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4.3 Turbine Blade Aerodynamics the endwall. Figure 32 shows the profile of such an endwall that is employed62. The figure also includes the profile height variations across the passage. Harvey et al and Hartland et al. provide guidelines for designing the non-axisymmetric contour profiles for the linear cascade63. The measured static pressure distributions and computed surface streamlines at a flat endwall and at the contoured endwall of figure 32 are presented in figure 33 for a linear blade passage. The other endwall profile of the passage is always flat in this case. The surface static pressure Ps on the contoured endwall in figure 33 increases near both the pressure side and suction side compared to the Ps at the same locations on the flat endwall. But, the pitchwise pressure gradient, that drives the cross-pitch flow, in the first 40% axial chord decreases for the contoured endwall compared to that for the flat endwall. This clearly affects the cross-pitch flow on the endwall as shown in figure 33. The turning of the streamlines near the leading edge is much lower on the contoured endwall than on the flat endwall. The distance of the saddle point from the leading edge is also smaller for the contoured endwall than for the flat endwall. This indicates that the leading edge horse-shoe vortex is smaller in size above the contoured endwall. Inside the passage, the streamlines are also turning less toward the suction surface and appear to be more parallel to the blade surface on the contoured endwall. This occurs as the strength of the cross-pitch flow near the contoured endwall is decreased. The consequences of the results in figure 33 are weaker passage vortex and lower total pressure loss across the blade passage with the non-axisymmetric contoured endwall. These will be shown next. Figure 34 shows the streamlines in a pitchwise plane located 9% axial chord down the passage. The blade profile and contoured endwall profile are identified as solid objects in the figure. The structure of the pressure side leg vortex at this location is very clear near the pressure side of the flat endwall case. On the other hand, the streamlines near the pressure side for the contoured endwall case have not completed the full revolution to create a vortex structure. This happens as the pressure side leg vortex is weakened and reduced in size by the contoured endwall. As the pressure side leg vortex is driven from the horse-shoe vortex, this also validates the assertion that the horse-shoe vortex reduces with the contoured endwall. The passage vortex can be identified in figure 34 at the suction side where the total pressure loss coefficients, Cpt are very high. The extent of the passage vortex can be considered for Cpt>0.40 in this case. Then, clearly the passage vortex size for the contoured endwall passage is about half of that for the flat endwall linear cascade. The magnitudes of Cpt also indicate that the passage vortex is much weaker for the contoured endwall as the Cpt are lower for the contoured endwall at the passage vortex location than for the flat endwall. As such, the mass-averaged total pressure loss across the passage reduces significantly with the contoured endwall. Several other profiles of the non-axisymmetric contoured endwall have been tested successfully in blade and vane passages64. The results are similar to what we have discussed so far. These endwall profiles reduce the total pressure loss across the blade passage by weakening the endwall cross flows and passage vortex. Endwall Film Injection: Coolant air injected through tiny holes in the endwall covers the endwall with a layer of film of cold air and protects the endwall from the hot gas streak in the blade passage (figure 35). Wall static pressure changes in the vicinity of the coolant injection holes as the coolant jet blocks the boundary Fig. 34. Streamlines and total pressure loss coefficients with and without non-axisymmetric contoured endwall showing pressure side leg vortex and passage vortex. Source: See Note 62. Fig. 35. Coolant injection through holes in endwall for film cool- ing. L= hole length and D= characteristic scale of hole shape. 382PDF Image | Turbine Blade Aerodynamics
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