Endwall Boundary Layer Development in an Engine Representative Four-Stage Low Pressure Turbine Rig

In a linear cascade of low pressure turbine, LPT, blades, the position and strength of the vortices forming the endwall flows depends on the state of the inlet endwall boundary layer, i.e., whether it is laminar or turbulent. The latter will determine, amongst other effects, the location where the inlet boundary layer rolls up into a passage vortex, the amount of fluid that gets swept up by the passage vortex and the interaction with the pressure side separation bubble, de la Rosa Blanco et al. [1]. As a consequence, the mass-averaged stagnation pressure loss and therefore the design of a low-pressure turbine are influenced by the state of the inlet endwall boundary layer. The paragraph above highlights the importance of determining the state of the boundary layer along the endwalls if an understanding of the endwall flows in a LPT at realistic engine conditions is sought. The results presented in this paper are taken from hot film measurements performed on the endwalls of selected nozzle guide vanes from the fourth stage of the Affordable Near Term Low Emission, ANTLE, LPT rig. These results are compared with those from a low speed linear cascade of similar LPT blades. In the cold flow four-stage LPT rig, a transitional boundary layer has been found on the platforms upstream of the leading edge of the blades. The boundary layer is more turbulent nearer the leading edge of the blade and for higher Reynolds numbers. As for the passage, for both the cold flow four-stage rig and the low speed linear cascade, the new inlet boundary layer formed behind the pressure leg of the horseshoe vortex is a transitional boundary layer. The transition process progresses from the pressure to the suction surface of the passage in the direction of the secondary flow.