Effect of mainstream oscillation on instantaneous behaviors for film cooling with different trench heights

Mainstream oscillation is inevitable in the practical gas turbine running environment. To study the effects of different oscillation configurations on instantaneous film cooling performance, a series of large eddy simulations are performed in this work. Time-solved experiments are carried out to validate the numerical methodology. Through the analyses of time-averaged characteristics, it is concluded that with the enhancement of oscillation intensity, the anti-counter rotating vortex pair gradually disappears, and the pattern of counter rotating vortex pair is also changed, which leads to a significant variation in time-averaged film effectiveness. The analyses of film cooling instability show that without mainstream oscillation, the local unsteadiness distributes as two symmetrical bands, which is resulted from the trajectory of the symmetrical vortical structure. With the introduction of mainstream oscillation, these two bands gradually disappear, and the level of cooling instability increases significantly with the oscillation amplitude and frequency. By statistical analyses, it shows that under heavily oscillated mainstream, the probabilities of transient film effectiveness approaching 0 or 1 increase, which implies that the events of non-coverage and full film coverage tend to happen frequently. In addition, the design with a larger trench height is beneficial, because it can increase the time-averaged cooling effectiveness, and at the same time, the unsteadiness still keeps at the same levels under the same conditions of mainstream oscillation.