EFFECTS OF ROTATING INLET DISTORTION ON MULTISTAGE COMPRESSOR STABILITY

In multispool engines, rotating stall in an upstream compressor will i mpose a rotating distortion on the downstream compressor, thereby affe cting its stability margin. In this paper experiments are described in which this effect was simulated by a rotating screen upstream of seve ral multistage low-speed compressors. The measurements are complemente d by, and compared with, a theoretical model of multistage compressor response to speed and direction of rotation of an inlet distortion. Fo r corotating distortions (i.e., distortions rotating in the same direc tion as rotor rotation), experiments show that the compressors exhibit ed significant loss in stability margin and that they could be divided into two groups according to their response. The first group exhibite d a single peak in stall margin degradation when the distortion speed corresponded to roughly 50 percent of rotor speed The second group sho wed two peaks in stall margin degradation corresponding to distortion speeds of approximately 25-35 percent and 70-75 percent of rotor speed . These new results demonstrate that multistage compressors can have m ore than a single resonant response. Detailed measurements suggest tha t the two types of behavior are linked to differences between the stal l inception processes observed for the two groups of compressors and t hat a direct connection thus exists between the observed forced respon se and the unsteady flow phenomena at stall onset. For counterrotation al distortions, all the compressors rested showed minimal loss of stab ility margin. The results imply that counterrotation of the fan and co re compressor, or LP and HP compressors, could be a worthwhile design choice. Calculations based on the two-dimensional theoretical model sh ow excellent agreement for the compressors, which had a single peak fo r stall margin degradation. We take this first-of-a-kind comparison as showing that the model, though simplified, captures the essential flu id dynamic features of the phenomena. Agreement is not good for compre ssors that had two peaks in the curve of stall margin shift versus dis tortion rotation speed. The discrepancy is attributed to the three-dim ensional and short length scale nature of the stall inception process in these machines; this includes phenomena that have not yet been addr essed in any model.