CONTROL OF TIP LEAKAGE IN A HIGH PRESSURE TURBINE CASCADE USING TIP BLOWING

Blowing from the tip of a turbine blade was studied experimentally to determine if total pressure loss could be reduced. Experiments were done with a linear cascade in a low speed wind tunnel. Total pressure drop through the blade row and secondary velocity fields in the passage between two blades were measured. Cases were documented with various blowing hole configurations on flat and squealer tipped blades. Blowing normal to the tip was not helpful, and in some cases increased losses. Blowing from the bottom of a squealer cavity provided little benefit. With a flat tip, blowing from holes located near and inclined toward the pressure side generally reduced total pressure drop by reducing the effect of the tip leakage vortex. Holes near the axial location of maximum loading were most helpful, while holes closer to the leading and trailing edges were not as effective. Higher jet velocity resulted in larger total pressure drop reduction. With a tip gap of 1.5% of axial chord, jets with a velocity 1.5 times the cascade inlet velocity had a significant effect. A total pressure drop reduction of the order 20% was possible using a jet mass flow of about 0.4% of the main flow. Jets were most effective with smaller tip gaps, as they were more able to counter the leakage flow.