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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 9 Typical high-pressure turbine blade with shower head and film-cooling<br />

holes.<br />

before emerging at the surface. All these features are indicated in Fig. 10,<br />

which shows a schematic diagram <strong>of</strong> the airfoil internal passages.<br />

The parameters affecting cooled blade design are (1) blade material<br />

characteristics and cost, (2) airfoil shape, (3) required cooling flow and the<br />

trailing-edge thickness, (4) allowable stress levels, (5) number <strong>of</strong> blades, and<br />

(6) vibratory environment. Minimizing the cooling air required for the<br />

airfoil is an important consideration in design. Cooling air flow (ejected at<br />

film-holes, trailing edge, etc.) and blade rotation generally affect (1) the<br />

aerodynamics <strong>of</strong> the blade and the overall efficiency <strong>of</strong> the system, (2)<br />

impingement heat transfer at the leading edge, and (3) the local heat transfer<br />

behind trip-strips and fins. In addition, blade-tip thickness and streamlines<br />

at the trailing edge are affected by the cooling flow issuing at the trailing<br />

edge. (For new designs, it may be necessary to use experience and<br />

extrapolate beyond the range <strong>of</strong> available data.) At present, CFD analysis<br />

does not provide accurate predictions <strong>of</strong> heat transfer and pressure drops in<br />

cooling passages with complicated internal geometry, especially where<br />

rotation is involved. Further, for complicated situations, considerable<br />

expertise and effort are needed to obtain solutions by using CFD. Hence, it<br />

is difficult to use CFD as a design tool. Improvements in CFD techniques<br />

Copyright © 2003 Marcel Dekker, Inc.

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