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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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further used to provide a film on the pressure and suction surfaces, thereby<br />

providing a design having high thermal efficiency.<br />

This above summary indicates that development <strong>of</strong> more effective<br />

internal cooling techniques that can counterbalance high external heat loads<br />

for the blade leading edge and tip section is required, particularly for<br />

moderate turbine inlet temperatures, when the application <strong>of</strong> film cooling<br />

can be avoided.<br />

Swirling Flow<br />

Extensive studies <strong>of</strong> heat exchangers show that one <strong>of</strong> the effective heattransfer<br />

augmentation techniques is based on swirling flow. Swirling flow is<br />

commonly referred to as a vortical flow that can be generated by a number<br />

<strong>of</strong> methods, including twisted tape inserts, coil wires, inlet guide vanes, and<br />

tangential injection <strong>of</strong> the fluid. Some <strong>of</strong> these methods generate swirl<br />

continuously along the entire length <strong>of</strong> the test section, <strong>of</strong>ten a cylindrical<br />

tube, whereas others are placed at the inlet with the decay <strong>of</strong> swirl along the<br />

tube. A technique based on tangential jet-induced swirl flow that has been<br />

recently developed [86, 87] is <strong>of</strong> particular interest for the leading edge <strong>of</strong> a<br />

blade. It utilizes a three-dimensional screw-shaped swirl flow induced by<br />

tangential slot jets swiping the interior <strong>of</strong> the leading edge as shown in Fig.<br />

21. Multiple jets along the channel provide energized in radial direction swirl<br />

flow motion, which is sustained over the entire height <strong>of</strong> the blade. Heat<br />

transfer along the concave surface is augmented by the unsteady Go¨rtler<br />

vortices formed in the near-wall region. The high-momentum jet, introduced<br />

Figure 21 Illustration <strong>of</strong> blade leading-edge swirl cooling.<br />

Copyright © 2003 Marcel Dekker, Inc.

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