431 Effect of tip clearance level

Effects of tip clearance level on the mass transfer on the tip surface are shown in the contour plots of fig. 4.39 and fig. 4.40. At tip clearance of r/C = 0.86%, high mass transfer rates occur immediately at the pressure edge of the tip surface, especially near the mid-chord of the surface, where the Sh number descends from a value of 10000 at the pressure side to about 3500 near mid-chord of the tip. For the tip clearance of 1.72%C, high mass transfer rates still exist on the second half of the tip surface with peak values at a small distance downstream of the pressure edge, and near the leading edge. These peaks are resulted from the reattachment of the leakage flow after separation bubble. A large low mass transfer zone also appears on the first half of the tip surface near the leading edge and close to the suction side, which is probably caused by the low rate of leakage flow passing through that area as shown from the surface flow visualization the leakage vortex separates from the tip-endwall around X/Cx = 0.2. There are small peaks of high mass transfer on the first half of the tip and near the pressure edge, inducing irregular mass transfer patterns.

At higher tip clearance of r/C = 3.45%, the high mass transfer region remains near the pressure edge close to the leading edge, while the low mass transfer region increases in size on the first half of the tip surface, probably induced by the leakage flow coming from the suction side at this relatively large tip clearance level. Relatively low mass transfer rates exist on the second half of the tip surface due to the accumulative and perhaps reverse flows occurring there. For the largest tip clearance of 6.90%C, the high mass transfer region appears along the pressure and the suction edge of the tip surface close to the leading edge. At the mid-chord surface and in the second half of the tip surface, low mass transfer is caused by the reverse flow. The low mass transfer region affected by the leakage flow from the suction side become larger in size on the first half of the tip surface.

Local Sh at different x/C locations along (y — y0)/C are plotted in fig. 4.41 and fig. 4.42, at different tip clearance levels. For t/C = 0.86%, maximum mass transfer rates always occur at the start of the naphthalene surface (y—yQ = 0), very close to the pressure side edge for all x/C locations, showing the leakage flow separates and reattaches on the tip surface immediately at the pressure side edge of the tip. At tip clearance level of 1.72%, mass transfer peaks exist at a small distance (« 0.02C) from the start of naphthalene surface for all x/C locations, probably indicating the leakage flow reattaches over a larger separation

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