Fixed-geometry centrifugal compressors have inherent limitations in the off-design operating range and performance that can be delivered. Variable inlet swirl, typically introduced with variable inlet guide vanes, has been a frequent tool implemented across many industries to extend the stable operating range and off-design efficiency. However, a review of available data in the literature reveals a large disparity in the effectiveness of variable inlet swirl for useful map movement. An ideal configuration responds to preswirl with a significant reduction in the mass flow at instability along with increased efficiency. Unfortunately, some cases reveal no movement in the surge line, with increases in inlet swirl only serving to reduce the stage efficiency.
This study investigates underlying causes for the variation in sensitivity to inlet swirl amongst various compressor designs, represented by form parameters. An analytical model is established and then verified using CFD and selected test data to explore fundamental causes for decreased sensitivity to inlet swirl. Analytical and computational results suggest a new understanding of the governing parameters for inlet swirl sensitivity, and thus provide insight into the scenarios when inlet swirl is an effective tool for manipulation of performance characteristics. Synthesizing the underlying trends, guidelines are developed for design selections and operational situations when inlet swirl can be effectively implemented as a means to extend the stable operating range and off-design efficiency.
