Turbine blade tip cooling and heat transfer

Abstract

Turbine blade tip leakage flow from blade pressure side to suction side over the tip surface increases the thermal loading to the blade tip, leading to a high local temperature and thus, is considered one of the primary sources of blade failure. Leakage flow can be reduced by using a recessed or squealer tip blade or by cooling the blade tip to incorporate film cooling. The performance of different recessed tip geometries were investigated and compared with plane tip performance. A transient liquid crystal technique was employed to measure detailed heat transfer coefficient distributions. Coolant injection from holes located on the blade tip, near the tip along the pressure side and combination cases were also investigated. Experiments were performed for plane tip and squealer tip for different coolant to mainstream blowing ratios of 1.0, 2.0, and 3.0. A transient infrared (IR) thermography technique was used to simultaneously measure heat transfer coefficient and film cooling effectiveness. Static pressure distributions on the shroud were measured to obtain some understanding about the leakage flow direction and its effect on surface heat transfer distribution. All tests were performed in a four bladed, stationary, linear cascade. The Reynolds number based on cascade exit velocity and axial chord length was 8.61$21510$1785 and the inlet and exit Mach numbers were 0.16 and 0.55, respectively. Tip gap-to-blade span ratio of 1% and recess depth-to-blade span ratio of 4.16% was used. Results showed that performance of the full pressure side squealer was the poorest and that a squealer tip performs better than the plane tip blade. Tip injection reduced heat transfer coefficient on the blade tip and an increase in blowing ratio caused a decrease in heat transfer coefficient for both plane and squealer tip blade. For pressure side film injection, film effectiveness was lower than tip injection. Some reattachment of jets was observed at blowing ratio of 3.0 for plane tip. For squealer tip, almost no film effectiveness was observed. In case of combination cooling, very high film effectiveness and heat transfer coefficient is observed at blowing ratio of 3.0 for both plane and squealer tip blade tip.