An experimental study of the effects of partial premixing and swirler configuration on noise generated by turbulent jet and swirling, flows and flames
The combustion generated noise in gas turbine engines is a barrier for further reduction in engine noise emissions. Combustion-generated oscillations interact with hardware and lead to combustion instabilities, performance degradation or structural damage. These engines employ dual- and multi-swirlers, and various degree of partial premixing. Motivated by these, the effect of partial premixing on sound generation was investigated for jet flames, and advanced combustor design based swirling flames. The effect of swirler configuration on noise generated by swirling flows and flames was explored. For jet flames, the effect of partial premixing was investigated by keeping fuel flowrate constant with gradual air addition and also with constant overall volume flowrate. For swirling flames, fuel was gradually added with constant air flowrate. Narrowband spectral measurements were obtained. The results indicate that the equivalence ratio plays a stronger role in combustion generated noise than the Reynolds number, Mach number, and firing rate. The partial premixing affects the sound generation primarily through impact on flame structure and combustion volume. The effect on sound generation depends on the stoichiometric air requirement of fuel. This results in a more pronounced effect in methane flames compared to ethane and ethylene flames because of lower stoichiometric air requirement of methane. Combustion affects high frequencies due to its effect on velocity field. Unlike premixed flames, swirling partially premixed flames strongly amplify upstream generated low frequency tones with their relative amplification a function of swirler configuration. The co-rotating double swirler configuration generates less noisy flames than single and counter-rotating double swirler configurations and thus is preferable. The addition of a second swirler reduces precession speed of precessing vortex core (PVC) and weakens its structure, with counter-rotation having a stronger impact. This explains the widest stability limit of counter-rotating double swirler configuration. The swirling flow spectra are dominated by high amplitude PVC tones which collapse at one Strouhal number termed as characteristic Strouhal number (Stch). The acoustically determined Stch can be used to determine the effective Swirl number of complex swirler configurations. The acoustic spectral analysis may be used for detection of flow anomalies in swirling flows and flames.
Mongeau, Purdue University.
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