We present highly resolved large eddy simulation (LES) of a realistic effusion cooling geometry. The test case features mixing between a high-density working fluid (CO2) and a low-density crossflow (air) to mimic the density stratification in typical gas turbine cooling flows. The coolant CO2 is fed from a plenum into the air channel through a series of 52 inclined orifices in a staggered arrangement. Highly detailed LES computations presented in this study resolved entire 52 cooling holes to accurately capture realistic jet-to-jet interactions which are critical in cooling film formation. Two different blowing ratios (M) are analyzed, comparing predictions obtained from wall-modeled and wall-resolved grids. M is defined as the density times the velocity of the coolant divided by that of the air channel. These values are chosen so that the coolant jets are attached to the wall (M=0.457) in one case, and detached (M=1.22) in the other. Results show that for the two blowing ratios analyzed, the wall-resolved adiabatic effectiveness based on CO2 concentration compares favorably with that obtained from pressure sensitive paint (PSP) measurements. The mixing and turbulence characteristics upstream and downstream of the jets are characterized using the probability density function of CO2 concentration and its impact on cooling effectiveness. It is found that the lower blowing ratio case provides more initial cooling than the higher blowing ratio case because the cooling jets attach to the surface and induce interactions among adjacent columns of jet streams. This provides more uniform film coverage and therefore higher initial cooling effectiveness. In the higher blowing ratio case, however, this interaction is significantly delayed due to lift-off of the coolant jets. PDF analysis shows interactions between adjacent cooling stream columns near the wall do not occur until the last jets. Although both the wall-resolved (WR) and wall-modeled (WM) cases show consistent trends with the PSP measurement, the wall-resolved cases show better quantitative agreement overall.

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