Statistical evaluation on cooling unsteadiness level for a cylindrical film-hole influenced by partial blockage under pulsed mainstream

In this work, instantaneous experiments and large eddy simulations are performed to evaluate the cooling unsteadiness level for cylindrical partially-blocked film-holes. The effects of mainstream pulsation and blowing ratio are discussed. Four statistical metrics are proposed as comprehensive criteria, including time-averaged film effectiveness, standard deviation, probability, and fluctuation range of instantaneous data. The results show that under all the conditions, time-averaged film effectiveness always decreases with the increase of blockage level, due to the intensive detachment phenomenon of cooling air. Cooling unsteadiness of partially blocked film-holes is mutually resulted from the turbulent fluctuation of near-wall vortices and instantaneous variation of boundary condition. Quantified by the standard deviation, the regions with high unsteadiness level distributes oppositely between the two conditions with and without pulsation, which attributes to the different dominating factors. Under the steady condition, the probability distribution of instantaneous film effectiveness conforms to normal distribution; but under the pulsed condition, it distributes more scattered with an enlarged fluctuation range. The probability of low instantaneous film effectiveness increases with blockage level. At large blowing ratio, mainstream pulsation is undesired to the unblocked film-hole, but becomes beneficial to the partially-blocked holes, because the probability of good film performance is higher.