Film cooling is used to protect turbine components from the extreme temperatures by diverting cold air from the compressor through film cooling holes onto component surfaces. The air arrives to the shaped film cooling holes by a complex network of internal channels. This often leads to complex flow conditions beneath film cooling holes known as “crossflow” when the coolant is running perpendicular to the axis of the film cooling hole. Previous studies have shown that the effects of crossflow can significantly degrade film cooling performance. This study demonstrated that the turning of flow into a film cooling hole generates an inlet separation that grows in size as the inlet velocity ratio increases. This separation bubble constricts the effective hydraulic diameter of the metering section, increasing the maximum velocity at the hole inlet while also feeding recirculating flow into the diffuser section of the shaped hole, generating large scale stall. Inlet separation has been reported before in film cooling holes, but serious investigation into modifying the inlet geometry to mitigate separation has only recently become viable with advances in additive manufacturing (AM) technology. By leveraging AM to modify the inlet of a standard shaped hole, experiments showed up to 40% improvement in crossflow fed film cooling. CFD was used to show the improvements were generated by significantly reducing inlet separation leading to better diffuser performance. While performance was significantly increased by inlet rounding, the coolant jet still exited the shaped hole diffuser with a significant lateral velocity. This jet skew helped spread the coolant but also left the diffuser vulnerable to mainstream ingestion. Using CFD and AM, hole inlets were designed to significantly reduce jet skew while leaving the diffuser parallel with the mainstream flow. This swept inlet allowed very precise control of the coolant path downstream of the shaped hole and did not suffer from coolant biasing within the diffuser. This study shows that modifications of the inlets to film cooling shaped holes, which significantly reduce separation at the inlet, result in significantly improved film effectiveness performance.


Experimental and Computational Investigation of Shaped Film Cooling Holes Designed to Minimize Inlet Separation

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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