Large eddy simulation on trenched film cooling with forward and backward injections under pulsed conditions

Considering the strong turbulence effect and inevitable environmental pulsation in a practical film cooling process, this work exhibits both time-averaged and transient performances of trenched film cooling with forward and backward cooling air injections by a series of large eddy simulations. The blowing ratio in the steady case is 1.5, while cosine and square waves are selected as pulsed boundaries with a Strouhal number of 0.254. The spatial-temporal evolution of flow and temperature fields is revealed, from which two important conclusions are obtained: 1) opposite to the results obtained under steady conditions, the pulsed backward injection decreases the time-averaged cooling effectiveness by over 15%, and the film covered area also shrinks remarkably compared to forward injections. This phenomenon is determined by the different flow mechanisms between the steady and pulsed conditions. 2) Under a steady blowing ratio, the film cooling unsteadiness is caused by the temporal fluctuation of the near-wall vortex, which may induce a fast change in temperature and component failure. For the pulsed forward injection, additional high film cooling unsteadiness appears in the lateral sides of the film hole inside the trench. For the backward injection, the pulsation does not result in an obvious increase in unsteadiness.