Flow and Thermal Performance of a Gas Turbine Nozzle Guide Vane with a Leading Edge Fillet

Complex three-dimensional vortex flows develop at the junction of a gas turbine airfoil and its casing (endwall). These flows increase the transfer of heat from the combustion gases to the metal parts and contribute to reduced aerodynamic efficiency. Past studies have shown that the use of a large fillet at the airfoil-endwall junction can reduce or eliminate the endwall vortex flow pattern. To determine the effects of a fillet on turbine surface conditions, wall shear stress and heat transfer coefficients were measured on the endwall of a low-speed linear turbine vane cascade. A shear stress measurement technique was developed and implemented for this study. High-resolution measurements of magnitude and direction provided quantitative information about the endwall flow features with and without a large fillet at the airfoilendwall junction. The fillet was shown to increase shear stress magnitude, but reduce turning of the flow at the endwall. Heat transfer coefficients were also measured with and without the fillet. Results indicated that the leading edge fillet changed the distribution of heat transfer on the vane endwall. Introduction A major factor in reduced aerodynamic efficiency and increased part temperatures in a gas turbine engine is a complex three-dimensional flow, known as secondary flow. This flow occurs where an axial turbine airfoil meets the inner or outer casing, known as the endwall (see Figure 1). In an axial gas turbine, secondary flows result in aerodynamic losses in a vane or blade stage, which can reduce the engine’s overall efficiency by up to 3%. Secondary flows also tend to convect hot mainstream gases onto the endwall, which result in higher local heat transfer coefficients and increased metal temperatures. A 25°C (50°F) increase in metal temperature can result in a reduction in part life by a factor of two. Controlling or eliminating secondary flow could increase the aerodynamic efficiency of the engine and reduce the required cooling, or the same amount of cooling could increase part life expectancy. While several options to eliminate secondary flow have been successfully investigated, a fundamental understanding of the flow remains elusive. More information about the flow and its interaction with the turbine surfaces is necessary to advance current engine designs. This paper will discuss the influence of a large fillet at the junction of the endwall and the airfoil, on the surface heat transfer and wall shear stress of a modern nozzle guide vane. Literature Review Secondary flow is not unique to gas turbines and has been investigated for symmetric airfoils and cylinders in crossflow. However, a couple of features of a turbine cascade are unique. First, the airfoils are not symmetric and turn the flow through large angles. This results in cross-passage pressure gradients that are not present for symmetric airfoils or cylinders. Second, the flow is accelerated in the turbine passage by a streamwise favorable pressure gradient. These factors, as well as the three-dimensionality of secondary flow, make it difficult to predict secondary flow development and progression. Langston et al. presented one of the first descriptions of endwall secondary flow in a gas turbine. Other researchers have presented secondary flow models with slight differences, but they all agree on the major components. The incoming boundary layer on the endwall has a constant static pressure profile in the spanwise (normal to wall, along span of airfoil) direction. However, the boundary layer has a nonuniform velocity profile because of the difference in velocity between fluid entrained near the wall and fluid in the mainstream, which corresponds to a non-uniform total pressure profile. As the boundary layer stagnates on the airfoil, the total pressure profile becomes a spanwise pressure gradient that drives the flow to the endwall. This turning creates a vortex that splits at the stagnation point and wraps into two legs around the