High-Speed Velocity Measurements of Film Cooling Applications at High-Turbulence Main Flow Conditions

High-turbulence flow conditions in gas turbine combustors create a rough environment for film cooling flows. Fundamental knowledge of the interaction between main flow fluctuations and cooling jet is essential to develop more efficient cooling geometries. To gain detailed insight into the turbulent flow structures, high-speed PIV measurements have been carried out in a closed loop, thermal wind tunnel facility. Velocity fields with a temporal resolution of 6 kHz and a vector spacing of 0.5 mm have been extracted from the recorded data. Measurements have been performed for a density ratio of DR = 1.2 and momentum ratios of I = 2, I = 8 and I = 15. A Makita style active turbulence grid has been used to create a main flow turbulence intensity of Tu = 18.5% and Tu = 21.5%. Mean flow fields and dominant flow structures that are present in the time-resolved velocity fields are analyzed and discussed. Finally, results for high-turbulence main flow conditions are compared with results from low-turbulence (Tu = 1%) thermographic PIV data, carried out in previous measurements. It can be observed that low-momentum jets are highly fragile. Both small and large vortices affect the jet trajectory. Depending on the current flow situation, the jet lifts off the surface or gets pushed towards the wall. The same is true for high momentum ratios and large eddies. On the other hand, small eddies cannot significantly affect high-momentum jets. The vortex gets deflected by the jet and its intensity is reduced while interacting with the high-turbulence shear layer fluctuations. All in all, turbulence can have a positive effect on the cooling film by steering the jet trajectory towards the surface. On the other hand, however, vortices can enhance the jet detachment and thus reduce the cooling efficiency of the film.