Impact of Inlet Swirl on High-Speed High-Flow Centrifugal Stage Performance

Cooling cycle in applications such as Liquefied Natural Gas makes use of centrifugal impellers associated with heavy gas. Increasing projects handled flow leads compressor manufacturers to try to increase flow coefficient and peripheral Mach Number of those stages to limit compressor size. As a consequence, severe aerodynamic problems linked to the blade aspect ratio, meridional channel curvature and inlet tip relative Mach number arise. Of the three, the onset of transonic or supersonic inlet conditions at tip is a matter of concern since it can significantly reduce stage range and performance. Very aggressive blade redesign is often not possible due to mechanical limitations. An accurate coupling of the impeller with the upstream stator parts may partly overcome the problem. The paper summarizes the design activity of a centrifugal stage designed for inlet pre-rotation, covering the most significant considerations made during its design, starting from 1D preliminary design until three dimensional CFD verifications. Finally, the paper draws conclusions on comparison with the results of an experimental campaign the purpose of which was to evaluate the benefits brought about by the new impeller design and different inlet configurations. The new impeller design allowed to reach still acceptable operating range even with relative Mach Number of 1.05 but with a noticeable efficiency decay. Use of Inlet Guide Vane with prewhirl, to bring Mach down to lower value, allowed to recover efficiency values without any further gain of operating range.