Large Tip Clearance Flows in High Pressure Stages of Axial Compressors

Abstract

This thesis investigates over tip leakage where the tip clearance is large. In the high pressure stages of axial compressors the tip clearance can be typically 6% of span and the total blockage due to tip clearance can consume in excess of forty percent of the annulus height. Experimental and computational investigations of large tip clearance in a linear cascade have been used to investigate this phenomenon.

Two cascade builds have been used the first (Build A) consisted of a controlled diffusion aerofoil of low stagger and thirty degrees flow turning. The second cascade (Build B) consisted of an engine representative design with high stagger and around ten degrees of flow turning. The diffusion factor of both cascades was around 0.3. The major findings are that: Large tip clearances have a smaller detrimental influence on single row performance than the previous research would have suggested, for Build B the loss at 10% tip clearance was the same as the 0% tip clearance loss, though the overall flow turning was much reduced. An increase in blade loading towards the tip was observed with both builds. Both these phenomenon were attributed to the small amount of movement of the over tip leakage vortex.

An engine representative level of inlet skew was implemented using upstream injection so to assess its influence. This was found to have a remarkably small influence on the performance of a single row with the tip clearance and geometry of the blading having a much greater influence.

Finally a circumferential grooved casing treatment was applied in the linear cascade but this was found not to be an appropriate tool for such an investigation.