Film Cooling: Breaking the Limits of Diffusion Shaped Holes

The study of gas turbine film cooling encompasses several thousands of technical papers since the 1960's. Yet today virtually all applications use only two types of film hole geometries, round holes and diffuser shaped holes. There has been no substantive improvement upon diffuser shaped film hole adiabatic effectiveness that has made its way into production turbine components. The present study is an examination of why this apparent limitation has not been overcome. Thirty examples of proposed discrete holes film cooling geometries, including the standard bearer of diffuser shaped holes, are viewed to discern what is possible, what the typical detractors to implementation are, and how the community might go forward to entitlement. Eight key factors are used to judge the application potential for each of the film hole geometries. In the end, while the diffuser shaped film hole is still the best option today, a few other geometries are identified as candidates for significant improvement in film effectiveness. Three examples of film cooling with increasing levels of improvement over diffuser shaped holes are presented for consideration.