A Flow Visualization Study of Axial Turbine Tip Desensitization by Coolant Injection From a Tip Trench

The effect of coolant injection from a tip trench was investigated in a large-scale rotating turbine rig. Coolant is injected into the tip gap from discrete injection holes, located in a tip trench and directed towards the pressure-side. Surface flow patterns are visualized by a mixture of oil and paint. The mixture is applied on the blade pressure-side and allowed to seep onto the tip platform. Injection rates of 0.4%, 0.5%, 0.6%, and 0.7%, at a gap height of 1.40% blade height were investigated. Flow patterns for a gap height of 0.72% blade height are compared to the larger clearance gap. The flow visualization technique successfully identifies flow features like pressure-side edge separation, and reattachment and recirculation on the tip surface. The location of the reattachment line from the pressure-side edge varies little along the length of the blade and occurs at about two gap heights from the pressure-side edge. At the large gap height the tip gap flow is fully separated over the last 5% of the blade axial chord. Surface oil flow lines are directed almost normal to the camberline along most of the tip surface. Flow patterns with injection indicate that the ejected coolant effectively blocks the leakage flow. The coolant jets are turned towards the blade suction-side and appear to form a film on the tip surface. Some of the visualization material is carried by the leakage flow into the passage and is deposited on the blade suction surface, thereby giving an indication of the inception and growth of the leakage vortex. Suction surface patterns with injection indicate that leakage flow may be entering the adjacent passage at multiple locations.