Effect of Blowing Ratio on the Nusselt Number and Film Cooling Effectiveness Distributions of a Showerhead Film Cooled Blade in a Transonic Cascade
This paper investigates the effect of blowing ratio on the film cooling performance of a showerhead film cooled first stage turbine blade. The blade was instrumented with double-sided thin film heat flux gages to experimentally characterize the Nusselt number and film cooling effectiveness distributions over the surface of the blade. The blade was arranged in a two-dimensional, linear cascade within a transonic, blowdown type wind tunnel. The wind tunnel freestream conditions were varied over two exit Mach numbers, Me=0.78 and Me=1.01, with an inlet freestream turbulence intensity of 12% , with an integral length scale normalized by blade chord of 0.26 generated by a passive, mesh turbulence grid. The coolant conditions were varied by changing the ratio of coolant to freestream mass flux, blowing ratio, over three values, BR=0.60, 1.0, and 1.5 while keeping a density ratio of 1.7.
Experimental results show that ingestion of freestream flow into the showerhead cooling plenum can occur below a blowing ratio of 0.6. Film cooling increases Nusselt number over the uncooled case and increasing the blowing ratio also increases Nusselt number. At a blowing ratio of 1.5 and Me=1.01 a large drop in effectiveness just downstream of injection on both the pressure and suction surfaces is evidence of jet liftoff. The blowing ratio of 1.0 was found to have superior heat load reduction over the blade surface at both freestream conditions tested. The blowing ratio of 1.0 reduced the heat load by as much as 39% and 32% at Me=0.78 and 1.01, respectively.