Flow and Thermal Performance of an Airfoil-endwall Fillet for a Gas Turbine Nozzle Guide Vane

Gas turbine engines are used in a variety of power generation applications, including providing thrust for the F-35 Lightning II Joint Strike Fighter, turning electrical generators in combined-cycle power plants, and powering the M1 Abrams Main Battle Tank. In the high-temperature region of the turbine section, a complex vortical (swirling) flow present near the junction of a turbine airfoil and its casing (endwall) tends to decrease aerodynamic efficiency and increase metal temperatures. Past research indicates a large fillet at the airfoil-endwall junction can help to mitigate the effects of the endwall vortical flow. Also, leakage flow through inherent gaps between individually manufactured turbine components can interfere with the endwall vortical flow. This research discusses the effect of two types of leading edge airfoil-endwall fillets, with and without leakage flow from a two-dimensional slot simulating the combustor-turbine gap. Measurements of aerodynamic loss were obtained at the exit of a nozzle guide vane cascade at matched engine Reynolds number conditions. Results showed that without leakage flow, the fillets slightly reduced aerodynamic loss. The addition of leakage flow from the combustorturbine interface gap increased losses by 14% for the linear-profile fillet but did not increase losses for the elliptical-profile fillet, indicating that a thorough understanding of the inlet flow conditions is critical in the design of a fillet. Introduction Gas turbines are a popular choice for motive power for aircraft because of their high thrust-to-frontal drag area and power-to-weight ratios. They also provide shaft power for electricity generation in power plants, where their relatively rapid response to varying electric load demands and minimal installation requirements is an advantage. Significant increases in gas turbine efficiency and power output have been achieved by increasing the temperature of the combustion products entering the turbine section, to the point that current turbines operate with combustion gas temperatures on the order of 300°C higher than the melting point of the metal components. Innovative cooling schemes, such as bleeding air from the compressor section through the core of a part and then ejecting it through holes in its surface, are required to maintain part integrity. The endwall region of a turbine vane or blade is particularly important to cool because of the presence of a complex flow that develops at the airfoil-endwall junction. Flow models, such as the one presented by Langston and depicted in Figure 1, describe an approaching boundary layer on the endwall that rolls up into a horseshoe vortex at the leading edge of the vane. The horseshoe vortex splits into suction and pressure side legs, and the pressure side leg develops into a larger passage vortex. These vortical structures, generally termed secondary flows, can be a large source of aerodynamic loss. Secondary flows also sweep coolant from the endwall and increase endwall heat transfer. Several methods to control or eliminate secondary flows have been tested, including blowing, threedimensional endwall contouring, and modification of the leading-edge endwall-airfoil junction. Adding a fillet to the endwall-airfoil junction has been shown to be particularly successful in increasing aerodynamic efficiency and reducing endwall heat transfer by inhibiting the formation of the horseshoe vortex. In an engine, gaps are present between individually manufactured components, and cool air from the compressor is allowed to leak through the gaps. This prevents hot gas ingestion, and can provide some