Daniel V. Brown

Combustion Noise of Auxiliary Power Units

Noise from auxiliary power units (APU) is an important contributor to the overall level of ramp noise. Currently, ramp noise is regulated by international governing bodies as well as by individual airport. A significant component of APU noise is combustion noise. In this study, the unique spectral shape of APU combustion noise is identified. It is found that the spectral shape is the same regardless of engine size, power setting and directivity. Also, it is practically the same as that of open flame combustion noise. The frequency at the peak of the combustion noise spectrum is found to lie in the narrow range between 250 to 350 Hz. The peak sound pressure level of a given APU varies as the square of the fuel consumption rate. In the literature, suggestions have been made concerning a second combustion noise mechanism arising from the passage of hot entropy spots through the exhaust nozzle or constriction. In this investigation, no evidence has been found to indicate the existence of a second APU combustion noise component.

Numerical Modeling and Experimental Validation of the Acoustic Efficiency of Treated Ducts on an Aircraft Auxiliary Power System

Over the last decade research dedicated to improving aircraft systems that contribute to ramp noise has significantly increased. In the same timeframe new regulations have been implemented that restrict noise generated by the aircraft while located on the ground. These requirements aim at protecting workers from risks associated to noise exposure. One major contributor to ramp noise is the Auxiliary Power Unit (APU) gas turbine engine. To meet ramp noise level targets, aircraft manufacturers have to ensure the APU and its associated ducts are correctly designed. Numerical and experimental works are required to define an optimum solution for acoustics, but also minimize the size and weight of the components. The aim of this paper is to present some of the work performed to optimize the acoustic designs of the inlet and exhaust ducts of a modern APU. On the inlet side, classical SDOF liners have been considered, while non-locally reacting treatments have been studied for the exhaust. For both ducts numerical studies were carried out at Airbus with Airbus France ACTIPOLE solver (Fast Multipole Boundary Element Method) and the commercial software ACTRAN/TM (Finite Element Method). Test campaigns were performed at Honeywell to validate in-duct attenuations and installed noise sources directivity predictions. Comparisons between predicted and measured results are provided.

Prediction of Transmission Loss Through the Treated Inlet Duct of an Aircraft Auxiliary Power Unit

Results are presented for a computational study of noise propagation through the treated inlet duct of an Auxiliary Power Unit, for which extensive test data were available. The acoustic treatment in the tested duct consisted of locally-reacting panels of different thicknesses. Measured impedance data were also available for the treatment panels. Several duct and treatment configurations were analyzed using the ACTRAN/DGM program. Predictions of transmission loss for these configurations were compared with measured data. The ACTRAN/DGM analyses correctly predicted the qualitative trends in transmission loss for the different duct treatment configurations. Although peak levels of transmission loss tended to be overpredicted, results indicated that ACTRAN/DGM could be used to assess the relative performance of different treatment designs.