Endwall Loss Reduction of High Lift Low Pressure Turbine Airfoils Using Profile Contouring: Part II — Validation

Abstract

The hypothesis, posed in Part I [1], that excessive endwall loss of high lift low pressure turbine (LPT) airfoils is due to the influence of high stagger angles on the endwall pressure distribution and not front-loading is evaluated in a linear cascade at Re = 100,000 using both experimental and computational studies. A nominally high lift and high stagger angle front-loaded profile (L2F) with aspect ratio 3.5 is contoured at the endwall to reduce the stagger angle while maintaining the front loading. The contouring process effectively generates a fillet at the endwall, so the resulting airfoil is referred to as L2F-EF (Endwall Fillet). Although referred to as a fillet, this profile contouring process is novel in that it is designed to isolate the effect of stagger angle on endwall loss. Total pressure loss measurements downstream of the blade row indicate that the use of the lower stagger angle at the endwall reduces mixed out mass averaged endwall and passage losses approximately 23% and 10% respectively. This is in good agreement with computational results used to design the contour which predict 18% and 7% loss reductions. The endwall flow field of the L2F and L2F-EF models is measured using stereoscopic particle image velocimetry (PIV) in the passage. These data are used to quantify changes in the endwall flow field due to the contouring. PIV results show that this loss reduction is characterized by reduced inlet boundary layer separation as well as a change in strength and location of the suction side horseshoe vortex (SHV) and passage vortex (PV). The endwall profile contouring also produces a reduction in all terms of the Reynolds stress tensor consistent with a decrease in deformation work and overall flow unsteadiness. These results confirm that the stagger angle has a significant effect on high-lift front-loaded LPT endwall loss. Low stagger profiling is successful in reducing endwall loss by limiting the development and migration of the low momentum fluid associated with the SHV and PV interaction.

Investigation of Low-Pressure Turbine Endwall Flows: Simulations and Experiments

“…26 The design intent was to have good low-Reynolds number performance compared to the Pratt and Whitney Pack B blade and high aerodynamic lift. The axial blade chord and span in the AFRL experiments 25 were C x =6in and H=3.5C x (21in), respectively. The pitchwise spacing was 1.221C x and thus 38% larger than for the Pack B blade.…”

“…The Reynolds number based on axial chord, C X , for the simulations was Re=100,000 and thus the same as in the experiments and earlier RANS calculations. 24,25…”

“…H. Location of Slices for Comparison with Sangston et al 25 Measurements Sangston et al 25 acquired Particle Image Velocimetry (PIV) data in an outflow plane at x=1.5C…”

“…by Sangston et al 25 is provided in Fig. 14. Compared to the experiment the passage vortex is slightly closer to the suction side of the blade.…”

“…The AFRL decided to investigate profile contouring to reduce the endwall losses (e.g., Lyall et al 24 and Sangston et al 25 ). A new geometry was designed that blends from a L2F profile inside the passage to a low-stagger L2F-LS airfoil near the endwall.…”