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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 32 Schematics <strong>of</strong> a hot cascade for airfoil-cooling validation.<br />

cascade local total temperatures and pressures, simulating engine<br />

conditions, are recommended to be measured with an automatic traversing<br />

Kiel probe and thermocouples.<br />

The similarity principles applied to the main-stream gas side <strong>of</strong> the hot<br />

cascade also have to be applied to the coolant side <strong>of</strong> the airfoil under<br />

investigation. Additionally, the gas-to-coolant mass flow rate and the gasto-coolant<br />

total temperature ðTg=TcÞ ratios must be matched. Emulation <strong>of</strong><br />

the forced convection heat-transfer process on gas turbine blade surfaces<br />

requires the similarity <strong>of</strong> Re, Pr, Ma, Tx/Tgo, and Cp/Cv. The Nusselt<br />

number in the turbine (inlet) and in the hot cascade (inlet) will be the same if<br />

the similarity <strong>of</strong> all <strong>of</strong> these parameters is enforced, because the Nusselt<br />

number has a direct functional dependency on the local Reynolds number,<br />

the Prandtl number, and the free stream-to-wall temperature ratio. It should<br />

be noted that the list <strong>of</strong> similarity parameters used results from the unsteady<br />

form <strong>of</strong> the momentum and energy equations. In addition, the free-stream<br />

turbulence also affects airfoil heat transfer and should be reproduced during<br />

heat-transfer simulation experiments. Ideally, the intensity <strong>of</strong> free-stream<br />

turbulence and a typical length scale <strong>of</strong> the characteristic turbulent eddies as<br />

well as the inherent periodic unsteadiness <strong>of</strong> the turbine environment should<br />

Copyright © 2003 Marcel Dekker, Inc.

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