Experimental investigation of the influence of flow structure on compound angled film cooling performance

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 to shaped geometries but high coolant spread. The current study incorporates flow structure measurements of film cooling injection flows inclined at 30 to a flat surface and lateral angles of 15, 45, 60 and 90 to the freestream. Blowing ratios of 1-2 and density ratios of 1-1.5 are studied. The density ratios are obtained by maintaining the freestream and coolant flows at different temperatures. Three dimensional velocity measurements are carried out through high resolution stereoscopic particle image velocimetry (PIV). 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 more lateral spread of the coolant. Infra red thermography measurements are made for the same operating points using the super position principle which allows calculation of adiabatic effectiveness and film heat transfer coefficient. The detrimental effects of jet lift-off at high momentum flux ratios are counteracted by greater coolant spread downstream by the asymmetric vorticity. The streamwise circulation decay with downstream distance for compound angled holes depends on the orientation angle and scales with a momentum flux ratio scaled downstream distance. The boundary layer pile-up is observed to increase with greater lateral orientation of the injection holes. Heat transfer is generally enhanced with greater compound angles and increasing momentum flux ratios. One reason for this enhancement is the increased wall-normal vorticity representing coherent unsteadiness in the near wall region. A blowing ratio pulsation of amplitude 0.3 is applied at two hole diameter based reduced frequencies of 0.01 and 0.017 for a 0 and 60 compound angled injection. The velocity flow structure shows that due to the lateral sweeping motion of the compound angled jet the wake zone is flattened and laterally spread, while the jet is closely attached to the wall. At high blowing ratios, pulsations do not have a very significant effect but cause some enhancement in cooling effectiveness due to lateral coolant spread for compound angled injection. The heat transfer coefficient increases with pulsations compared to a steady jet for all downstream distances as the pulsations enhance coherent unsteadiness. Aerothermal losses are also quantified for pulsating and steady, streamwise and 60 oriented compound angled film cooling jets. The entropy change, derived from pressure and temperature measurements, is calculated by using modified reference conditions to better reflect the losses in both the jet and the freestream. Mass averaged entropy and kinetic energy loss coefficients seem to be apt quantities for comparing aerothermal performance of streamwise and compound angled injection. It is observed that the mass averaged entropy loss coefficient of a streamwise jet doubles when it is pulsated whereas that of a compound angled jet increases by around fifty