Analysis of the Unsteady Flow Field Inside a Fan-Shaped Cooling Hole Predicted by Large-Eddy Simulation

Film cooling is a common technique to manage turbine vane and blade thermal environment. Optimization of the film cooling efficiency through efficient cooling hole shape design and appropriate control of operating conditions is still an active academic and industrial topic. In this context, the analysis of the flow issued by a film cooling hole is an important and fundamental research axis to develop proper understanding of film cooling physics. Recently, a lot of emphasis has been put on the creation of empirical models to substitute for the flow calculation inside cooling holes. The reliability of such models however relies on the proper understanding of the flow inside the hole. Using Large Eddy Simulation (LES), it can be shown that the flow in the perforation has a significant impact on the film cooling effectiveness.

The present work attends to understand such a mechanism by specifically addressing the 7-7-7 fan-shaped cooling hole [1]. In the current study, the LES flow predictions are first validated against the experimental data base available from Penn State University in terms of aerodynamics and adiabatic effectiveness. The flow features inside the hole are then more specifically studied with the objective of identifying the dominant flow characteristics and vortex structures. To do so, mathematical techniques such as the Fast Fourier Transforms (FFT) and Dynamic Mode Decomposition (DMD) are used to quantitively access the flow modal structure inside the hole. From this analysis, one retains that after entering the hole from the plenum side, the flow turns sharply forming a separation region and an internal shear layer is formed at the interface of this separation zone. As a consequence, periodically shed vortices appear due to the the roll-up of this shear layer. This vortex shedding is predominant in the cylindrical part of the hole but once the hole expands and the cross-section changes, flow adaptation occurs. Indeed, the lateral and forward hole expansion is followed by a decrease of the in hole flow velocity. This leads to a reorganization of the flow with vortex paring and breakdown as well as the establishment of a second separation region. The main outcome of this complex flow is a rather uniform turbulent flow profile formed at the hole exit. In agreement with the literature, such a process suppresses the vortices shed near the hole-entry which if escape from the cooling hole have been shown to not always be beneficial to the film cooling performance.  

[1] R. P. Schroeder and K. A. Thole, Adiabatic effectiveness measurements for a baseline shaped film cooling hole, GT2014-25992. in Proceedings of ASME Turbo Expo 2014: Turbine Technical Conference and Exposition, June 16 – 20, 2014, Düsseldorf, Germany