A Study of Large Tip Clearance Flows in an Axial Compressor Blade Row

The tip clearance flow of industrial axial compressor rotors has a significant impact on compressor performance. Most tip clearance flow research work has been undertaken in the earlier low-pressure transonic stages of compressors. The main differences between the earlier (low-pressure) and later (high-pressure) stages include blade profile, stagger angle, Mach number, blade length and tip clearance. The tip clearance in the later stages of an industrial axial compressor is relatively large due to mechanical constraints and short blading. The stagger angle is much lower and so the tip clearance flow is at a higher angle to the (negative) axial direction. In the present work, a computational method has been employed to investigate tip clearance flow from 1% span to 10% span for blading such as that found in the later stages. A pinch tip model is used to model the blade tip in a cascade with a stationary and moving end-wall. It has been found that the tip clearance flow rolls up into a vortex much later than in the earlier stages. The migration of the tip clearance vortex across the passage is much less than for the earlier stages and also the induced vortex is much weaker. Comparisons between a cascade with fixed and moving end-walls are made, the main difference being that the tip clearance flow is stronger with a moving end-wall. The 1% tip clearance flow structure with stationary end-wall is shown to be different from all other cases investigated.