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4.3 Turbine Blade Aerodynamics of the fillet profiles have been tested and are shown in figure 24. As can be seen in the figure, two types of basic construction of fillet profiles can be identified: (i) profile with varying height from the blade surface to the endwall and (ii) profile of bulb with surface thickness at the outer periphery49. The thickness of the type (i) fillet profiles reduces to zero as they extend out from the blade surface to the endwall. These fillets may blend with either the endwall or blade wall or with both the endwall and blade surface as they wrap around the leading edge extending inside the passage. The fillet profiles of the type (ii) blend with the blade surface as they wrap around the leading edge, but meet the endwall with a finite thickness. Type (ii) fillets simply thicken the blade profile near the leading edge at the endwall. All types of fillets studied until now have asymmetric profile with respect to the leading edge and have their highest point located at the leading edge. The height of this highest point from the endwall i.e. the maximum height of the fillets is typically one boundary layer thickness of the incoming flow. The studies mentioned above show that type (i) fillets are the most effective in reducing the secondary flows in the blade passage. These fillets reduce the size and strength of the leading edge horse-shoe vortex. Consequently, the strength of the passage vortex is reduced. The high total pressure losses due the passage vortex then also decrease across the blade passage. Figure 25 shows the horse-shoe vortex structure at the leading edge with a fillet profile of type (i) employed at the leading edge. The profile height varies linearly to zero from the blade surface to the endwall and blends with the endwall and blade wall inside the passage on the pressure side and suction side (Fillet 150). The blade passage is the same as that in figure 8. The wedge shaped object on the left of the flow visualization image of figure 25 is the fillet profile. The size of the horse-shoe vortex is about half in the flow visualization and about one-fifth in velocity vector plot with the fillet compared to the case without any fillet. Note that the flow visualization is observed at a low speed to avoid any smearing and diffusion of smoke. The flow area at the leading edge is reduced in the passage with the fillet. For the incompressible flow, this will cause the boundary layer fluid to be displaced from the leading edge plane. Also, the adverse pressure gradient along the leading edge plane (due to the stagnation) is reduced by the fillet slope. All these factors are responsible in reducing the size of the horse-shoe vortex with the fillet. The turbulent kinetic energy is also reduced significantly in figure 25 compared to what is observed without the fillet. This indicates that the strength of the horse-shoe vortex is also reduced by the fillet. There is also no apparent structure of the leading edge corner vortex in the secondary velocity vectors with the fillet. As the horse-shoe vortex is reduced, the Fillet 1 is expected to reduce the passage vortex size and strength downstream in the blade passage. Figure 26 shows the passage vortex at a plane 92% axial chord (near the exit) with and without fillet in the same blade passage. Comparing the velocity vectors in figure 12 (Plane I) and figure 26, it can be seen that the location of the passage vortex center with the Fillet 1 moves little higher above the endwall than without the fillet. In an upstream location near the suction side in the blade passage, the suction side leg vortex is reduced in size and weakens with the Fillet 1 compared to that without the fillet51. The significant differences are observed in the total pressure loss contours of figure 26. The high total pressure loss region (Cpt>0.45) can be considered as the signature of the passage vortex. The Cpt contours presented here are measured Fig. 26. Passage vortex and total pressure loss at 92% axial chord with and without fillet. Source: See Note 25. Fig. 27. Secondary velocity vectors and turbulent kinetic energy (k) at pressure side (Plane PS1) of a linear vane cascade with and without fillet. PS= pressure side. Source: See Note 49. (Zess) 378PDF Image | Turbine Blade Aerodynamics
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