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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Surface Curvature<br />

Curvature plays a very important role in achieving required film-cooling<br />

effectiveness. For the same blowing rate ðM ¼ 0:5Þ, the suction-side<br />

effectiveness is higher than that for an equivalent flat plate by up to<br />

100%; on the pressure side the cooling effectiveness is reduced by around<br />

50%. The effect is evident for all coolant-to-gas density ratios. The observed<br />

curvature effects on film-cooling effectiveness have been attributed to the<br />

balance <strong>of</strong> the forces exerted on the injected fluid jet by the static pressure<br />

and the centrifugal force along the path <strong>of</strong> the injected jet. These forces have<br />

been shown, for low-momentum jets, to act on the injected jet in the<br />

direction that moves the jet toward the surface on the suction side and away<br />

from the surface on the pressure side. For high-momentum jets, the opposite<br />

occurs. Hence, for low-momentum jets, the general trend for film-cooling<br />

effectiveness is low on the pressure side and high on the suction side, relative<br />

to a flat plate. These trends are reversed for high-momentum jets. Suction<br />

surface data exhibited the characteristic limiting value caused by blow-<strong>of</strong>f<br />

and observed with flat-plate data. Clearly then, it is necessary for the turbine<br />

cooling designer to have a sufficiently large database or appropriately<br />

calibrated numerical methods to be able to assess the impact <strong>of</strong> film cooling<br />

during the design process.<br />

Pressure Gradient<br />

Very little data are available to define the effect <strong>of</strong> pressure gradient on filmcooling<br />

performance. Its effect is generally combined with surface curvature<br />

when airfoils are tested under simulated main-stream conditions. In this<br />

situation it is not possible to separate the two interrelated effects. However,<br />

it is generally understood that both favorable (accelerating) and adverse<br />

(decelerating) main-stream pressure gradients can cause a slight degradation<br />

<strong>of</strong> film-cooling effectiveness as compared to that for zero main-stream<br />

pressure gradient flow (constant velocity).<br />

Main-Stream Turbulence<br />

Early studies on film cooling were all performed under low free-stream<br />

turbulence conditions. In an actual gas turbine, airfoils experience<br />

combustor-generated turbulence as well as the unsteady wake effect<br />

generated by upstream airfoil trailing edges. These effects have received<br />

more attention in recent years because the results obtained under lowturbulence<br />

conditions may lead to significant overprediction <strong>of</strong> the filmcooling<br />

effectiveness. Free-stream turbulence and unsteady wake may<br />

damage the film protection on a surface due to greater mixing <strong>of</strong> the injected<br />

Copyright © 2003 Marcel Dekker, Inc.

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