Stereoscopic particle image velocimetry of the flow field in the rotor exit region of a forward-blade centrifugal turbomachine

This article reports the measurements of the complicated flow phenomena in the rotor exit region of a multi-blade centrifugal fan with stereoscopic particle image velocimetry (PIV). The main purpose is to enhance the quality of raw PIV data for future turbulence analyses. The instantaneous and averaged quantities are sensitive to phase-matching of the raw data since the blade-to-blade distance is short. An image processing based procedure is devised that identifies the rotor margin and blade tips location and selects the phase-resolved images. The large measurement area is divided into four fields of view. PIV data post-processing optimizes the relative position of different fields of view and enhances matching between them by detecting the phase-resolved images. The accuracy and reliability of measurements are examined by error and convergency analyses. The results are used to analyse the interactions between jet and wake structures as well as between rotor exit flow and the volute stream near the rotor. Jet/wake structures, non-uniform flow patterns, and high vorticity layers with opposite value in both sides of each blade are novel measured patterns in centrifugal fan which also occur in other centrifugal turbomachines. Such data are a necessary prerequisite for the accurate turbulence modelling of the complicated jet/wake flow and the mixing in the volute.