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Handbook of Turbomachinery Second Edition Revised - Ventech!

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mass flow rate (e.g., M ¼ 1 for a single row <strong>of</strong> holes and M ¼ 0:5 for a<br />

double row <strong>of</strong> holes) the staggered row configuration provides higher filmcooling<br />

effectiveness than the single-row configuration at blowing rates that<br />

result in ‘‘blow-<strong>of</strong>f’’ in single-row configurations. At lower blowing rates,<br />

M < 0:5, the film-cooling effectiveness obtained for both configurations is<br />

about the same. A further advantage in the use <strong>of</strong> staggered rows is derived<br />

from the spanwise, more uniform film cooling. It has been shown that the<br />

use <strong>of</strong> double rows <strong>of</strong> staggered holes or shaped holes in order to eliminate<br />

blow-<strong>of</strong>f is advantageous for suction surface applications. To improve<br />

pressure surface effectiveness, use <strong>of</strong> multiple rows with film accumulation<br />

from preceding rows is required.<br />

Application <strong>of</strong> shaped holes that are diverged in the lateral direction<br />

normal to the direction <strong>of</strong> the main-stream flow can significantly improve<br />

attachment <strong>of</strong> the film jet to the surface. The most common shaped hole<br />

geometry consists <strong>of</strong> a circular metering hole, which controls the film flow<br />

rate, with a flared diffuser section at the discharge <strong>of</strong> the hole. For values <strong>of</strong><br />

M ¼ 0:5, the shaped hole film effectiveness continues to increase with<br />

increasing blowing rate, whereas the cylindrical hole effectiveness exhibits a<br />

sharp decline, particularly in the region near the holes. This sharp decline in<br />

effectiveness is attributed to jet blow-<strong>of</strong>f. Additionally, the shaped hole<br />

contributes to improved lateral distribution <strong>of</strong> the secondary flow so that the<br />

film-cooling effectiveness between holes is significantly greater than that<br />

obtained with straight cylindrical holes. The improved effectiveness is<br />

attributed to the reduced injection velocity <strong>of</strong> the secondary flow due to the<br />

larger discharge area. This lower velocity causes the jet to stay closer to the<br />

wall rather than penetrate into the main stream and accounts for the higher<br />

film effectiveness than obtained at high blowing rates for the same hole<br />

geometry.<br />

It is apparent from the preceding discussion that the use <strong>of</strong> shaped<br />

holes or double hole rows in order to eliminate blow-<strong>of</strong>f only appears to be<br />

advantageous for suction surface applications.<br />

Where a turbine airfoil is cooled with the aid <strong>of</strong> films, there is an added<br />

benefit in that the film-cooling holes can constitute quite a powerful<br />

convection heat-transfer system locally. While this is not usually a sufficient<br />

reason in itself to warrant the provision <strong>of</strong> a film-cooling hole row (or rows)<br />

in a design, it nevertheless provides a useful bonus when film cooling is<br />

added. This effect is particularly beneficial in the leading-edge region <strong>of</strong> an<br />

airfoil when a shower head configuration is used. Radially inclining the film<br />

holes increases the local convection cooling effect by increasing the length <strong>of</strong><br />

the holes.<br />

In summary, the main factors affecting the performance <strong>of</strong> film<br />

cooling include hole size, shape, spacing and angle, number <strong>of</strong> rows, row<br />

Copyright © 2003 Marcel Dekker, Inc.

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