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4.3 Turbine Blade Aerodynamics Near the leading edge at the endwall, the pressure surface streamlines are inclined toward the endwall indicating the flow is driven by the horse-shoe vortex. The separation lines created by the oil streaklines on the suction surface of figure 7 reveals some interesting features of the boundary layer behavior. The separation lines divide the flow on the suction surface into three regimes: (i) two dimensional laminar boundary layer regime, (ii) turbulent boundary layer regime, and (iii) three dimensional flow regime. (i) Two dimensional laminar regime: This regime extends from the leading edge to the lowest suction pressure on the suction surface and between the S2s separation lines near the two endwalls in figure 7. The surface streamlines are seen to be nearly parallel to the endwall in this regime. The laminar boundary layer starting at the leading edge undergoes a high acceleration on the suction surface. According to Hodson and Dominy, the over- acceleration in the boundary layer causes a two dimensional separation bubble near the blend point of the circular leading edge and the suction surface15. This separation bubble extends across most of the span, but it is not apparent in the bottom surface flow visualization of figure 7. The suction surface leading edge separation bubble is shown by the flow visualization in Gregory-Smith et al.16. Following the re-attachment behind the separation bubble, the laminar boundary layer accelerates along the suction surface and continues to grow until the separation line S3s. (ii) Turbulent regime: This regime is limited by the re-attachment line following the separation at S3s and trailing edge and between the S2s lines. The laminar boundary layer separates at the lowest suction pressure located at axial distance at S3s because of the adverse pressure gradient (see figure 3) and forms another closed separation bubble. The boundary layer undergoes transition and becomes turbulent as it re-attaches behind the separation bubble on the suction surface. The turbulent boundary layer grows along the suction surface and may separate again due to the adverse pressure gradient near the trailing edge to form the trailing edge wake. (iii) Three dimensional flow regime: This regime is indicated by the region between the separation line S2s and endwall. The regime begins at the location where the suction side leg of the leading edge horse-shoe vortex and pressure side leg vortex from the adjacent blade meet on the suction surface. The pair then emerges as the passage vortex which then moves toward the mid-span as it follows the suction surface toward the passage exit. The suction surface boundary layer separates along the S1s and S2s lines near endwalls in figure 7 as the passage vortex and suction side leg vortex climbs up the suction surface. The distinct appearance of the separation line S2s indicates that the suction side leg vortex maintains its existence in the axial development of the passage vortex which will also be shown in further detail in the next section. The inclination of the surface streamlines toward the mid-span in this regime is caused by the entrainment of the boundary layer fluids (both at the endwall and the suction surface) by the passage vortex. Note that the surface streamlines are symmetric about the mid-span of the blade surface in figure 7. The patterns become asymmetric in three- dimensional cascade by the influence of radial forces as will be shown in further sections. The locations of the separation bubbles and separation lines on the blade surface are strongly influenced by the inlet flow angle and Reynolds number or Mach number of the incoming flow. For the high speed compressible flow (with the Mach number>0.70), the flow expands and accelerates along the passage creating local supersonic region at the passage throat17. As a result, a series of weak compression fans are developed at the suction surface near the throat. Detemple-Laake also shows that at transonic and supersonic flow, shocks are formed across the span at the trailing edge of the blade surface18. The shock at the suction surface trailing edge is deflected by the wake from the adjacent blade trailing edge. The shock at the pressure side trailing edge is reflected at the adjacent blade suction surface as a sequence of compression-expansion-compression waves. At all Mach numbers tested (exit Mach number ranges between 0.70 and 1.3), schlieren photographs show that flow separates locally from the blade pressure surface and suction surface forming separation bubbles similar to the subsonic flow pattern19. The separation lines for the suction side leg vortex and the passage vortex on the suction surface move nearer to the mid-span as the Mach number is increased. The suction side leg vortex is deflected by the shock from the adjacent blade pressure side trailing edge and moves closer to the passage vortex at supersonic flow. The endwall pressure distributions for high speed compressible flows show the same behavior as that at the low speed flows. Static pressure on the endwall increases slightly at the trailing edge due to the expansion at the trailing edge. 4.3-4 Development and Structure of Secondary Flows in the Passage We have shown in the earlier section that the secondary flows in the turbine vane/blade passage are dominated by the vortex flows located in the hub endwall region. So far, these vortex flows have been deduced from pressure distributions, near-wall streamlines and saddle points or surface oil-flow visualizations. The vortex flows have been identified as the suction side leg and the pressure side leg originating from the leading edge horse-shoe vortex that eventually merge in a complex way to form the passage vortex. The three vortex structures (horse-shoe, pressure side leg, suction side leg) are the primary sources of the vortex flows in the passage. In addition, smaller corner vortices are induced at the corner of blade edge at the endwall. Vortices are also induced on the suction side near the meeting point of the pressure side leg vortex and suction side leg vortex flows and are advected with these legs along the suction surface toward the passage exit. This section will discuss the structure and development of the three primary vortex flows along the passage at different axial locations by presenting the flow visualization images, streamlines, pressure losses, vorticity, turbulence intensity, and flow turning angles. The induced vortex flows will be identified later in the section. Leading edge horse-shoe vortex: The leading edge horse-shoe vortex is formed at the junction of an endwall and the blunt leading edge of the blade. As the flow approaches the leading edge stagnation line, static pressure rises across the flow from the endwall. The static pressure increases more in the free-stream region above the boundary layer since the free-stream velocity is higher compared to the velocity in the boundary layer20. This spanwise pressure gradient in the vicinity of the leading edge causes a vortex roll-up, known as the leading edge horse-shoe 368PDF Image | Turbine Blade Aerodynamics
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