Particle Image Velocimetry Characterization of High-Speed Centrifugal Compressor Impeller-Diffuser Interaction

High-density instantaneous and rotor phase-locked velocity field measurements are made in the centrifugal compressor diffuser vaneless space by means of the particle image velocimetry technique. The technique is challenged in this application by a small length scale and large velocities yielding very small flow time scales and demanding the use of extremely small flow tracking particles. The experiments reveal a detailed picture of the rotor phase-locked average impeller exit and diffuser inlet flowfield as is useful for unsteady flow modeling. Strong passage velocity gradients exist across the vaneless space up to the diffuser vanes, showing that the phase-locked averaged impeller exit flow presents a highly unsteady non-mixed-out velocity field for the downstream vane row. This unsteady vane intake is thus characterized by a strong fluctuation in incidence angle with the passing blade event. Measured time variation in the velocity field indicates a significant time-varying potential field due to incidence fluctuations.