Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 20 Impingement geometry definition.
of holes onto a flat surface, and the second impingement from a row of holes
onto a concave surface.
A method for producing relatively large forced convection heattransfer
coefficients on a surface is by the use of a multiplicity, or array, of
jets impinging normal to the surface from a perforated parallel plate placed
close to the surface. As the air jets approach the impinged surface, they
make a 908 turn and thereby become what are called wall jets. As the wall
jets from two adjacent impinging jets approach each other, their interference
forces the flow to separate from the surface and form a stream, often of
relatively low velocity, flowing past the impinging jets to reach the exit
where the air is removed. This spent flow is called ‘‘cross flow’’ and tends to
deflect the impinging jets from their initial direction and can thereby reduce
the average convective heat-transfer coefficient.
Design features approximating a square array of jets impinging on a
flat plate with cross-flow air exhausting to one side can be based on the
correlation of Kercher and Tabakoff [82]. This correlation is
Nud ¼ havdh=k ¼ f 1f 2ðrV3dh=mÞ m ðZn=dhÞ 0:09 ðmCp=kÞ 1=3
Copyright © 2003 Marcel Dekker, Inc.