A study on film cooling uniformity effectiveness trends for various coolant hole geometries

Shaped holes have become a dominant geometry in the field of film cooling because of their superior performance over that of cylindrical holes. Holes that promote lateral diffusion increase the spread of the coolant, resulting in better film coverage and an ability to cool even at high blowing ratios. Compound-angle (CA) shaped holes are commonly used to counter the effects of surface curvature on airfoils and circumvent geometric constraints. Previous studies have concluded that increasing the diffusion angle in film cooling holes leads to an improvement in film cooling effectiveness. Discharge coefficient and film cooling effectiveness measurements are conducted to characterize these effects. Computational Fluid Dynamic (CFD) results are also used to compare and validate these test results. Until recently, the uniformity of the coolant flow has not typically been quantified The current study, however, calculates a film cooling uniformity coefficient (CUC) for each of these testcases, defined in previous literature. A combination of CFD and experimental data are used in this study to explore the characteristics of the CUC and effectiveness of a variety of coolant hole geometries. Five different film hole geometries are investigated: (i) cylindrical (CYL), (ii) cylindrical trenched (CYLT), (iii) CA fan (FAN), (iv) CA fan trenched (FANT), and (v) CA fan with a 30 degree injection angle with respect to the free stream velocity (FANA). All coolant hole geometries have a L/D of 7.5, and an injection angle of 35 degrees. Geometries (i), (ii), and (iii) are analyzed using CFD analysis, and geometries (iv) and (v) are experimental data. Temperature sensitive paint, TSP, is the technique used to acquire the temperature distribution downstream of the cooling holes, and thus, ascertain the laterallyaveraged film cooling effectiveness. Data is obtained for blowing ratios ranging from 0.5 to 2.0. The CFD approximation of the CYL configuration was very accurate for the laterally averaged effectiveness at low X/D. However, the maximum effectiveness prediction was higher, and the CUC was slightly lower than the experimental data. At high M, the FAN, FANT, and FANA geometries showed significantly higher laterally averaged and maximum effectiveness values than the cylindrical hole for low X/D values. As X/D increases, the CYL showed a steady increase in the CUC, reaching near perfect uniformity at X/D > 50.