Effects of flow compressibility and density ratio on film cooling performance

An experimental investigation was performed to examine the effects of flow compressibility and density ratio of coolant to mainstream flows on the performance of film cooling injected from a row of cylindrical holes over a flat plate. The experimental study was conducted in a transonic wind tunnel. A pressure-sensitive paint technique was used tomapthe corresponding adiabatic film cooling effectiveness distributions over the surface of interest, based on a mass flux analog to traditional temperature-based cooling effectiveness measurements. It was found that, at the relatively low blowing ratio of M ≤ 0.40, the flow compressibility had almost no effects on the film cooling effectiveness over the surface of interest. At relatively high blowing ratios ofM ≥ 0.85, the film cooling effectiveness for the test case with compressible, transonic mainstream flow was found to become marginally better than that with the mainstream being incompressible. Although the density ratio was found to have limited effects on the film cooling performance at relatively low blowing ratios, it was found to affect the film cooling effectiveness substantially at relatively high blowing ratios. A denser coolant flow would result in a better film cooling protection to the surface of interest.