Experimental and numerical investigations on transient film cooling performances with trenched holes considering mainstream oscillation

By a series of time-resolved experiments and large eddy simulations, the present work exhibits the transient behaviors and unsteady characteristics of film cooling with different trench configurations. The inevitable mainstream oscillation in turbine operation is considered, with a Strouhal number of 0.0128 and an amplitude of 0.50. The results show that: 1) The main instantaneous features are well captured by both experimental and numerical methods, and the transient behaviors of film cooling with trench design can be accurately predicted by the present large eddy simulation, even considering mainstream oscillation. 2) Without mainstream oscillation, film cooling unsteadiness is mainly caused by the temporal evolution of near-wall vortices, and the trench configurations exhibit a strong correlation on the unsteadiness level. The design with a higher trench height can evidently enhance the near-wall stability of film cooling while keeping a high time-averaged film effectiveness. 3) Film cooling unsteadiness is significantly aggravated by the mainstream oscillation. Under the present oscillation condition, the standard deviation of instantaneous film effectiveness is increased over 350.0 %, compared to the non-oscillation data. Meanwhile, the regions with high unsteadiness level are revealed with mainstream oscillation, which is mainly caused by the temporary coverage of cooling air in the target surface.