Page 411 - Mechanical Engineers' Handbook (Volume 4)

400 Cooling Electronic Equipment

Table 11 Values of B and n for Use in Eq. (67)
Reynolds
Number Range B n
1000–6000 0.409 0.531
6000–30,000 0.212 0.606
30,000–100,000 0.139 0.806

Forced Convection in Tubes, Pipes, Ducts, and Annuli
For heat transfer in tubes, pipes, ducts, and annuli, use is made of the equivalent diameter
4A
d (68)
e
WP
in the Reynolds and Nusselt numbers unless the cross section is circular, in which case d e
and d d.
i
31
In the laminar regime where Re 2100,
1/3
Nu hd /k 1.86[RePr(d /L)] ( / ) 0.14 (69)
w
e
e
with all fluid properties except evaluated at the bulk temperature of the fluid.
w
For Reynolds numbers above transition, Re 2100,
0.8
1/3
Nu 0.023(Re) (Pr) ( / ) 0.14 (70)
w
and in the transition region, 2100 Re 10,000, 32
1/3
2/3
Nu 0.116[(Re) 2/3 125](Pr) ( / ) 0.14 [1 (d /L) ] (71)
w
e
33
London has proposed a correlation for the flow of air in rectangular ducts. It is shown
in Fig. 13. This correlation may be used for air flowing between longitudinal fins.

Figure 13 Heat-transfer and friction data for forced air through rectangular ducts. St is the Stanton
number, St hG/c p .