Large eddy simulation of compressible, shaped-hole film cooling

Shaped-hole film cooling is an essential cooling technology in modern gas turbine engines. The performance of a given film cooling design, and thus the life of the cooled part, is dependent on the scenario parameters that define the environment in which it operates, including Reynolds and Mach numbers, upstream boundary layer and turbulence characteristics, blowing ratio, density ratio, pressure gradient, surface curvature, etc. Of these, one parameter that has received relatively little attention in the open literature is Mach number. This work evaluates the role of Mach number in shaped-hole film cooling performance by comparing results of two large eddy simulations of a plenum-fed 7-7-7 hole cooling a flat plate at freestream Mach numbers of 0.25 and 0.5, with all other relevant scenario parameters fixed. This comparison shows important differences between the two cases. To begin, the performance, as measured by laterally-averaged adiabatic effectiveness, is dramatically different, with the high Mach case performing up to 50% worse. This decrease in performance at high Mach is due to the coolant jet separating from the surface and a large scale oscillation that leads to significant asymmetry in the time averaged flow, features which are absent in the low Mach case. These changes appear to be caused by changes to the flow in the cooling hole, which is also asymmetric in the high Mach case. This asymmetry leads to entrainment of hot gas into the hole on one side. Further, in the high Mach case, the flow in the hole is mildly supersonic and has sequence of weak shocks, which are, of course, absent in the low Mach scenario. (C) 2019 Published by Elsevier Ltd.