The Effect of Airfoil Thickness on the Efficiency of Low-Pressure Turbines

Abstract

The effect of airfoil thickness on the efficiency of low-pressure (LP) turbines has been investigated experimentally in a multistage turbine high-speed rig. The rig consists of three stages of a state of the art LP turbine. The stages are characterized by a very high hade angle, reverse cut-off design, very high lift, and very high aspect ratio airfoils. Two different sets of stators have been designed and tested. The first set of stators is made of airfoils with a thickness to chord ratio around 10% along the span with the exception of a small areas close to the end walls. In those areas, the thickness has been increased above the previous value to reduce the secondary flows. These types of airfoils have been referred to in the literature as “spoon” airfoils. The second set of stators has been designed to have the same spanwise distribution of pressure coefficient (Cp) on the suction surface than the first set. However, the thickness to chord ratio was increased along the span up to values around 20% to raise the velocity of the flow and to remove any separation bubble on the pressure side. The resulting shape of the profiles is representative of “hollow” airfoils. The velocity triangles, chord distribution, leading and trailing edge locations, and flowpath have been maintained between both sets. They have been tested with the same blades and at the same operating conditions with the intention of determining the impact of the profile thickness on the overall efficiency. The turbine characteristics: sensitivity to speed, specific work, Reynolds number, and purge flows have been obtained for both sets. The comparison of the results suggests that the efficiency of both types of airfoils exhibit the same behavior; no significant differences in the results can be distinguished.