Handbook of Turbomachinery Second Edition Revised - Ventech!

1. Reduce the effect of main-stream temperature dilution by utilizing
the maximum potential of internal cooling and discharging the
spent air at temperature that is close to the allowable metal
temperature.
2. Minimize pressure drop in the internal cooling passages of the
airfoil to enable discharge of the spent cooling air on the pressure
side of the airfoil upstream of the throat or through the trailing
edge at a velocity that matches the main-stream velocity. This
results in reduced aerodynamic losses and improved turbine
performance.
These design rules are often compromised when the internal cooling is
applied for the leading edge, which is usually the highest thermally loaded
part of an airfoil. Internal cooling of the blade leading edges is often
preferred for modestly high inlet temperatures due to their lower
manufacturing cost and elimination of stress concentration versus film
cooling. More effective internal cooling techniques based on impingement
or swirl cooling are required for this region of the airfoil. These techniques
typically require a larger pressure drop of the cooling air and air has to be
discharged in less favorable locations for aerodynamic penalties—on the
suction side or at the trailing edge of the airfoil at a velocity lower than the
main stream.
Somewhat limited information can be found in the open literature
concerning the effect of film cooling on aerodynamic losses, particularly for
the typical turbine airfoil flow that is characterized by large pressure
gradients and multiple rows of film injection. Figures 5and 6present a
summary data [8, 9] obtained for typical cooling flows and penalties
associated with different locations of cooling discharge along a nozzle vane
and a turbine blade. These data clearly illustrate a relatively small impact of
the air discharged at low main-stream Mach number locations like
stagnation point or a significant portion of the pressure side. However, a
very strong adverse effect can be observed at high main-stream Mach
number locations such as the suction side, particularly closer to the throat of
the passage.
An analytical treatment for the mixing loss due to film blowing, based
on the hypothesis that the injected coolant mixes with the main-stream flow
rather than remaining entrained in the boundary layer, is recommended by
Hartsel [10], providing the following simplified one-dimensional total
pressure loss equation:
Where
DP=P? ¼ gmc=m?Ma 2 =2ð1 þ Tc=T? 2Vc=V? cos aÞ
Copyright © 2003 Marcel Dekker, Inc.