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However, the fact that Sherwood numbers for the case of a moving endwall were consistently lower than those for the case of no motion indicates that there does exist a slight effect of endwall motion that acts to reduce Sh. A possible explanation for this phenomenon is that at small clearances, viscous effects become sufficiently strong as to slightly drag the leakage vortex towards the suction side and alter the driving pressure gradients. The leakage flow may be slightly reduced in magnitude, resulting in smaller velocities and smaller mass transfer coefficients. The movement of the reattachment point towards the pressure edge corroborates this.

At t/C = 0.86%, the picture is the same as that for t/C = 0.6% on the first half of the blade, as seen in figure 5.8. Reattachment occurs at roughly the same «/-position as for t/C = 0.6%. However, there is virtually no effect of endwall motion on the Sherwood numbers on the first half of the blade. Sherwood numbers are lower for the case of endwall motion, as for the smallest clearance, with the difference lying within the experimental error limits of this study. However, figure 5.9 shows that there is a slight effect on the second half of the blade, with the Sherwood numbers slightly increasing with the introduction of wall motion. This is somewhat difficult to explain, since both the flow visualization and the pressure measurements point to a reduced flow velocity near the trailing edge at this clearance level as compared to t/C = 0.86%. However, it was confirmed that the data were repeatable. The reattachment point seems to shift towards the suction side, indicating a slight increase in the size of the separation bubble, which is not observed from the flow visualization.

As the clearance is increased to t/C = 1.72% (figs. 5.10 and 5.11), the trend of reducing Sherwood numbers continues. As the flow moves downstream towards the trailing edge, the point of maximum Sherwood number shifts towards the suction side, indicating an increase in the size of the separation bubble near the trailing edge. At the very last x/C location shown, there is no maximum of Sh, indicating that the bubble covers the entire width of the tip. There is no effect of endwall motion at this clearance level.

At r/C = 3.45% (figs. 5.12 and 5.13) , the overall Sherwood numbers decrease further. For this clearance level, the separation bubble covers the entire width of the tip starting from x/C = 0.42, as seen by the continuous increase of Sh from pressure side to the suction side. As expected, based on the previously discussed clearance levels, there is no effect of endwall motion on the mass transfer.

At t/C = 6.9% (figs. 5.14 and 5.15), the size of the separation bubble become larger. Near the leading edge, two peaks of Sherwood number are observed, corresponding to the two reattachment lines caused by flow entering from pressure and suction sides. The size of the midchord separation bubble increases, and there is no observable effect of endwall motion on the mass transfer.

A summary of the area-averaged Sherwood number on the tip surface for each of the cases studied is given in table. 5.1. It can be seen that except for the case of smallest clearance, the difference in Sherwood number caused by wall motion is within the error limits of this experiment (see appendix C).

r/C

Run

Shm, EES = 0%

Run

Shm, (EES = 100%)

Percent change

T/C = 0.6%

0 0

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