PIV Measurement of Shock Waves in a Transonic Centrifugal Compressor

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In a single-stage high-pressure-ratio centrifugal compressor, the velocity relative to the impeller and to the diffuser usually exceeds the velocity of sound. That is, generation of shock wave is unavoidable. The objectives of the present study are to measure the flow including the shock wave in the impeller inlet or between diffuser cascade vanes using a double-pulse Particle Image Velocimetry (PIV) and to clarify the relationship between the intensity of shock wave and the flow rate. A conditional sampling technique is applied to get a relative position between a rotating impeller and stationary vanes, and then an instantaneous structure of shock wave is discussed. Furthermore, a temporal change in shock wave structure will be discussed with stall or surging.(1) PIV diagnostics using a small wind-tunnel, simulating an impeller blade passage or a diffuser intervanes, was constructed. The minimum time interval of 1 μs and the maximum velocity of 55 m/s were established.(2) A laser light sheet projector was designed for sensing of submicron particles.(3) PIV measurement at the impeller inlet was succeeded, where the maximum absolute velocity was about 92 m/s. There is not enough data to discuss details yet, but a velocity vector map indicated the existence of a shock wave.(4) Many know-hows are accumulated on a laser-sheet lighting, a particle seeding, a laser pulse interval, a technique to determine the location of impeller blades and so on.(5) A detailed measurement of pressure field between cascade diffuser vanes was carried out. The system to create the structure of unsteady pressure field including shock waves due to rotor-stator interaction was constructed.