Analysis of the Instantaneous Film Cooling Effectiveness on Showerhead Film Cooling Using Fast Response Pressure Sensitive Paint

The instantaneous effect of unsteady wakes on film cooling effectiveness using a fast response pressure sensitive paint was studied. Testing was performed in a five-passage, low-speed wind tunnel using five times scaled versions of typical high-pressure blades (axial chord of 17 cm). The blowing ratios studied include 0.8 and 1.2, with density ratios of 1.0 and 1.5 at each blowing ratio. Upstream unsteady wakes were produced by a spoked wheel-type wake generator. The wake generator, utilizing a varying number of rods and rotational speeds, produced Strouhal numbers of 0.1 and 0.3 while the wind tunnel inlet mainstream Reynolds number was held at a constant value of 300,000 (mainstream velocity of 21 m/s) for every test case. For each Strouhal number, blowing ratio and density ratio studied, several positions within the period between rods, and the resulting wakes, were recorded. Results show that the location of the wake along a blade's surface had a significant impact on the film cooling effectiveness in the immediate location of the wake. An overall fluctuation of up to 69% in film cooling effectiveness was observed within the wake period. Conclusions also show that while mainstream flows with higher Strouhal numbers tend to decrease film cooling effectiveness, the fluctuation of film cooling effectiveness at higher Strouhal numbers was reduced when compared to results utilizing lower Strouhal numbers. A comparison of results from tests at different density ratios showed a much smaller change in the instantaneous film cooling effectiveness between test cases.