Characterization of oscillations during premix gas turbine combustion

The use of premix combustion in stationary gas turbines can produce very low levels of NOx emissions. This benefit is widely recognized but turbine developers routinely encounter problems with combustion oscillations during the testing of new premix combustors. Because of the associated pressure fluctuations, combustion oscillations must be eliminated in a final combustor design. Eliminating these oscillations is often time-consuming and costly because there is no single approach to solve an oscillation problem, Previous investigations of combustion stability have focused on rocket applications, industrial furnaces, and some aeroengine gas turbines. Comparatively little published data is available for premixed combustion at conditions typical of an industrial gas turbine. In this paper we report experimental observations of oscillations produced by a fuel nozzle typical of industrial gas turbines. Tests are conducted in a specially designed combustor capable of providing the acoustic feedback needed to study oscillations. Tests results are presented for pressures lip to IO atmospheres, with inlet air temperatures up to 588 K (600 F) burning natural gas fuel. Based on theoretical considerations, it is expected that oscillations can be characterized by a nozzle reference velocity, with operating pressure playing a smaller role. This expectation is compared to observed data that shows both the benefits and limitations of characterizing the combustor oscillating behavior in terms of a reference velocity rather than other engine operating parameters. This approach to characterizing oscillations is then used to evaluate how geometric changes to the fuel nozzle will affect the boundary between stable and oscillating combustion.