Tip leakage aerodynamics over stepped squealer tips in a turbine cascade

Tip gap flow physics and aerodynamic loss generations for two stepped squealer tips of a “Higher Pressure-side rim and Lower Suction-side rim” (HPLS) tip and a “Lower Pressure-side rim and Higher Suction-side rim” (LPHS) tip have been investigated in a turbine cascade. For a fixed tip gap height-to-chord ratio of h/c = 2.0%, oil film flow visualizations are performed on the casing wall as well as on the cavity floor, and three-dimensional flow fields downstream of the cascade are measured with a five-hole probe. For the HPLS tip, the leakage inflow over the pressure-side rim cannot reach the suction-side rim in the upstream region due to the presence of an inlet flow intrusion, and there exists a strong near-wall flow heading toward the trailing edge all over the cavity floor. On the other hand, the LPHS tip has a mid-chord leakage flow penetration into the blade flow passage, and also provides a downstream leakage flow penetration deeper than that for the HPLS tip. Its cavity floor can be divided into a backward flow region and a wide separation bubble. Aerodynamic loss for the HPLS tip, which is nearly identical to that for the cavity squealer tip, is lower than those for the LPHS and plane tips in a considerable degree.