Experimental Transonic Centrifugal Compressor Investigation: Loading Effects on Deterministic Diffuser Velocity Fields

The future of small gas turbines relies on higher-efficiency and higher-pressure-ratio centrifugal compressors for lighter, more efficient, and reliable designs. An efficient diffuser is essential to the performance, durability, and operability of a modern compressor stage. The diffuser entry flow in a high-speed centrifugal compressor is highly unsteady and complex, featuring shocks, boundary-layer/shock interactions, and large incidence variations imposed on the diffuser vanes. The observedflow structures are not only unsteady, but they are also strongly dependent on the steady compressor-stage loading. To characterize the variation in diffuser flow at different operating points of a modern transonic centrifugal compressor, particle image velocimetry experiments are conducted in the Purdue University high-speed centrifugal compressor facility. The data at all loading conditions demonstrated that the flowfield in the diffuser is characterized by a much more complicated structure than that associated with steady uniform diffusion. The semivaneless-space acceleration region’s size is shown to be weakly dependent on loading, whereas the diffuser throat structures are strong functions of the loading condition.