EFFECTS OF NONAXISYMMETRIC TIP CLEARANCE ON AXIAL COMPRESSOR PERFORMANCE AND STABILITY

the effects of circumferentially nonuniform tip clearance on axial com pressor performance and stability have been investigated experimentall y and analytically. A theoretical model for compressor behavior with n onaxisymmetric tip clearance has been developed and used to design a s eries of first-of-a-kind experiments on a four-stage, low-speed compre ssor. The experiments and computational results together show clearly the central physical features and controlling parameters of compressor response to nonaxisymmetric tip clearance. It was found that the loss in stall margin was more severe than that estimated based on average clearance. the stall point was, in fact, closer to that obtained with uniform clearance at the maximum clearance level The circumferential l ength scale of the tip clearance (and accompanying flow asymmetry) was an important factor in determining the stall margin reduction. For th e same average clearance, the loss in peak pressure rise was 50 percen t higher for an asymmetry with fundamental wavelength equal to the com pressor circumference than with wavelength equal to one-half the circu mference. The clearance asymmetry had much less of an effect on peak e fficiency; the measured maximum efficiency decrease obtained was less than 0.4 percent compared to the 8 percent decrease in peak pressure r ise due to the asymmetric clearance. The efficiency penalty due to non axisymmetric tip clearance was thus close to that obtained with a unif orm clearance at the circumferentially averaged level. The theoretical model accurately captured the decreases in both steady-state pressure rise and stable operating range which are associated with clearance a symmetry. It also gave a good description of the observed trends of: ( i) increasing velocity asymmetry with decreasing compressor flow, and (ii) decreasing effect of clearance asymmetry with decreasing dominant wavelength of the clearance distribution. The time-resolved data show ed that the spatial structure of the prestall propagating disturbances in the compressor annulus was well represented and that the stability limiting process could be linked to the unsteady structure of these d isturbance modes. The model was also utilized for parametric studies t o define how compressor performance and stability are affected by the circumferential distribution of clearance, steady-state compressor pre ssure-rise characteristic, and system dynamic parameters. Sensitivity to clearance asymmetry was found to fall off strongly with the (asymme try-related) reduced frequency and to increase with peak pressure rise and increasing curvature of the characteristic near the peak.