Flow and Thermal Performance of a Leading Edge Endwall-airfoil Fillet for a Gas Turbine Nozzle Guide Vane

Gas turbine engines have a high power-to-weight ratio, making them ideal for generation of aircraft thrust, and can have excellent electrical power generation efficiencies when used in a combined cycle power plant. A complex vortical flow at the junction of the nozzle guide vane airfoil and its casing (endwall) in the turbine section tends to decrease aerodynamic efficiency and increase metal temperatures. This research discusses the combined effect of a large leading edge endwall-airfoil fillet and leakage flow from a two-dimensional slot simulating the combustorturbine gap. High-resolution measurements of endwall heat transfer and adiabatic cooling effectiveness of gap leakage flow were obtained in a large-scale cascade at engine Reynolds number conditions. Cool leakage flow from a combustor-turbine interface gap upstream of the endwall causes increased heat transfer, but also cooler wall temperatures. A fillet in the presence of upstream leakage flow displaces the coolant, but also reduces the heat transfer coefficient augmentation caused by the leakage flow. Introduction Increases in gas turbine efficiency and power output can be achieved by increasing the temperature of the combustion products entering the nozzle guide vane in the turbine section. However, the high heat loads to the nickel superalloy material of the nozzle guide vane require significant cooling to maintain part strength and durability, given that the combustion gas temperature can be on the order of 300°C higher than the superalloy melting temperature. Air bled from the compressor section of the engine can be routed to the turbine parts for internal and external cooling. 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 coefficients. Several methods to control or eliminate secondary flows have been tested, including blowing, endwall contouring, and modification of the leading-edge endwall-airfoil junction. Adding a fillet to the endwallairfoil 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 cooling to the part surface. A particular gap of interest is between the combustor and the turbine. Leakage flow from this gap interacts with the secondary flow, and at high flow rates the coolant may interfere with secondary flow development. This paper presents experimental measurements of the combined effect of combustor-turbine interface leakage and a large leading-edge fillet on endwall heat transfer and adiabatic cooling effectiveness. The objective of this study is to explore the interaction of the fillet with combustor-turbine gap leakage flow.