Assessment of an Hybrid RANS/LES Approach for the Prediction of Aerodynamic Losses on a Linear Cascade of Turbine Blades with Free-Stream Turbulence

Authors

Benjamin François - ONERA, France
Fabien Gand - ONERA, France

Abstract

Turbine blades are highly aerodynamically loaded, which lead to secondary flows in the vicinity of the hub/shroud walls. These secondary flows are the cause of significant aerodynamic losses in the turbine row. In addition, necessary rotor-stator wheel space in turbine implies cavities in the main flow passage. Purge air is injected through these cavities to prevent hot gas injection in the inter-stage disk. These cavities and purge flows amplify the aforementioned secondary flows and the associated aerodynamic losses. Finally, turbine row operates in a flow with a high intensity of free-stream turbulence, inherited from the upstream combustion chamber and/or upstream turbine rows. Hence, the accurate prediction of such flows with numerical simulations is quite challenging. For the design engineer, the CFD methods need also to be fast to be compliant in an industrial design cycle. RANS simulations offer a very good compromise between numerical cost and accuracy but the choice of the most appropriate turbulence model remains delicate and the modeling of free-stream turbulence raises an ill-defined mathematical problem for the eddy-viscosity turbulence models. Large-Eddy Simulations offer more accurate predictions but they require high density meshes at the walls, and thus an important computational time. Hybrid RANS/LES approaches intend to combine both advantages of the two approaches of RANS and LES: they alleviate the mesh efforts in the attached boundary layers compared to LES methods using a RANS solving and they switch to LES in the detached flow and free-shear areas. The present work proposes to evaluate an in-house hybrid RANS/LES approach, named Zonal Detached Eddy Simulation (ZDES), on the aforementioned flow topology. The selected configuration is the low Mach number linear cascade of turbine blades from the EU project MAGPI. The configuration includes a cavity and a purge flow located upstream of the blades and the free-stream turbulence in the main flow is significant. The objective of this work is to evaluate the accuracy of the ZDES for predicting the aerodynamic losses on the proposed configuration. The results will be compared with experimental data and RANS simulations. The influence of the modeling of the free-stream turbulence, of the mesh density will be discussed in the paper.