Effects of Oscillations in the Main Flow due to Thermo Acoustic Fields in a Gas Turbine Combustor on Film Cooling

Abstract

The objective of this study is to understand the effects of oscillations in the main flow and the film cooling jets caused by the thermoacoustic fields formed in a gas turbine combustor on film cooling. As a first step, CFD simulations are performed for the case of steady mainstream and steady film cooling jets for validation of models and simulations and compared with other studies trying to predict adiabatic effectiveness under similar operating conditions. Based on the knowledge gained on the capability and limitations of different turbulence models for the steady simulations, simulations were extended to unsteady main flow and unsteady cooling jets. The unsteady simulations are performed using URANS-realizable k-ε turbulence model and LES-Smagorinsky-Lilly model. Initially, oscillations due to the combustion instabilities are approximated to be in sinusoidal form. For unsteady main flow and cooling jet simulations, results from the Seo et al. [3] experimental study were selected for comparison with CFD results. The effects of different frequencies (2, 16, 32 Hz) on film cooling are investigated. In each case, average blowing ratio was M=0.5. The results show that if the frequencies of the main flow and the cooling jet flow are increased, the adiabatic centerline effectiveness is decreased and the heat transfer coefficient is increased. Some representative results are: if the frequency of the main flow is increased from 0 Hz to 2 Hz, 16 Hz, or 32 Hz for L/D=1.6, the centerline effectiveness is decreased about 10%, 12%, or 47% and the spanwise-averaged heat transfer coefficient is increased around 1%, 2%, or 4% respectively. If the frequency of the mainstream and the jet flow is increased, the amplitude of the pressure difference between the mainstream and the plenum is increased and the amplitude of coolant flow rate oscillation is increased. Additionally, rectangular or triangular wave forms are used for mainstream and coolant jet flow in order to see the effect on the results and total 36 cases are simulated and effects of changing wave form are investigated. In each case, coolant flow rate was the same as sinusoidal wave forms. It seems like rectangular wave form for main flow at 2 Hz has a negative effect on film cooling performance whereas the same wave form for coolant jet at 32 Hz has a positive effect.