Effects of Oscillations in the Main Flow on Film Cooling at Various Frequencies at a Low Blowing Ratio

The objective of this study was to investigate the effects of oscillations in the main flow on film cooling at various single frequencies at a low average blowing ratio of M = 0.5. The oscillations in the main flow could be a result of combustion instabilities that have been one of the major concerns for gas turbine industry. Understanding the effect of the instabilities on film cooling is important for better design of the gas turbine engines. The frequencies from 268 to 2144 Hz were identified as the dominant frequencies from a Fourier analysis of a combustor instability data on pressure oscillations. Lastly, the experimental data on gas turbine combustor instabilities is applied to the main flow using Fourier Series and the results are compared to those at single frequencies. Numerical simulations are carried out using LES Smagorinsky-Lilly and URANS k-epsilon models. This study is focused on film cooling effectiveness and heat transfer coefficient which are very important in calculation of the blade temperatures. The results show that as the frequency of the main flow goes from 0 to 180 Hz, the film cooling effectiveness is decreased due to enhancement of jet lift off with increasing frequency. However, when the frequency goes from 180 to 268 Hz, the film cooling effectiveness climbs up sharply because a thin coolant film near the wall is overlapped by large vortices containing the coolant. If the frequency changes from 268 to 1072 Hz, the effectiveness drops because the large vortices generated catch up with each other and they start overlapping and they are moved away from the wall. Main flow frequencies from 1072 to 2144 Hz cause an increase of the film cooling effectiveness since the coolant jet could not respond to these very high frequencies and the coolant behavior starts to return to that at 0 Hz gradually along with the effectiveness. In terms of heat transfer coefficients, when the oscillation frequency climbs from 0 to 536 Hz, the spanwise-averaged Stanton number ratio (St_m/St_o) increases due to growing disturbances in the flow. If the frequency is increased from 536 to 2144, the spanwise-averaged Stanton number ratio is decreased. When the oscillation frequency exceeds 536 Hz, the mixing between the hot mainstream and the coolant is reduced because jets do not respond to the flow oscillations as quickly by very short period.