EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE UNSTEADY FLOW FIELD AND TONE GENERATION IN AN ISOLATED CENTRIFUGAL FAN IMPELLER

摘要

In spite of low circumferential Mach number the sound of iso-lated centrifugal fan impellers is sometimes dominated by distinctive tones at blade passing frequency (BPF) and integer multiples. This paper reports on an experimental and numerical investigation in-tended to unveil the tone generating mechanism. The sound spectra from three impellers operating at a large range of speed were meas-ured and decomposed into Strouhal and Helmholtz number dependent functions. This led to the preliminary conclusion that the BPF related tones are exclusively flow-induced. Based on hot-wire and blade pressure fluctuation measurements and a subsequent correlation analysis, coherent flow structures different from the one associated with the principal azimuthal flow pattern due to the blades were de-tected. Eventually, numerical three-dimensional unsteady flow simu-lation and experimental flow visualization revealed an inlet vortex. It takes on a helical form, with the vortex core slowly varying its posi-tion with respect to the impeller center. As the blades cut through that quasi-stationary helical vortex they encounter blade force fluctuations producing the BPF tones. The slow spin of the vortex core and the slow variation of vortex strength were identified as the reason for the amplitude modulation of the BPF tone.