Surface film cooling characteristics of rectangular holes along the trailing edge of a transonic turbine blade tip

The present study investigates film cooling in the trailing edge region of a transonic turbine blade tip, with an arrangement which is significantly different from all previous blade tip investigations. In particular, five rectangular film cooling holes, located along the tip near the blade trailing edge and positioned downstream of a squealer recess region, are employed. Each film cooling hole is oriented at a 45° angle relative to the blade tip surface. Heat transfer coefficient ratio data, and adiabatic film cooling effectiveness data are provided for two tip gaps over a range of blowing ratios. Tip gap magnitudes are 1.4 mm and 0.8 mm, which correspond to 1.16% and 0.66% of the true blade span, respectively. Surface heat transfer characteristics are measured using a transient impulse-response approach with infrared thermography. Along most of the trailing edge tip surface, adiabatic film cooling effectiveness values are locally augmented for all blowing ratios BR and for both tip gaps considered, which evidences substantial film coolant concentrations near the blade tip surface. Local values of adiabatic film cooling effectiveness often increase with increasing blowing ratio and are consistently higher for the smaller tip gap. In general, only minor differences between the 0.8 mm and 1.4 mm tip gap heat transfer coefficient ratio data sets are present, with marginally higher values of heat transfer coefficient ratios for the smaller tip gap. Together, heat transfer coefficient ratio and adiabatic film cooling effectiveness data suggest that the thermal protection offered by the 0.8 mm tip gap along the trailing edge tip region is superior to that of the 1.4 mm tip gap for a range of blowing ratios.