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4.3 Turbine Blade Aerodynamics Figure 29 shows the effects of the endwall profiling through the passage aft of a vane cascade on the secondary flows57. The data are presented in a plane near the passage exit where the endwall profile has become almost flat. The low streamwise velocity magnitudes adjacent to the pressure side in figure 29 are located within the boundary layer on this side. The high velocities near the contour endwall are the results of flow accelerations along the endwall. The two concentrated low velocity regions (U/U2<0.90) adjacent to the suction side near the flat and contour endwalls are located in the passage vortex at this plane. The passage vortex region near the contour endwall is about half the size of that near the flat endwall. The velocity vector plot in figure 29 reveals that fluids are being displaced from the contour endwall region toward the mid-span (y/S=0.50) as all the vectors in this region are pointing toward the mid- span. The vectors pointing toward the suction side near y/S=1.0 indicate that the cross-pitch flow is much stronger at the flat endwall than at the contour endwall. As this component of the flow is one of the major contributors for the growth of the passage vortex, the passage vortex is smaller near the contour endwall. The passage vortices can be identified by the small turning vectors creating an apparent clockwise and counter- clockwise motion near the contour endwall and flat endwall, respectively. The static pressure distributions near the endwalls along the passage in figure 30 illustrate further the effects of the profiled endwall on the cross- pitch and secondary flows. The contoured profile for the data in figure 30 is similar to that in figure 29 except the profile now extends across the entire passage length. The other endwall of the passage is flat without any contouring. In figure 30, the pressure distribution at the flat wall is similar to what is observed on the non-profiled endwalls in a linear vane passage. At the contoured endwall side, the contour lines of constant pressure near the leading edge are aligned more in the pitch direction than in the axial direction. In contrast, the constant pressure lines at the flat wall side near the leading edge are aligned more in the axial direction. The pressures at the contoured endwall are higher than those at the flat endwall for the first 40% axial chord. The pressures then are lower at the contoured endwall than at the flat endwall for the latter 60% axial chord. Thus, the pressure gradient at the contoured endwall is more parallel to the vane surface than to the pitch direction. The pressure gradient at the flat endwall is more parallel to the pitch direction than to the vane surface. As a consequence, the cross flow in the pitch direction is stronger on the flat endwall than on the contoured endwall. The reduced strength of the endwall cross flow then suppresses the growth of the passage vortex as mentioned earlier. The total pressure loss at the passage exit is reduced when the passage vortex near the contoured endwall is weakened and reduced in size. This is illustrated in the total pressure measurements in figure 3158. The planar vane cascade in the figure employs flat endwalls at both the hub and tip while the tip wall is axially contoured and the hub wall is flat for the contoured vane cascade. The contouring here extends across the entire passage length. The data in figure 31 are presented in a plane located 10% axial chord downstream of the passage exit. Hence, the endwall profile is flat at this location and the total passage height (z/S) is same for both the planar cascade and contoured cascade. The passage vortex regions in the figure can be identified by the highly concentrated circular contour lines near the endwalls. The parallel contour lines about y/P=0.50 indicate the wake region. The loss distributions are almost symmetric about the mid-span location z/S=0.50 for the planar cascade and the passage vortices are located away from the endwall regions as expected. While the loss distributions are asymmetric in the contoured cascade, the passage vortex loss region about z/S=0.10 from the flat wall side is similar to the passage vortex loss region in the planar cascade. However, the core loss region of this passage vortex has shifted closer to the flat wall side and further away from the suction side trailing edge. Fig. 30. Static pressure distributions near endwalls in a linear vane passage with endwall profiling extending from leading edge to trailing. Source: See Note 55. (Shih) Fig. 31. Total pressure loss, Cpt distributions at 1.10Cax for a linear vane cascade with and without endwall profiling. Source: See Note 55. (Dossena) 380PDF Image | Turbine Blade Aerodynamics
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