Large-eddy simulation of film cooling flows at density gradients

The present paper investigates the impact of the velocity and density ratio on the turbulent mixing process in gas turbine blade film cooling. A cooling fluid is injected through an inclined pipe at alpha = 30 degrees into a turbulent boundary layer at a freestream Reynolds number of Re-x = 400,000. This jet-in-a-crossflow (JICF) problem is investigated using large-eddy simulations (LES). The governing equations comprise the Navier-Stokes equations plus additional transport equations for several species to simulate a non-reacting gas mixture. That is, gases of different density are effused into an air crossflow at a constant temperature. An efficient large-eddy simulation method for low subsonic flows based on an implicit dual time-stepping scheme combined with low Mach number preconditioning is applied. The comparison of the numerical findings with experimental velocity data from two-component particle-image velocimetry (PIV) measurements shows an excellent agreement. The results evidence the dynamics of the flow field in the vicinity of the jet hole, i.e., the recirculation region and the inclination of the shear layers, to be mainly determined by the velocity ratio. However, evaluating the cooling efficiency downstream of the jet hole the mass flux ratio proves to be the dominant similarity parameter, i.e., the density ratio of the jet and crossflow fluid has to be considered. (C) 2007 Elsevier Inc. All rights reserved.