Handbook of Turbomachinery Second Edition Revised - Ventech!

swirl cooling technique provides internal heat-transfer augmentation
comparable with the impingement coupled with film discharge of the spent
air and is more effective than impingement with cross flow or trip-strips.
Application of this technique might allow extension of the range of internal
blade cooling for moderately high turbine inlet gas temperatures, further
avoiding the use of film cooling for the leading edge.
Film Cooling
There is relatively little published data on the effects of film cooling on
rotating turbine blades. Most of the studies were performed for the
stationary environment. A number of ongoing studies address the effects of
rotation on film flow and heat transfer.
Very early work by Dring et al. [89] in a low-speed rotating test facility
examined a single film-cooling hole placed on the suction and pressure sides
of a rotor blade. Contrary to the nature of a film on a nonrotating aerofoil, it
could be seen that the pressure-side coolant becomes entrained into the
spanwise secondary flow and migrates toward the tip. On the suction side of
the aerofoil, the coolant behaves as though rotation has only a small effect.
The tests were for cases where the density ratios of 1.0 and 4.0 were examined.
It was observed that the higher-density ratio gives rise to more effective
cooling as is the case for nonrotating airfoils. The experiments by Dring have
demonstrated that film cooling on rotating blades behaves differently from
nonrotating vanes. Unfortunately, however, the data are too limited to derive
any design correlation for film cooling on rotating airfoils.
More recently, the work by Takeishi et al. [90] on a full-scale rotating
turbine has also shown that pressure-side film-cooling behavior on rotating
blades is quite different from that found on nonrotating airfoils. It was
observed that on the suction side the film effectiveness values are similar.
However, in the case of the rotating blades the pressure-side film
effectiveness decays very rapidly for a film near the leading edge. In a
similar work, Abhari & Epstein (1992) [91] have confirmed these findings.
They showed that at both low and high blowing rates the film has almost no
effect on reducing the heat transfer on the pressure surface. At low blowing
rates the suction side is relatively well cooled, but at high blowing there
appears to be film lift-off and poor film cooling. From these limited data, the
effect of rotation can be summarized as follows:
The effectiveness of a leading-edge film flowing along the suction
surface compares well with stationary blade data.
The effectiveness of a leading-edge film flowing along the pressure
surface decreases more rapidly compared to stationary blade data.
Copyright © 2003 Marcel Dekker, Inc.