CHARACTERIZATION OF OSCILLATIONS DURING PREMIX GAS-TURBINE COMBUSTION

The use of premix combustion in stationary gas turbines can produce ve ry low levels of NOx emissions. This benefit is widely recognized but turbine developers routinely encounter problems with combustion oscill ations during the testing of new premix combustors. Because of the ass ociated pressure fluctuations, combustion oscillations must be elimina ted in a final combustor design. Eliminating these oscillations is oft en time-consuming and costly because there is no single approach to so lve an oscillation problem, Previous investigations of combustion stab ility have focused on rocket applications, industrial furnaces, and so me aeroengine gas turbines. Comparatively little published data is ava ilable for premixed combustion at conditions typical of an industrial gas turbine. In this paper we report experimental observations of osci llations produced by a fuel nozzle typical of industrial gas turbines. Tests are conducted in a specially designed combustor capable of prov iding the acoustic feedback needed to study oscillations. Tests result s are presented for pressures lip to IO atmospheres, with inlet air te mperatures up to 588 K (600 F) burning natural gas fuel. Based on theo retical considerations, it is expected that oscillations can be charac terized by a nozzle reference velocity, with operating pressure playin g a smaller role. This expectation is compared to observed data that s hows both the benefits and limitations of characterizing the combustor oscillating behavior in terms of a reference velocity rather than oth er engine operating parameters. This approach to characterizing oscill ations is then used to evaluate how geometric changes to the fuel nozz le will affect the boundary between stable and oscillating combustion.