The Effect of Diffuser Shape for Film Cooling Holes With Constant Expansion Angles and Area Ratio

Abstract

Shaped film cooling holes are used in gas turbine components to deliver coolant to the high temperature surfaces of turbine blades and vanes to improve their durability. In general, shaped holes are created by expanding the outlet of the hole, resulting in a large area at the outlet of the hole that diffuses the flow. It has been shown in past studies that increasing the diffuser outlet to meter inlet area ratio causes a lower average momentum of the coolant jet at the hole exit, thereby producing better cooling performance. Instead of increasing the size of the diffuser section by increasing the area ratio, the present study focuses on changing the cross-section shape of the diffuser. This is done to mimic changes observed in the diffuser shape of conventionally manufactured film cooling holes. The present study utilizes 10-10-10 diffuser expansion angles and maintains a constant diffuser to meter area ratio. However, the diffuser shape is varied by changing the diffuser edge angle, κ, located between the diffuser sidewall and the diffuser downstream wall. Three film cooling hole shapes were tested using three different diffuser edge angles, resulting in a narrow outlet, a wide outlet, and a standard outlet film cooling hole. Each hole shape was tested in a large wind tunnel with coolant supplied to the film cooling holes at three different blowing ratios by a co-flow and counterflow delivery channel, similar to the delivery method in a turbine vane with an internal baffle. In addition, the film cooling holes were tested with simulated diffuser roughness. Adiabatic effectiveness measurements indicate that film cooling hole performance is most impacted by diffuser roughness. The film cooling hole shape arising from the diffuser edge angle directly impacts the sensitivity to blowing ratio and coolant feed direction. Therefore, it is recommended that manufacturing of film cooling holes focus on reducing roughness in the diffuser for the highest performance. It is also recommended that the tolerance of the film cooling hole shape be biased towards wider film cooling holes to minimize sensitivity to the blowing ratio and coolant feed direction.