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bubble close to the pressure side edge of the tip. For the two largest tip clearances, the peaks of mass transfer Sh move further away from the pressure side edge and toward the suction side upstream of x/C = 0.41, showing a even larger separation bubble on the first half of the tip surface. The reason for a valley of Sh occurring around 0.05(y — y0)/C from x/C=0.13 to 0.33 for the largest tip clearance is not very clear. It may have something to do with the interaction between the leakage flow from suction side and the pressure side at larger tip clearance levels. After x/C = 0.41, the separation bubble is perhaps large enough to cover the tip thickness, leading to the accumulation or even reverse flow on second half of the tip surface. In fact, the mass transfer patterns for the two largest tip clearance are changed after x/C = 0.41, with peaks of mass transfer near the suction side edge of the tip, showing the effect of reverse flow.

Local Sh at different x/C locations are plotted in fig. 4.43 and fig. 4.44 against (y—y0)/r, at different tip clearance levels. For x/C < 0.43, we can clearly observe that the leakage flow reattaches at about 2 — 3r from the beginning of the naphthalene surface for tip clearance levels 0.86-3.45%C, the smaller the clearance level, the higher the peak value. For the largest tip clearance of 6.90%C, the reattachment occurs closer to the pressure edge. After x/C = 0.43, we can see that the reattachment of the leakage lies at about 2r for almost all x/C locations and at all clearance levels. For the smallest tip clearance of 0.86%C, the entrance flow effect on the mass transfer is quite clear, with the maximum at the leakage flow entrance 3-4 times as high as that of further downstream. For other larger tip clearances, this entrance flow effect is also evident but less effective. It also shows for the two largest tip clearances, the separation bubble is so big in the second half of the tip surface that it covers the whole tip. The second peaks at large (y — y0)/r at larger tip clearance level for x/C < 0.21 are probably caused by the second separation bubble on the suction side near the leading edge, as shown in the flow visualization.

The averaged Sherwood number along x/C for different tip clearances is plotted in fig. 4.50(a). We can see that the averaged mass transfer rate for t/C = 1.72% is higher than that of t/C = 0.86%. Similar result was also obtained by Teng et al. (2000) and Azad et al. (2000) and can be explained by the combined effects of shear flow and total leakage flow rate inside the tip clearance. For the two largest tip clearance, the first peak near the leading edge is due to the high mass transfer on the pressure and the suction edge of the tip, while the valley and peak around x/C = 0.5 — 0.65 is caused by the separation and reverse flow. The peaks around x/C = 0.4 for all cases are caused by the in accurate measurement near the injection hole of the naphthalene casting.

4.3.2 Effect of mainstream stream Reynolds number and turbulence

At the same tip clearance level of t/C = 0.86%, the effects of mainstream Reynolds number and turbulence intensity on the mass transfer on the tip surface are displayed in contour plots fig. 4.4 and 4.45. The mass transfer pattern is quite similar for all Reynolds number cases, with higher mainstream Reynolds number resulting in higher mass transfer rates. The high mainstream turbulence intensity seems not affecting the mass transfer pattern very much but reduces the local mass transfer rate.

The local mass transfer Sh along the span at different x/C locations is shown in fig. 4.46

and fig. 4.47 in terms of (y — y0)/r. The entrance flow characteristics is obvious at this smallest tip clearance levels. The figures show that the mass transfer Sh generally increases with increasing mainstream Reynolds number, while the high mainstream turbulence intensity leads to lower mass transfer rate compare to that of lower turbulence level at a similar Reynolds number. At low mainstream Reynolds number (Reex = 4.65 x 105 or high mainstream turbulence level (Tu=12%), the separation bubble occurring close to the pressure edge of the tip is apparently larger and the peaks of mass transfer locate at a small distance downstream of the start of naphthalene surface on the tip. In the normalized local Sh plots in fig. 4.48 and fig. 4.49, mass transfer Sherwood numbers over Re®x collapse to one curve very well for all cases with different Reynolds numbers at almost all x/C locations, which suggest that though the leakage flow may separate laminarly due to the large acceleration rate at inlet of the clearance, the leakage flow after reattachment is turbulent inside the small tip clearance of 0.86%C.

Averaged Sherwood number along x/C for different Reynolds numbers and turbulence levels is shown in fig. 4.50(b). As state earlier, the higher mainstream Reynolds number, the higher averaged mass transfer rates on the tip surface. The high mainstream turbulence intensity suppress the mass transfer rate on the tip surface compared with that of lower turbulence levels at similar mainstream exit Reynolds number.

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