High Lift and Aft Loaded Profiles for Low Pressure Turbines

This paper shows how it is possible to reduce the number of blades in LP turbines by approximately 15% relative to the first generation of high lift blading employed in the very latest engines. This is achieved through an understanding of the behaviour of the boundary layers on high lift and ultra high lift profiles subjected to incoming wakes. Initial development of the new profiles was carried out by attaching a flap to the trailing edge of one blade in a linear cascade. The test facility allows for the simulation of upstream wakes by using a moving bar system. Hot wire measurements were made to obtain boundary layer losses and surface mounted hot films were used to observe the changes in boundary layer state. Measurements were taken at a Reynolds number between 100,000 and 210,000. The effect of increased lift above the datum profile was investigated first with steady and then with unsteady inflow (i.e. with wakes present). For the same profile, the losses generated with wakes present were below those generated by the profile with no wakes present. The boundary layer behaviour on these very high lift pressure distributions suggested that aft loading the profiles would further reduce the profile loss. Finally, two very highly loaded and aft loaded LP turbine profile were designed and then tested in cascade. The new profiles produced losses only slightly higher than those for the datum profile with unsteady inflow, but generated 15% greater lift.