Simulations of Slot Film-Cooling With Freestream Acceleration and Turbulence

Slot film cooling in an accelerating boundary layer with high free-stream turbulence is studied numerically using Large Eddy Simulations (LES). Recent cooling designs of turbine airfoils (such as double-wall cooling) enable slot cooling configurations that are known to provide improved cooling effectiveness over discrete hole cooling systems. Calculations are done for a symmetrical leading edge geometry with the slot fed by a plenum populated with pin fins. To generate the inflow turbulence, the Synthetic Eddy Method (SEM) is used by which the turbulence intensity and length scales in each direction can be specified at the inflow. Different levels of turbulence are imposed at the inflow cross-plane. For the inflow at the plenum, an a priori simulation has been performed in the plenum with pin fins, and the velocity signals are stored at a plane downstream of the pin fins over a sufficient period of time, and are used as the inflow boundary condition in the plenum. Calculations are done for a Reynolds number of 250,000 and freestream turbulence levels of 0.7%, 3.5%, 7.8% and 13.7% are reported. These conditions correspond to the experimental measurements of Busche and Ames (2014). Numerical results show good agreement with experiment data and show the observed decay of thermal effectiveness with turbulence intensity. The turbulence and non-uniformity exiting the slot are shown to play an important role in the cooling effectiveness distributions downstream of the slot. To provide a better understanding of the flow physics and heat transfer, the mean flowfield and turbulence statistics are studied. Generation of freestream structures is observed at the leading edge, and the amplification of the corresponding fluctuations downstream is identified as one of the parameters influencing the slot cooling performance. Predictions show the higher growth rate of the thermal boundary layer with increasing turbulence which is a clear indication of the increase in turbulent thermal diffusivity and reduction of the effective turbulence Prandtl number. The self-similar temperature profiles deviate from those measured under low freestream turbulence condition.