axial location of the bluff body, and the width of the bluff body. The flow rates of air and fuel are varied widely to traverse conditions of generation of low-amplitude broadband noise to those of excitation of high-amplitude discrete tones. The observed dominant frequencies are expressed in terms of non-dimensional numbers such as the Helmholtz number and the Strouhal number, representing the natural acoustic modes and the free-field vortex shedding process, respectively. It is seen that, as the flow conditions are varied, the amplitude of the dominant frequency steeply rises, accompanying an abrupt jump in the dominant frequency from a constant Helmholtz number regime to a constant Strouhal number regime. High-speed images of CH* chemiluminescence of the flame zone exhibit periodic shedding of vortical structures at the same dominant frequencies as in the observed acoustic spectra. These results show that the natural acoustic modes shift to follow the vortex shedding trend with the Reynolds number of the air-flow, under conditions of excitation of loud discrete tones. Higher fuel flow rates are observed to be capable of exciting higher acoustic modes to exhibit the above behaviour. A larger location of the bluff body away from the fuel injection location, and a lesser width of the bluff body, tend to excite acoustic oscillations of lower amplitude.