The Augmentation of Internal Blade Tip-Cap Cooling by Arrays of Shaped Pins

The objective of the present study is to demonstrate a method to provide substantially increased convective heat flux on the internal cooled tip cap of a turbine blade. The new tip-cap augmentation consists of several variations involving the fabrication or placement of arrays of discrete shaped pins on the internal tip-cap surface. Due to the nature of flow in a 180 deg turn, the augmentation mechanism and geometry have been designed to accommodate a mixture of impingementlike flow, channel flow, and strong secondary flows. A large-scale model of a sharp 180 deg tip turn is used with the liquid crystal thermography method to obtain detailed heat transfer distributions over the internal tip-cap surface. Inlet channel Reynolds numbers range from 200,000 to 450,000 in this study. The inlet and exit passages have aspect ratios of 2:1, while the tip turn divider-to-cap distance maintains nearly the same hydraulic diameter as the passages. Five tip-cap surfaces were tested including a smooth surface, two different heights of aluminum pin arrays, one more closely spaced pin array, and one pin array made of insulating material. Effective heat transfer coefficients based on the original smooth surface area were increased by up to a factor of 2.5. Most of this increase is due to the added surface area of the pin array. However, factoring this surface area effect out shows that the heat transfer coefficient has also been increased by about 20-30%, primarily over the base region of the tip cap itself. This augmentation method resulted in negligible increase in tip turn pressure drop over that of a smooth surface.

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