323 Surface flow on the tipendwall with tip clearance

With the presence of tip clearance, the surface flow patterns are changed appreciably at different tip clearance levels.

The surface flow patterns on the tip-end wall at the tip clearance level of t¡C = 0.86% in fig. 3.7(a) is similar to that of without tip clearance. The incoming flow approaching the blade leading edge, splits into two parts at the saddle point (Al), forming the horseshoe vortex system composed of pressure side leg (Vph separating along S2p) and suction side leg (Vsh separating along S2s) as in the case of zero tip clearance. Similarly, the pressure side leg of the horseshoe vortex (Vph) and the endwall boundary layer flow starting along AR are accelerated by the traverse pressure gradient toward the suction side of neighboring blade, as shown by the stress pattern on the surface. The suction leg of the horseshoe vortex (Vsh) lifts up around the 15%^ from the leading edge and moves around the passage vortex (Vp) along the suction surface (S2s). The other part of boundary layer flow starting along AR is accelerated into the tip clearance due to the pressure difference between the pressure and the suction sides. Large shear stresses exist on the endwall along the pressure edge of the tip apparently caused by the leakage flow passing through the narrow passage between the endwall and the separation bubble formed on the pressure edge of the tip surface. The leakage flow then separates on the endwall, forming the dark low stress streak along the pressure edge of the tip on the endwall, then it reattaches and passes through the clearance, and separates along LS, which coincides with the suction side edge from 15% to 70%^, to become the tip leakage vortex (Vti) before meeting the passage vortex (Vp) separating along PS. The flow between LS and PS has relatively large shear stresses, and the leakage vortex separation line LS becomes weak and finally merges with the passage vortex separation line PS near the exit of the cascade, probably showing that the tip leakage vortex starting from about \h%Cx is rather weak at this small clearance level.

At the tip clearance of 1.72%C (fig. 3.7(b)), the surface flow pattern is close to that of 0.86%C, though the horseshoe vortex system (separation lines S2p and S2s) is more difficult to identify. It can be found that more boundary layer fluids on the endwall are sucked into the tip clearance along AR as it moves from near the pressure side in previous case to about one third of pitch into the passage from the pressure side. A wider high-shear stress zone exists near the pressure edge of the tip on the endwall, perhaps indicating a larger separation bubble on the tip along the pressure edge. Along the separation line on the endwall immediately after the high shear stress zone near the pressure edge of the tip, there is a small accumulating spot around 0.75Cx, probably showing the existence of a low pressure zone there. The tip leakage vortex {Vti separating along LS) apparently starts separating along the suction edge of the tip from around 0.2Cx, and the area between the leakage flow separation (LS) and the passage vortex separation (PS) widens as they move further downstream into the blade passage. It is interesting to see the leakage flow on the endwall opposite to the first half of the tip surface splits into several small branches after reathachment. The surface flow patterns at two small tip clearance levels are similar to the results from Sjolander and Amrud (1987) and Moore and Tilton (1988), which suggested that the separation on the endwall is caused by the adverse pressure gradient created by the convergent-divergent passage formed by the separation bubble on the tip pressure edge, and that the separation on the endwall is laminar due to the acceleration of the leakage flow into the clearance.

At t/C = 3.45% (fig. 3.7(c)), the horse vortex system is no longer evident. Part of the incoming boundary layer flow is driven into the tip clearance from the suction side near the leading edge, mixing with the boundary layer flow from the pressure side along AS. The dividing line AR moves further into the blade passage, indicating more endwall boundary layer flow is sucked into the pressure side of tip clearance. The high shear stress zone along the pressure edge of the tip becomes even larger and the leakage flow separates on the endwall as it gets through the clearance in several large groups around the mid-chord tip surface, to form a complicated flow pattern, before separating along LS into the tip leakage vortex (Vti). The leakage vortex separates near the suction edge around 0.3Cx and soon deviates away from the suction edge into the blade passage, at the same time the separation of passage vortex (Vp separating along PS) also moves further downstream in the blade passage, possibly indicating a weaker and underdeveloped passage vortex compared to the tip leakage vortex, since more fluids are sucked into the tip leakage flow to become the leakage vortex instead of the passage vortex.

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