CFD SIMULATION OF JET PULSATION EFFECTS ON FILM COOLING OF FLAT PLATES

摘要

In this paper fourteen different CFD cases for CFH (cylindrical film hole) and LDIFF (film hole with laterally diffused exit) geometries were conducted to study the film cooling of flat plates. Those cases included different blowing ratios: 0.5, 1.25 and 1.5 and both steady flow and pulsed jets. In the jet pulsation cases the Duty Cycle was taken 50% and the Strouhal number ranged from 0.0119 to 1.0 for the CFH geometry and from 0.0119 to 0.38 for the LDIFF geometry. Fluent commercial code with realizable k - ε turbulence model was used in this study to investigate how the pulsed jet performance was affected by varying: 1) pulsation frequency,2) blowing ratio and 3) jet geometry.For the CFH geometry (B=0.5) the pulsed jet showed lower film cooling effectiveness than the steady state for all cases examined. However, the frequency effects varied according to the downstream location from the jet exit. Immediately near the jet trailing edge the effectiveness increased as the frequency increased. Downstream (x/D above3) the effectiveness for both St=0.0119 and 0.38 almost agreed while lower effectiveness were noted for St= 0.19. For St=1.0, the effectiveness was above the other frequencies (for all x/D values) but still below the steady state ones. As for the LDIFF geometry (B=1.25) the effect of frequency was negligible and the pulsed jet showed lower film cooling effectiveness than the steady state.Two different blowing ratios (0.5 and 1.5) were examined for the CFH geometry. The pulsation had different effects in the two cases. At B=0.5, lower effectiveness performance everywhere was obtained for pulsed cases compared to steady ones. For B=1.5 pulsation results were highly dependent on the frequency. For low frequency (St=0.0119) the effectiveness was below the steady state one for all x/D values. For higher frequency (St=0.38) the effectiveness was higher than thesteady state one for all x/D values. As for St=0.19 and 1.0 the results were in between the above two frequencies.A spatially averaged effectiveness was developed to enable comparing "overall" performance of all cases examined. This was done by choosing an area downstream of the jet that covered from the jet "trailing edge", x/D = 0 to x/D =10 and also covered a 1/2 pitch on both sides of the jet in the spanwise direction. Using the defined spatially averaged effectiveness with 50% of the coolant (Duty Cycle) an overall reduction in the film cooling effectiveness was found to be: 52.73% for the LDIFF (B=1.25), 38.12% for the CFH (B=0.5) and an overall enhancement of 14.77% for the CFH (B=1.5). Knowing that low effectiveness in case of B=1.5, CFH geometry was caused by the jet "lift-off, results above clearly indicate that jet pulsation will be more effective for cases with detached jet under steady state conditions.Although pulsation didn't bring overall benefit to film cooling, there were cases where pulsed jets helped to increase effectiveness over the steady state conditions. Therefore, present results might be useful for evaluation of the effect of pulse frequency on film cooling effectiveness in real life applications, where jets pulse naturally due to the pressure fluctuations in the engine.