Combustion Noise Investigation

Improved methods for predicting both direct and indirect combustion noise from aircraft engines are developed and experimentally evaluated by conducting rig experiments and by comparing with data from several full scale engines. Comparison of predictions with full scale engine data indicated that direct combustion noise is the dominant source for the P and WA engines investigated. The direct combustion noise prediction system includes expressions for acoustic power level, peak frequency and full-scale engine acoustic transmission loss due to combustor/duct coupling and turbine attenuation. These expressions are derived in terms of readily available performance and geometry parameters from the burner and turbine. New parameters introduced by the prediction system include the effects of fuel nozzle number and burner length. Predictions are shown to be in good agreement with data obtained from component rig tests on several JT8D type burner configurations (including single and multiple fuel nozzle, conventional and low emission designs). In addition, when transmission losses are accounted for, the predictions are also shown to be in good agreement with observed combustion noise levels and peak frequencies from four P and WA turbofan engines (i.e. the JT8D-109, JT9D-7A, JT9D-70 and the prototype JT10D). Predicted combustion noise directivity patterns and spectra shapes are determined empirically, using the data from both the rig tests and these four engines. Results from the analytical and experimental combustion noise investigations are used to identify combustion noise reduction methods obtainable through modifications in burner design and/or performance parameters.