Measurement and Prediction of the Tonal Noise of a Centrifugal Compressor at Inlet

Abstract

The paper addresses two aspects of the tonal noise generated by a subsonic, centrifugal compressor. The first aspect is an experimental investigation, based on a modal detection in the inlet duct, proving that the wake-interaction noise from the impingement of the impeller blade wakes on the diffuser vanes is the dominant mechanism. The second aspect is the analytical modelling of the sound transmission from the impeller outer radius through the inter-blade channels and its recombination as helical modes in the inlet duct. The propagation inside the inter-blade channels is described by a slowly-varying bent duct approach, for which a simplified description of the channel geometry is considered. The modal pressure patterns predicted analytically are compared with a numerical simulation using a commercial software, and the discrepancies suggest that significant modal coupling occurs, ignored in the analytical solution. The presented work is part of a global predicting methodology in which different analytical models are chained from the sources to an external observer, successively dealing with the modal excitation of the impeller channels from sources distributed on the diffuser, the sound transmission through the channels, the sound recombination at inlet and the radiation by the inlet duct termination. More specifically the coupling procedure between the inter-blade channel modes and the helical modes in the air-inlet duct is discussed, as only an element of the analytical tool-box currently under development.