Acoustic source strength determination of turbocharger in an unfavourable acoustic environment

Abstract: The aim of the M.Sc thesis work is to specify a measurement method suitable for deter-mining the sound power levels and especially to quantifying the levels at the compressor blade pass frequency of a turbocharger in the new turbo performance rig located at Scania CV AB, Södertälje.Intensity and pressure based methods are widely used to determine the sound power levels. The thesis work focuses on pressure based methods since intensity measurements has a limitation in high frequencies and the intensity scanning in the rig is not allowed when the test rig is being operated. Unlike the intensity based methods the major drawback of using the pressure based methods is the influence of test environment on the sound pressure measurements. Since the room is not completely anechoic and reflections from various objects in the room may lead to wrong estimation of sound power levels. In order to understand the influence of test environment at the four chosen microphone positions several measurements were performed both in compliance with international standards and also to test assumptions on the acoustics characteristics of the room.Other than the turbocharger itself the test environment also includes three main auxiliary equipments; a cooling fan, a burner and an oil conditioning system which may contribute to the background noise at the microphone locations. A detailed study has been conducted to understand the influence from these additional sound sources during the measurements. It was concluded that the background sound do not a˙ect the measured results in the frequency range of interest. Measures were taken to isolate radiation from connecting pipes by shielding them with sound absorbing material.Based on the results from the test environment measurements and the background noise analysis the international standard ISO 3744 (Determination of sound power levels in an essentially free field over a reflecting plane) is recommended to determine the sound power levels of the turbocharger.For a constant shaft speed it was found that the highest A-weighted sound power levels were observed when the turbocharger was running close to surge followed by peak eÿciency and choke conditions on the compressor map. There is one limitation associated with the calculated sound power level and that is, the estimated sound power level is uncertain since it is based on only 4 microphone positions and thereby is not capturing the details of the compressor directivity.As future work, a setup with a large number of microphones surrounding the test speci-men is recommended which would help to determine the directivity hence improving the accuracy of the measurements. Also further studies on the sensitivity of the microphone positions, the arrangement of the auxiliary equipment in the room and the influence by the inlet and outlet pipes used in the real installation is recommended.Keywords: Turbocharger, surge, choke, peak eÿciency, compressor map, compressor blade pass frequency, sound power levels.