Numerical study of gas turbine nozzle guide vane external heat transfer and showerhead film cooling effectiveness by means of hybrid RANS-LES CFD modelling

Abstract

An accurate estimation of gas turbine nozzle guide vane metal temperature locally on its surface is crucial for predicting vane lifetime. The stagnation region at the vane leading edge is, typically, exposed to the highest thermal load, thus, the showerhead configuration of film cooling is often applied on the leading edge. Widely used lean-­burn combustors implement swirlers, leading to a complex flow field over the nozzle guide vane as the swirling profile is often preserved at the inlet of the guide vane. The nozzle guide vane external heat transfer coefficient in the industry is often computed either with boundary layer codes or RANS methods. However, higher accuracy hybrid RANS­LES models are of interest since such models are becoming more and more available due to improved capabilities to facilitate the increase of computational resource demands. This study aims at introducing hybrid RANS-­LES modelling to investigate swirling inflow effects on showerhead film cooling performance.

The implemented methodology and various turbulence models are first validated by replicating an experimental case of a linear cascade of blades. The uncooled blade external heat transfer coefficient distribution is compared to the experimental data for cases with turbulence intensities of Tu =1% and Tu =8%. The SST k − ω and lag elliptic blending k − ε turbulence models successfully replicated the HTC distribution but the γ − Reθ transition model was required to capture transition on the suction side for cases with Tu =1%. The showerhead film cooling effectiveness is investigated with both RANS and hybrid RANS-­LES models for a configuration of three rows of staggered cooling holes and blowing ratios of 1.4 and 1.9. Uniform inflow and two profiles of swirling inflow are analysed, with the strongest swirl being coupled with a radial temperature profile. Results indicate that the RANS­-LES hybrid model is promising in capturing the unsteadiness of the horseshoe vortex. Mixing between mainstream and coolant jets, and the inherent unsteadiness of the mixing shear layers can only be captured with a scale resolving simulation. The swirling inflow lead to an altered stagnation line and imposed a radial pressure distribution close to the leading edge, which interfered with the coolant distribution on the
leading edge. The coolant jet contact with the vane surface area is found to be the most important parameter affecting the cooling effectiveness since a coolant lift­off led to a significant reduction in cooling effectiveness. A lift­off was often observed in areas where the swirling velocity component misaligned to the mainstream flow direction. A positive contribution to cooling effectiveness was recorded in regions where the swirl component aligned to the mainstream flow direction. The increase in cooling effectiveness was observed due to locally increased momentum of the main flow that aids coolant staying attached to the surface. The benefits of hybrid RANS-­LES modelling are established, and the
results signify the importance of accounting for an unsteady stagnation line during the design of the showerhead film cooling.