Film Cooling Measurements for Novel Hole Configurations

The effect of film cooling slot exit area and shape is studied. Detailed heat transfer coefficient and film effectiveness measurements are obtained simultaneously using a single transient IR thermography technique. Two designs with a crescent shaped exit and a slot exit are considered. The results show that the crescent shaped exit provides significantly higher film cooling effectiveness than the cylindrical hole exit at all blowing ratios. The converging slot exit provides similar effectiveness as the crescent for higher blowing ratios. However, the crescent shape also enhances heat transfer coefficients significantly. Overall effectiveness for both crescent and converging slot exits are clearly superior to the standard cylindrical hole. With increases in turbine inlet temperature, gas turbine designers are looking to improve cooling effectiveness without raising the amount of coolant usage. Typically film cooling is used to protect the blade outer surface by injecting cooler air through holes or slots on the blade surface. Inclined cylindrical holes have been used due to convenience of manufacturing. The cylindrical holes are extremely ineffective for higher coolant-to-mainstream velocity ratios due to jet lift-off and lack of surface protection. Also, the jets do not spread laterally resulting in non-existing coverage for areas between the holes. Two dimensional slots perform better but it is difficult to maintain structural integrity with a transverse slot running along the blade span. The focus of this investigation is to investigate advanced cooling hole geometries on film cooling heat transfer and cooling effectiveness over flat and turbine airfoil surfaces. The basic aim of these proposed geometries is to allow more uniform spreading of the jet along the surface resulting in uniform higher cooling. Two hole exits are studied, the first one is a crescent shaped hole exit that increases the coolant flow surface area and also reduces the uncovered area. The hole is not been investigated in any previous study and is a novel contribution by the present authors. The second is a converging slot where the flow enters a cylindrical hole and exits from a converging 2-D slot. In the present study, a transient infrared thermography technique is used for obtaining both heat transfer coefficient and film effectiveness from a single test. The transient IR technique is based on the two-equation, single test proposed by Vedula and Metzger [1] and was demonstrated successfully by Ekkad et al.[2]. In this test, two images with surface temperature distributions are captured at two different times during the transient test. The test is typical of a transient liquid crystal technique experiment wherein the test surface at ambient is suddenly exposed to a hot mainstream and cold coolant jet. The surface temperature response is captured by the IR camera at different time instants during the transient test. Simultaneous η and h distributions are investigated and presented, on the flat surface downstream of injection, for various blowing ratios for cylindrical holes and for the crescent and converging slot exits.