Assessment of turbulent models for a low-pressure turbine guide vane heat transfer with high free-stream turbulence

Detailed heat transfer distribution along gas turbine stator vane are essential for component mechanical integrity and life predictions. Among the different parts subjected to hot gas flow, both endwall and airfoil heat transfer coefficient (HTC) evaluations are crucial for cooling system optimization. The numerical appraisal of HTC distribution along the stator vane is particularly challenging because the flow is strongly affected by secondary effects. Large three-dimensional flow structures introduce remarkable spatial variation of heat transfer on the blade platform, both along streamwise and spanwise directions, making the use of simplified modelling approaches questionable in terms of reliability. Furthermore, the simultaneously HTC distribution assessment on both airfoil and endwall represents another crucial aspect of numerical investigation, especially when the airfoil HTC profile evaluation is heavily influenced by the turbulent transition onset, which is another one of the most interesting and difficult CFD demands. The aim of the present contribution is to describe the efforts devoted to the assessment of CFD analysis as a predictive tool for local heat transfer distributions on both endwall and airfoil surfaces. Several experimental and numerical studies about heat transfer evaluation of gas turbine stages have already been carried out over the last few years, however analysis were generally performed at low free-stream turbulence conditions. The main goal of this paper is to extend the validation of these numerical procedures towards higher and more realistic free-stream turbulence intensity up to levels found at the combustor exit (≃ 20%), increasing the challenge for high fidelity computational methods. An accurate numerical procedure was developed and validated exploiting an experimental investigation done by Radomsky and Thole, available from literature. They provide a large set of aerodynamic and heat transfer data of a low-pressure linear cascade with low and high inlet turbulence levels. The analysis, focused on steady state computations, is principally devoted to the turbulence modelling assessment, with particular emphasis on the transitional models appraisal. Obtained results showed that the aerodynamics of both passage and endwall are well captured independently of the turbulence modelling. A larger impact of turbulence models was found on both pattern and averaged value of heat transfer: transitional models mainly affect the airfoil HTC profiles where they improve the CFD heat transfer results as they detect the turbulence transition onset. However, they are not capable to replicate the fully turbulent condition on the endwall and they consequently slightly underestimate the HTC distribution.