Handbook of Turbomachinery Second Edition Revised - Ventech!

tangentially to the inner wall, behaves initially as quasi-impingement and
becomes unstable due to interaction with the concave surface of a circular
channel. The swirl motion results in a longer path for coolant air to travel
for a given axial length. Highly turbulent flow near the surface leads to a
thin boundary layer. Additional cooling benefits can be obtained due to
centrifugal forces, which push the cool high-density air toward the hot
surface. The Go¨rtler vortices are known to significantly enhance heat
transfer by its downwash and upwash vortical motion near the wall and by
continuous interaction and intermingling with each other.
A comparison of the three different swirl cooling configurations versus
impingement cooling with and without cross flow and versus smooth and
trip-strip augmented channel surface is presented in Fig. 22. For the purpose
of this comparison, the Nusselt number based on hydraulic diameter was
calculated and averaged over +60 angle from the leading edge. Reynolds
numbers were calculated based on the assumptions that all of the flow went
through the cross-sectional area of the leading-edge chamber. For
impingement cooling, Chupp and Helm’s correlation (1968) was used for
a given geometry: D=dn ¼ 4, and xn=dn ¼ 3. Kercher and Tabakoff’s data
(1970) were used for cross-flow effects on the impingement cooling. The heat
transfer in a channel with normal trip-strips was obtained from the
correlation of Webb et al. (1971) [88]. It can be seen from Fig. 21 that the
Figure 22 Comparison of internal blade-cooling techniques.
Copyright © 2003 Marcel Dekker, Inc.