Experimental investigation into the impact of crossflow on the coherent unsteadiness within film cooling flows

Highlights: ► The presence of coherent unsteadiness in film cooling flows has been investigated. ► Experimental investigation with both cylindrical and shaped holes. ► Two coherent unsteady structures are shown to be present in the downstream jet. ► Coherent unsteadiness is also seen inside the cooling hole tube. ► Crossflow at the cooling hole inlet impacts on the coherent unsteadiness. -- Abstract: Film cooling is regularly used to cool the surface of components within the turbine stage of an aero engine. This enables them to withstand the high air temperatures that are required for maximising aero engine cycle efficiency. It is known that the mixing of a film cooling flow with the main high temperature air flow through a turbine passage is an unsteady process, with coherent unsteady features occurring across a range of blowing ratios. Upon an aero engine the cooling holes on a turbine blade commonly have a crossflow at the hole inlet. Previous work has shown that crossflow at the hole inlet modifies the time-mean flowfield downstream of a cooling hole compared to the case without crossflow. The current paper investigates the impact of spanwise orientated crossflow on the coherent unsteadiness within film cooling flows. Both cylindrical and shaped cooling holes, located on a blade pressure surface, are studied. The range of blowing ratios considered is 0.7–1.8 and the crossflow velocity is up to 0.8 times the bulk jet velocity. High Speed Photography and Hot Wire Anemometry are used to observe the presence of coherent unsteadiness, both immediately downstream of the hole exit and within the cooling hole tube. The results show that the coherent unsteadiness downstream of the hole exit is persistent and its occurrence is not significantly affected by the magnitude of spanwise crossflow. Within the cooling hole tube the existence of coherent unsteadiness is presented for the first time, inside both cylindrical and shaped holes, with a Strouhal number of 0.6–1.2. The pattern of this in-hole coherent unsteadiness is seen to change with increasing the crossflow velocity