APU-Noise Reduction by Novel Muffler Concepts

Abstract

For a GTCP36-28 auxiliary power unit (APU), a set of mufflers has been designed and tested for some representative operating conditions. The first muffler design uses cavities of different sizes in conjunction with a bias flow for efficient broadband sound absorption. The second design - also expected to perform well over a large frequency range - makes use of a variable perforation and some porous absorber material. The acoustic damping performance of the mufflers was assessed using a downstream section of dedicated microphone probes. Individual spectra and circumferential averages have been computed and are used for a comparison to a hard-walled duct section of the same length. Results show a reasonable broadband absorption for most configurations. For one operating point, significant differences were found while comparing the performance of the cavity muffler with and without bias flow. The results suggest, that a small amount of air - less than initially expected - is sufficient to obtain the desired noise reduction.