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Handbook of Turbomachinery Second Edition Revised - Ventech!

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correspondingly high coolant-side thermal effectiveness so as to achieve<br />

acceptable temperature distributions. At least seven distinctly different<br />

regions <strong>of</strong> blade heat transfer can be identified:<br />

1. Stagnation point<br />

2. Laminar boundary layer<br />

3. Transitional boundary layer<br />

4. Turbulent boundary layer<br />

5. Shock/boundary-layer interaction<br />

6. Separation with reattachment<br />

7. Separation without reattachment<br />

Since the heat transfer is closely coupled to fluid mechanics phenomena,<br />

each <strong>of</strong> these regions involves a separate analysis valid for that particular<br />

region. The heat transfer depends on Mach number, Reynolds number, freestream<br />

turbulence, free stream-to-wall temperature ratio, blade curvature<br />

and roughness, material, and the gas properties.<br />

In heat-transfer applications, the use <strong>of</strong> dimensional analysis is<br />

important as the heat transfer depends on a large number <strong>of</strong> parameters.<br />

Dimensional analysis enables reduction <strong>of</strong> the large numbers <strong>of</strong> variables to<br />

a manageable number <strong>of</strong> nondimensional groups. This, in turn, enables<br />

experimentalists to design experiments to carry out a parametric study.<br />

There is considerable empiricism in the analysis <strong>of</strong> convective heat<br />

transfer because <strong>of</strong> complicated geometry involved and complex interaction<br />

between the flow and heat-transfer fields. The following groups can be<br />

identified by a dimensional analysis:<br />

Reynolds number ReL ¼ rVL=m, based on a reference length or<br />

Red ¼ rVd=m, based on hydraulic diameter<br />

Mach number Ma ¼ V=a<br />

Prandtl number Pr ¼ mcp=k<br />

Eckert’s number Ec ¼ðg 1ÞMa 2 T=2DT<br />

In heat transfer (internal as well as external) applications, several additional<br />

nondimensional groups are important. The overall heat transfer is related to<br />

the temperature difference between the fluid ðT?Þ and the solid wall ðTwÞ<br />

through Newton’s cooling law: QðxÞ ¼hðxÞAðTw T?Þ ¼ kAðqT=qyÞ w .<br />

The heat transfer coefficient h can be nondimensionalized as Nusselt<br />

number Nu ¼ hðxÞ L=k ¼ LðqT=qyÞ w =ðTw T?Þ<br />

As indicated earlier, the boundary layer acts as a resistance to the heat<br />

transfer and this property is characterized by the shear stress. The<br />

nondimensional group called the Stanton number St represents the<br />

Copyright © 2003 Marcel Dekker, Inc.

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