Design Modification of a Passive Tip-Leakage Control Method for Axial Turbines: Linear Cascade Wind Tunnel Results

Tip-leakage losses can contribute up to one third of the overall losses in unshrouded axial turbine blades. A passive tip-leakage flow control method is used to reduce the tip-leakage loss. Taking into account a modified discharge coefficient model, an inclination of the injection against the tip-leakage flow direction is said to have an even better effect on reducing the tip-leakage loss. To prove the effect, linear cascade measurements have been carried out at three different gap widths from 0.85% to 2.50% chord length. The used geometry is an up-scaled turbine blade tip cross section with weak turning. A single blade is fitted with an injection channel which is inclined by 45° against the tip-leakage flow direction. The flow field of the modified blade was measured 0.31 axial chord length downstream of the cascade using a pneumatic five-hole probe. The tip-leakage loss is reduced by passive tip-injection and further by inclined injection. The reduction can be significant at small gap widths. Detailed results are presented for a gap width of 1.40% chord length.