Large-eddy simulations of leading edge film cooling: Analysis of flow structures, effectiveness, and heat transfer coefficient

A numerical investigation is conducted to study leading edge film cooling with large eddy simulation (LES). The domain geometry is adopted from an experimental setup of [Ekkad, S.V., Han, J.C., Du, H., 1998. Detailed film cooling measurement on a cylindrical leading edge model: Effect of free-stream turbulence and coolant density. Journal of Turbomachinery 120, 799-807.] where turbine blade leading edge is represented by a semi-cylindrical blunt body with compound angle of injection. At blowing ratio of 0.4 and coolant to mainstream density ratio of unity, a laminar constant velocity and fully-turbulent coolant jet are studied. In both cases, the results show the existence of an asymmetric counter-rotating vortex pair in the immediate wake of the coolant jet. In addition to these primary structures, vortex tubes on the windward side of the jet are convected downstream over and to the aft- and fore-side of the counter-rotating vortex pair. All these structures play a role in the mixing of mainstream fluid with the coolant. The fully-turbulent coolant jet increases mixing with the mainstream in the outer shear layer but does not directly influence the flow dynamics in the turbulent boundary layer which forms within two coolant hole diameters of injection. As a result, the turbulent jet decreases adiabatic effectiveness but does not have a substantial effect on the heat transfer coefficient. The span-wise averaged adiabatic effectiveness agrees well with experiments for a turbulent coolant jet, without which the calculated effectiveness is over-predicted. On the other hand, the heat transfer coefficient which is only a function of near wall turbulence, shows good agreement with experiments for both coolant jet inlet conditions.