322 Surface flow near the endwall with no tip clearance

The surface flow visualizations without tip clearance on the pressure, suction, and endwall surfaces are shown in fig. 3.6. The incoming flow is from left to right for the side surfaces with the mid-span at the bottom.

The endwall surface flow pattern is shown in fig. 3.6(a). The incoming turbulent boundary layer (shown by the arrow), blocked by the blade leading edge, splits around the saddle point A1 and lifts up along the separation curves S2p and S2s to form the pressure and the suction leg of horseshoe vortex (Vph and Vsh in fig. 3.16). The less obvious separation lines Sip and Sis are due to the boundary layer separation ahead of the horseshoe vortex (Sieverding, 1985) and usually are difficult to detect, but is clearly shown in Wang et al. (1997) that both the pressure leg of the horseshoe vortex and this boundary layer separation vortex become part of the large passage vortex (Vp in fig. 3.16). The suction leg horseshoe vortex (Vsh) is swept onto the suction surface not far from the leading edge (« 15%CX) and then lifts off, moving around the passage vortex, Vp, and toward the trailing edge along S2s. The pressure side horseshoe vortex (Vph), however, is accelerated by the traverse pressure gradient, displayed by the shear stress pattern formed between S2p and R5, toward the suction surface of the neighboring blade, and becomes part of the passage vortex separating along the suction surface (S4). Suction side corner vortices (Fsc in fig. 3.16) are identified by the separation line S3, formed by the incoming endwall boundary layer meeting with the suction surface downstream of 40%^. Large shear stress is shown in this region. Pressure side corner vortices (Vpc in fig. 3.16) can be detected by the reattachment line R5, where a new endwall boundary layer starts to develop and is driven to the neighboring blade.

On the suction side, the surface flow pattern is also complicated (fig. 3.6(b)). The suction leg of the horseshoe vortex (Vsh) separates along the separation line S2s as it lifts off from the endwall and moves toward the trailing edge. The trace of the passage vortex (Vp) including the pressure side horseshoe vortex (Vph) is shown by S4 as it separates above the suction leg of horseshoe vortex from the surface, inducing large shear stress on the blade surface. Further downstream on the suction surface, the pattern under S4 is difficult to track, which could be caused by the wall vortex (Vwip in fig. 3.16) induced by the passage vortex (Wang et al., 1997) or by the suction leg of the horseshoe vortex moving around the passage vortex. The sense of rotation for the passage vortex can be interpreted from the inclination of the streamline on the surface. A reattachment line R3 shows the existence of suction side corner vortex (Fsc) with an opposite sense of rotation with respect to the passage vortex. Beyond the S2, the surface flow is essentially two dimensional from the leading edge until it separates at S7, and later reattaching at R7 after transition.

The surface flow pattern on the pressure side (fig. 3.6(c)) displays a high shear stress region (light colored) close to the leading edge (away from endwall), followed by a low shear stress region (deep dark) induced by flow separation (S6). In this separation zone (0.050.15 Sp/C), the oil-particle trace is not clear, showing little movement of fluid particles. The reattachment line (R6) is also not very obvious in the surface flow pattern, and is possibly located somewhere downstream the dark separation region that displays relative higher shear stresses. Downstream of the separation and reattachment region, the flow is essentially two dimensional and follows the streamline till the trail edge. The existence of leading edge corner vortex (Vplc in fig. 3.16) is obvious in the flow pattern. The incoming boundary layer obstructed by the blade leading edge becomes stagnant and induces a low shear region at the junction of the leading edge and the endwall. Further downstream, the interaction of the endwall boundary layer and the pressure surface boundary layer results in weak separation line R5 near the endwall, showing existence of a corner vortex (Vpc in fig. 3.16).

The secondary flow system near the endwall interpreted from the present surface flow visualization is similar to those from previous studies ( Sieverding (1985), Hodson and Dominy (1986), Goldstein and Spores (1988), Chung and Simon (1990), Wang et al. (1997)).

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