Influence of Flow Structure on Compound Angled Film Cooling Effectiveness and Heat Transfer

Film cooling in turbine blades involves injecting cold air through small holes over the surface of the blade to thermally protect it against the incoming hot freestream. Compound angled film cooling, in which the injected jet is angled laterally with respect to the streamwise flow direction, is used in industrial designs owing to their lower cost of manufacture compared with shaped geometries but a high coolant spread. The current study incorporates flow structure measurements of film cooling injection flows inclined at 30 deg to a flat surface with lateral angles of 15 deg, 60 deg, and 90 deg to the freestream. Blowing ratios of 1–2 and density ratios of 1–1.5 are studied. Three dimensional velocity measurements are carried out through high resolution stereoscopic particle image velocimetry. It is observed that the typical counter-rotating vortex pair structure associated with streamwise coolant injection is replaced with a single large vortex, which causes a more lateral spread of the coolant. Infrared thermography measurements are made for the same operating points using the super position principle, which allows calculation of adiabatic effectiveness and heat transfer coefficient. The adiabatic effectiveness is high at low blowing ratios for compound angled injection due to greater proximity of the coolant jet to the wall. At higher blowing ratios, the detrimental effects on effectiveness due to jet lift-off are counteracted by the greater coolant spread due to asymmetric primary vorticity. The heat transfer coefficient is also enhanced especially in the downstream region for high compound angles. The average heat transfer coefficient due to very large compound angles is not very sensitive to changing momentum flux ratios.