Vane Endwall and Fillet Summary

For a modern gas turbine engine, high efficiencies can be achieved by increasing the temperature of the gas exiting the combustor and entering the turbine section. The first stage stator turbine vane encounters the high temperature gases thus being subjected to the highest thermal and mechanical stress. The vane endwall platform of the turbine vanes experiences high thermal loading due to the complex flow structures that form. Several past studies have reported on detailed measurements which include endwall static pressure measurements, endwall heat transfer measurements, film-cooling, and flowfield measurements. These measurements were made using a scaled-up turbine vane cascade fabricated for a low speed wind tunnel.

The endwall flow pattern is quite complex encompassing a vortex that forms in the leading edge region and a vortex that forms as the flow progresses through the passage. Early studies by our group identified a viable geometric modifications to the leading edge-endwall juncture to eliminate the horseshoe vortex that develops at the vane leading edge. The geometric modifications studied for this work included the addition of a leading edge fillet between the juncture of the endwall and vane similar to what occurs on a shark’s dorsal fin. The initial study was carried out using computational fluid dynamics (CFD) simulations of the upstream and passage flows within the unmodified turbine vane for a baseline turbulent boundary layer that had a thickness of 9% of the total span. The CFD simulations for the leading edge fillet indicated that an effective fillet for eliminating the horseshoe was one that was one boundary layer thickness high by two boundary layer thicknesses long and was asymmetric about the stagnation line of the turbine vane. To verify the CFD results with the leading edge fillet design, experimental flowfield measurements were performed in a low-speed wind tunnel. The flowfield measurements were performed with a laser Doppler velocimeter in four planes orientated orthogonal to the vane. Good agreement between the CFD predictions and the experimental measurements verified the effectiveness of the leading edge fillet at eliminating the horseshoe vortex and the development of the passage vortex. The experimental results also verified the existence of the corner vortex. The fillet study resulted in a patent and is now commonly used in many turbine designs.

Several past studies have evaluated many different designs for endwall film-cooling hole patterns provided by leading gas turbine engine companies. In addition, a flush, two-dimensional slot was included in many simulations which is present at the combustor-turbine interface. The coolant leading the interface slot has been found to exit in a non-uniform manner leaving a large, uncooled ring around the vane. Film-cooling holes are effective at distributing coolant throughout much of the passage, but at low blowing rates are unable to provide any benefit to the critical vane-endwall junction both at the leading edge and along the pressure side. At high blowing ratios, the increased momentum of the jets induced separation at the leading edge and in the upstream portion of the passage along the pressure side, while the jets near the passage exit remained attached and penetrated completely to the vane surface.

Adiabatic effectiveness measurements for a film-cooled vane endwall with a preceding slot.