315 Neartip static pressure distribution

To measure the effect of tip clearance on the static pressure distribution, a row of pressure taps closest to the blade tip is used, which is 2.2cm (0.12%C) away from the blade tip. The repeatable static pressure coefficients near the tip are plotted in fig. 3.5 together with the midspan data and the inviscid solution at different clearance levels.

The effect of secondary flows are the apparent cause for the difference between the data for zero tip clearance and those for mid-span, especially close to the trailing edge, as shown in fig. 3.5(b). For measurements with tip clearances, the unloading effect (decrease in pressure difference) from the tip clearance can be clearly found from the plot, even though the pressure taps are not very close to the tip. From the leading edge up to S/C ps 0.5, the blade loading (i.e. the pressure difference) decreases with increase of tip clearance, as the pressure coefficients increase on the suction side but decreases on the pressure side. In effect, very close to the leading edge (S/C < 0.05), the loading for the two largest tip clearances become negative, indicating that there may be leakage flowing into (instead of out of) the tip clearance from the suction side (fig. 3.5(e) and (f). The loading decreases substantially for the all tip clearances in the second half of the blade, where the pressure coefficients increase on both the pressure and the suction surfaces compared with the mid-span data (except for the largest tip clearance), but with larger changes on the suction side, resulting in larger unloading effect at larger tip clearances due to the presence of leakage vortex on the suction side. However, for the largest tip clearance (t/C = 6.90%), the pressure coefficients reduces drastically downstream of Sp/C = 0.6 on the suction surface, which is possibly caused by the blocking of the strong tip leakage vortex, leading to the reverse of unloading. Similar effect also occurs at smaller clearances (t/C = 0 — 3.45%) close to Sp/C ps 0.9.

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