Trailing-edge cooling for gas turbines

The trailing-edge section of modern high-pressure turbine airfoils is an area that requires a high degree of attention from turbine performance and durability standpoints. Aerodynamic loss near the trailing edge includes expansion waves, normal shocks, and wake shedding. Thermal issues associated with trailing edge are also very complex and challenging. To maintain effective cooling ensuring metal temperature below design limit is particularly difficult, as it needs to be implemented in a relatively small area of the airfoil. To date, little effort has been devoted to advancing the fundamental understanding of the thermal characteristics in airfoil trailing-edge regions. Described in this paper are the procedures leading to closed-form, analytical solutions for temperature profile for four most representative trailing-edge configurations. The configurations studied are 1) solid wedge shape without discharge, 2) wedge with slot discharge, 3) wedge with discrete-hole discharge, and 4) wedge with pressure-side cutback slot discharge. Comparison among these four cases is made primarily in the context of airfoil metal temperature and resulting cooling effectiveness. Further discussed in the paper are the overall and detail design parameters for preferred trailing-edge cooling configurations as they affect turbine airfoil performance and durability. Also described in this treatment is a current experimental investigation of heat transfer over a trailing-edge configuration. The trailing edge is preceded with an internal cooling channel of pedestal array. The pedestal array consists of both circular pin fins and oblong shaped features or exit teardrops. Downstream to the pedestal array, the trailing edge exits in a pressure side cutback partitioned by the oblong-shaped teardrops. The local heat-transfer coefficient over the entire wetted surface in the internal cooling chamber has been determined using a "hybrid" measurement technique based on transient liquid crystal imaging. The hybrid technique employs the transient conduction model in a semi-infinite solid for resolving the heat-transfer coefficient on the end-wall surface uncovered by the pedestals. The heat-transfer coefficient over a pedestal can be resolved by the lumped capacitance method with an assumption of low Biot number. The overall heat transfer for both the pedestals and end-walls combined shows a significant enhancement compared to the case with thermally developed smooth channel. Near the most downstream section of the suction side, the land, caused by pressure side cutback, is exposed to the stream mixed with hot gas and discharged coolant. Both the adiabatic effectiveness and heat-transfer coefficient on the land section are characterized by using this hybrid liquid-crystal technique.