Active control of tip clearance flow through casing air injection in axial turbines

Abstract

This paper presents a numerical investigation of an effective method for controlling tip clearance flow in axial turbines. Cooling air is injected into the rotor passages from the casing wall through 10 discrete holes per rotor pitch, in a direction that is opposite to the tip clearance flow. Emphasis is played on the analysis of the complicated, three‐dimensional flow structures within the tiny tip gap, which is caused by the interaction between tip clearance flow and the high pressure injection flow. The results indicate that due to the obstruction by injection, less passage flow is entrained into tip clearance in all the cases considered. With air injection, the associated losses of both the tip clearance vortex and the tip passage vortex are reduced significantly. The heat transfer condition is improved noticeably caused by the cooling air. Besides that, it can be found that the more downstream the injection is distributed, the more the tip clearance vortex is influenced and the less thermal benefits obtained from the cooling jet. In the current study, the optimum isentropic efficiency occurs when 1% of the passage mass flow is injected at 30% of the axial chord from the leading edge, which has been improved by 0·3341%.