Suction surface near the tip clearance

Effects of tip clearance level on the mass transfer on the suction surface are plotted in fig. 4.27 and fig. 4.28 in the form of contour and surface, respectively. Prom the plots, we can see that the triangular high mass transfer region caused by the secondary flows decreases in size as the tip clearance level increases. At tip clearance of 0.86%C, the tip leakage vortex exiting from the suction side of the tip clearance produces a small high mass transfer zone centered around Ss/C = 0.7 close to the tip edge. The effects of secondary flows still exist near the edge of the triangular region and seem to be pushed away from the tip edge. For the tip clearance of r/C = 1.72%, the region affected by the tip leakage vortex extends both downstream and in the span direction with its peak at Ss/C = 0.5 very close to the tip edge, while the region affected by the secondary flows becomes smaller, though still detectable. At higher tip clearance of 3.45%C, the effect of secondary flows disappears and the smaller triangular region is mainly affected by the tip leakage vortex with two peaks of high mass transfer, probably indicating multiple tip leakage vortices.

Local Sh and Sh/Reli2 at different Ss/C locations along the span are shown in fig. 4.29 and fig. 4.30, respectively, at different tip clearance levels. As can be found in fig. 4.29, the effect of tip clearance on the mass transfer on the suction surface is not evident till Ss/C = 0.18 and the mass transfer distribution near the tip edge are rather flat, though higher than that with no tip clearance, indicating almost no existence of leakage vortex near the leading edge of the blade. Further downstream, the effect of tip clearance can be seen that the smaller tip clearance induces larger mass transfer rate near the tip. This peak near the tip edge is caused by the tip leakage flow reattaches on the surface and the second peak occurs at Ss/C = 0.83 — 1.15 for the two smaller tip clearance levels probably indicates the passage vortex separating from the surface, pushed away by the tip leakage vortex. For the two larger tip clearances this second peak is not evident and the triangular region become smaller as the effect of secondary flows disappears. The same trend can be observed for the normalized Re in fig. 4.30, where the mass transfer data collapse outside of the triangular region at all streamwise locations.

Spanwise and streamwise averaged Sh on the suction surface are plotted in fig. 4.31 for different tip clearance levels. In fig. 4.31(a), the effect of tip clearance is evident: the smaller the tip clearance, the higher Sh for Ss/C > 0.5. The variation of streamwise averaged Sh vs. Z/C in fig. 4.31(b) clearly shows the effect of tip leakage vortex (Vti) especially at smallest tip clearance of 0.86%C.

At the same tip clearance level of r/C = 0.86%, the effects of mainstream Reynolds number and turbulence intensity on the mass transfer on the suction surface can be seen as contour and surface plots in fig. 4.32 and fig. 4.33, respectively. We can find that the high mass transfer region caused by the tip leakage vortex increase in size at the highest mainstream Reynolds number (Reex = 7.18 x 105), while the effect of secondary flow as well as the size of the triangular high mass transfer region decrease at the high mainstream turbulence level (Tu = 12%). The high mainstream turbulence level also causes the earlier transition on the suction surface away from the tip.

The local mass transfer Sh vs. Z/C at different curvilinear locations is shown in fig. 4.34. We can find that near the leading edge (Ss/C < 0.09), the higher the mainstream Reynolds number, the higher mass transfer rate can be obtained, and the leakage flow has almost no effect on the mass transfer near the tip. Downstream of Ss/C = 0.09, the effect of Reynolds number is not quite evident outside the triangular region. The mass transfer curves in the triangular region follow the same trend at different Reynolds numbers: the first peak near the tip is caused by the leakage vortex reattachment while the second peak is probably induced by the passage vortex separation from the surface. However, the effect of high mainstream turbulence level on the mass transfer rate is not important until Ss/C = 0.94, where transition to turbulence begins. A very similar trend is revealed by the normalized Sh plot in fig. 4.35. Mass transfer curves for different Reynolds numbers at low turbulence level collapse downstream of the leading edge and outside the triangular region. The effect of mainstream turbulence level is also plotted in fig. 4.36. Again the high turbulence level can effectively reduce the size and mass transfer peak of the triangular region of high mass transfer.

Spanwise and streamwise averaged Sh on the suction surface are displayed in fig. 4.37 at the same tip clearance of 0.86%C for different Reynolds numbers and mainstream turbulence intensities. In fig. 4.37(a), we can find that mass transfer rates increase with mainstream Reynolds number while high turbulence at the same exit Reynolds number doesn't cause high mass transfer on the suction surface in streamwise direction. From fig. 4.37(b) for streamwise averaged Sh vs. Z/C, the high mainstream turbulence induces much higher Sh for Z/C > 0.4 due to the early turbulent transition.

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