Free-Form Versus Ruled Inducer Design in a Transonic Centrifugal Impeller

The detailed design of the inducer of a high pressure ratio transonic radial compressor impeller with a design inlet tip relative Mach number of 1.4 is considered. Numerical analysis has been used to compare a datum impeller with ruled inducer design with a number of different free-form design concepts, generated following the same aerodynamic design philosophy. The datum stage and one with a free-form inducer, referred to as ‘barrelled forward swept’, with forward swept leading edge near the tip and increased chord at mid-span, have been manufactured and tested. The tests were performed with the same stationary components, including the casing, vaned diffuser and the volute. The design with a barrelled forward sweep of the inducer allows the designer more control of the strength and position of the passage shock at the inlet while meeting mechanical constraints. Interestingly, the performance is also enhanced at off-design points at lower tip-speeds. The measurements show that the stage tested with the swept impeller achieves higher efficiency of between 0.5% and 1.6% compared to the datum design, depending on the operating speed. The CFD simulations are used to further study the flow at part speeds, in order to explain the causes of the observed performance differences at off design conditions.