Gas Turbine Blade Tip and Near Tip Heat Transfer with Film Cooling

Abstract

Gas turbine blade tips experience very high thermal loads due to high temperature combustion gases and tip leakage flows. This leads to the tip region being very susceptible to experiencing failures. To better understand these effects, experiments were performed in a suction-type low speed wind tunnel cascade to determine heat transfer coefficients and film cooling effectiveness distributions on a highly-loaded 2-D gas turbine blade model in the tip and near-tip regions. The blade shape was generated to match the pressure coefficients of the operating turbine blade design. The model featured multiple tip and pressure side film cooling holes and a squealer tip geometry. The Reynolds number based on axial chord and cascade inlet velocity for all experiments was 80,000. The heat transfer coefficients and film cooling effectiveness values are presented for the tip, pressure side, and suction side at several blowing ratios for each view. The tip gap was fixed at .85% span for all experiments. Tip heat transfer coefficients are shown to be largest near the leading edge for all blowing ratios. Large differences exist in both the heat transfer coefficient and film cooling effectiveness contours between the upper two and the lower two blowing ratios as ―lift-off‖ of the tip purge jets is observed with increasing blowing ratio. Pressure side heat transfer coefficients show subtle differences in the form of increasing heat transfer coefficient with blowing ratio near and downstream of the coolant holes. Film cooling effectiveness is largest immediately downstream of the pressure side holes and on the suction side squealer rim. The suction side heat transfer coefficients exhibit only subtle differences with changes in blowing ratio. The film cooling effectiveness on the suction side is seen to gradually increase with blowing ratio; primarily in the region of the leakage vortex. Blowing ratio has a significant effect on the tip and pressure side heat transfer coefficients and film cooling effectiveness. While on the suction side, the effect is minimal on heat transfer coefficients, but significant on film cooling effectiveness.