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Structural vibration and noise radiation of a single-stage centrifugal compressor excited by the internal unsteady flow is investigated experimentally in the first part of our work [Zhou et al. Appl Acoust 2018;131:112–28]. Experimental results for acceleration of vibration and sound pressure under different rotational speeds indicate that the straight discharge pipe connecting the volute is forced to vibrate the most intensively at the blade passing frequency (BPF) and acts as the major contributor to the compressor noise emission. In this paper, a pipe micro-perforated panel (PMPP) is optimally designed with the genetic algorithm to suppress vibration and noise radiation of the straight discharge pipe section. A maximum sound pressure level (SPL) reduction of 20 dB for the BPF component and 8 dB for the overall noise are observed under the tested working conditions, while the influence of the PMPP on the aerodynamic performance of the compressor is negligible. The PMPP may be a promising method for controlling the tonal component of vibro-noises radiated from the centrifugal compressor.

This paper presents an experimental study on the noise radiated from a centrifugal compressor unit, which mainly consists of a single-stage compressor connected with inlet and outlet pipes, a variable-frequency driving motor, a transmission gearbox and a bearing box. Sound pressure level spectra were measured at three rotational speeds each with several mass flow rates. The experimental results reveal that the overall sound pressure level increases with the rotational speed but varies slightly with the flow rate. The largest overall sound pressure level occurs at the measuring point near the motor, implying that the motor is one of the most important noise sources of the test rig. The frequencies of the tonal components are respectively consistent with the shaft frequency, the blade passing frequency of the compressor impeller and the interaction frequency of the magnetic flux wave with rotor slots in the variable-frequency motor. For sampling points near the connected pipes, the broadband noises mainly arise from the internal random pressure fluctuation through a curve fitting analysis of the squared sound pressure versus the dimensionless frequency. A coherence analysis between the acoustical and vibrational signals implies that the structural vibration induced by the internal pressure loads is the primary contributor for the compressor noise. Particularly, the outlet pipe with the highest vibration level should be a major concern for noise reduction of the centrifugal compressor.