Effect of unsteady wake, free stream turbulence, tip geometry on blade tip flow and heat transfer

Abstract

A comprehensive investigation of the effect of various tip sealing geometries is presented on the blade tip leakage flow and associated heat transfer. The linear cascade is made of four blades scaled up HPT turbine in a low speed wind tunnel facility with the two corner blades acting as guides. The tip section of a HPT first stage rotor blade is used to fabricate the 2-D blade. The wind tunnel accommodates an 116 degree turn in the flow through the blade cascade. The mainstream Reynolds number based on the axial chord length at cascade exit is 4.83e5. The center blade has a tip clearance gap of 1.5625% with respect to the blade span. Static pressure measurements are obtained on the blade surface, the tip surface, and the shroud. Several tip sealing techniques are investigated in this study. Crosswise trip strips are used to reduce leakage flow and associated heat transfer by placing the trip strips in different orientation. Cylindrical pin fins are investigated and compared to the trip strip geometries. A full and three partial squealers are investigated. The partial squealers are placed on the suction side, pressure side and midchord of the blade tip. Detailed heat transfer measurements are obtained using a steady state HSI-based liquid crystal technique. The upstream wake effect is simulated with a spoked wheel wake generator placed upstream of the cascade. A turbulence grid placed even farther upstream generates the required free-stream turbulence of 4.8%. The effect of periodic unsteady wake effect is investigated by varying the wake Strouhal number from 0.0 to 0.2, and to 0.4. Results show that the squealers and the trip strips placed against the leakage flow direction produce the lowest heat transfer on the tips compared to all the other cases. Results also show that the full squealer has a strongest effect on the overall reduction of tip heat transfer.