325Surface flow on the blade neartip surfaces

In fig. 3.9 on the suction side at small tip clearances (r/C = 0.86%, 1.72%), the surface flow patterns are close to that of without tip clearance. Separation curves S2s and S4 due to the suction leg of the horseshoe vortex (Fsc) and the passage vortex (Vp) can be clearly identified. However, S4 starts later on the suction surface, perhaps indicating the late development of the passage vortex due to the presence of tip clearance. The separation and reattachment/transition zone (S7, R7) decreases with increase in tip clearance level. When compared with the case of zero tip clearance, a major difference is the disappearance of the corner vortex (Fsc) reattachment line (R3) at suction surface. It is replaced by the reattachment line of the leakage vortex (Vti along LR). For larger tip clearances (3.45%C and 6.90%C), however, the separation curves of the suction leg of the horseshoe vortex and the passage vortex are difficult to track and totally disappeared at the highest tip clearance level 6.90%C, at the same time it can be observed that the region affected of the tip leakage vortex (Vti) grows with the increase in tip clearance level, from very close to the tip edge at small clearance level to almost one third of suction surface height at the largest clearance level, which suggests that the leakage vortex is strong and probably pushed the passage vortex (Vp) away from the suction surface. The tip leakage vortex (Vti) flow patterns on the suction surface at the two largest tip clearances are similar to that described by Sjolander and Amrud (1987): the surface flow first deviates from the tip at 0.4C and then is driven toward the tip on the second half of the suction surface. This pattern is perhaps caused by the roll-up of the leakage vortex and could incorporate boundary layer flow on the suction surface into the leakage vortex at large tip clearance levels.

On the pressure surface in fig. 3.10, the separation (S6) and reattachment (R6) zone still exists at all four tip clearance levels. The flow inside the separation zone is difficult to interpret, and larger shear stresses can be observed in reattachment zone at larger tip clearance levels. Downstream of this zone and away from the tip, the surface flow is still essentially two dimensional. No corner vortex (Vplc and Vpc) exists near the junction of the leading edge close to blade tip, as in the case of without tip clearance, even for the smallest tip clearance level of 0.86%C. The sink flow effect of tip clearance can be clearly seen by the traces of leakage flow accelerated toward the tip edge. The larger the tip clearance, the larger angle of the leakage trace to the tip edge, at almost right angle for the largest tip clearance (r/C = 6.90%), indicating that the boundary layer flow on the pressure surface can become part of the leakage flow. It can also be observed that at the two largest tip clearance levels, the flow is strongly accelerated from the separation zone near the tip into the tip clearance, which may cause high rates of local heat/mass transfer.

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