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 tu rbines by approximately, 15 percent relative to the first generation of hig h lift binding employed in the very latest engines. This is achieved throug h an understanding of the behavior of the boundary layers on high lift and ultra-high lift profiles subjected to incoming wakes. Initial development o f 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 simulat ion of upstream wakes by using a moving bar system. Hot wire measurements w ere made to obtain boundary layer losses and surface-mounted hot films were used to observe the changes in boundary layer state. Measurements were tak en at a Reynolds number between 100,000 and 210,000. The effect of increase d 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 profi le with no wakes present. The boundary layer behavior on these very high li ft pressure distributions suggested that aft loading the profiles would fur ther reduce the profile loss. Finally, two very highly loaded and aft loade d LP turbine profiles were designed and then rested in cascade. The new pro files produced losses only slightly higher than those for the datum profile with unsteady inflow, but generated 15 percent greater lift.