Handbook of Turbomachinery Second Edition Revised - Ventech!

correlated by
Nu a d ¼ 0:63ðGd=mÞ0:7ðdh=XnÞ 0:5 ðdh=DÞ 0:6 exp 1:27ðZn=dhÞðdh=XnÞ 0:5 ðdh=DÞ 1:2
h i
h i
Nu star
d ¼ 0:44ðGd=mÞ0:7 ðdh=XnÞ 0:8 exp 1:27ðZn=dhÞðdh=XnÞ 0:5 ðdh=DÞ 1:2
for the ranges:
Re from 3610 3 to 1:5610 4 ;
Zn=dh from 1:0 to10:0;
Xn=dh from 4:0 to16:0;
D=dh from 1:5 to16:0:
A typical heat-transfer coefficient distribution away from the stagnation
point was found to be only a weak function of the dimensionless hole
spacing parameter, Xn=dh, and practically independent of Re. Although
this correlation is good for the vane type of geometry, industry experience
has shown that this correlation should be modified for the typical blade
impingement geometry because a more restricted exit of the impingement
flow with the greater possibility of interaction between the impingement
and cross flow in a blade. Industry test experience of actual blade
geometries has shown that the numerical multiplier of 0.5 should be used
in place of 0.63 (reduction of 20%) for the ranges
Zn=dh from 1:5 to4:0; Xn=dh from 2:1 to9:7; D=dh from 2:0 to5:8:
The work of Metzger et al. [84] covered a wide range of geometric variables,
and in particular the effects of concave surface radius on heat-transfer
performance. Only average heat-transfer coefficients were measured.
For details of application of the correlation, the reader is referred to
the referenced work.
As stated earlier, a leading-edge impingement system should be
designed to avoid cross-flow effects, which result in jet bending and the
consequent degradation of impingent heat-transfer performance. This is
generally achieved by exhausting the spent impingement flow in a chordwise
direction. In the application of impingement cooling, special consideration
must be given to the manner in which the spent impingement air is
discharged from the impingement cavity. In many designs, the spent
impingement flow is discharged through radially angled film cooling holes in
the leading edge and through tangential film cooling holes on airfoil
sidewalls. The design is particularly effective since the high impingement
heat transfer at the leading edge is further complemented with convective
cooling in the radial holes. Additionally, the spent impingement air is
Copyright © 2003 Marcel Dekker, Inc.