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Handbook of Turbomachinery Second Edition Revised - Ventech!

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A considerable amount <strong>of</strong> early film-cooling experimental work was<br />

performed using two-dimensional slot configurations. A number <strong>of</strong> these<br />

studies were reviewed in detail, together with a comprehensive development<br />

<strong>of</strong> theoretical analyses relating to the subject <strong>of</strong> film coolant. Most <strong>of</strong> the<br />

empirical expressions for slot film-cooling effectiveness have been developed<br />

using heat sink models for two-dimensional incompressible flow filmcooling<br />

injection. Several are discussed in detail in [46]. The equation<br />

developed by this work is<br />

Z c ¼ 1:53=f1 þ 0:329 Re 0:2<br />

s<br />

½X=ðMsÞŠ 0:8 Kg<br />

where K ¼ 1 þ 1:5610 4 Res sin b with the slot Reynolds number<br />

Res ¼ ms=m.<br />

The above correlation is recommended as a generalized slot filmcooling<br />

correlation.<br />

Film Accumulation<br />

For most high-temperature turbine airfoil cooling designs, film-cooling<br />

schemes are employed on the leading edge as well as along both the suction<br />

and pressure surfaces. This is because the coolant film must be periodically<br />

renewed as decay <strong>of</strong> the film and mixing with the hot gas stream bring the<br />

adiabatic wall temperature back to the gas-stream temperature. In this<br />

situation, where there are two or more film rows provided on an airfoil<br />

surface, it is necessary to be able to estimate the combined effects <strong>of</strong> these<br />

films. The following equation for the overall film-cooling effectiveness is<br />

recommended for use where there is a film-cooling additive effect:<br />

Zf ¼ Z1 þ Z2ð1 Z1ÞþZ3ð1 Z1Þð1 Z2Þþ þZnð1 Z1Þð1 Z2Þ ð Þð1 Zn 1Þ<br />

where n is the number <strong>of</strong> film rows.<br />

This simple superposition model is based on the assumption that the<br />

accumulated film effectiveness due to multiple rows <strong>of</strong> holes can be<br />

represented from data or correlations for a single row <strong>of</strong> holes. It was<br />

derived from the concept that the gas temperature used to determine the film<br />

effectiveness downstream <strong>of</strong> an injection location should be the adiabatic<br />

film temperature due to all injections upstream <strong>of</strong> that location. Industry<br />

comparison <strong>of</strong> this method with multiple row data has, in general, shown<br />

good agreement. The film superposition equation was further studied for<br />

multiple row <strong>of</strong> injection holes over an airfoil suction surface. An excellent<br />

agreement between measured and predicted overall film-cooling effectiveness<br />

was demonstrated in a number <strong>of</strong> studies. It should be noted that the<br />

Copyright © 2003 Marcel Dekker, Inc.

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