
270 Heat Transfer: Thermal Management of Electronics
Note that as L → ∞, the average Nusselt number approaches to its fully developed
value of 7.54.
The average Nusselt number for the combined entry and fully developed laminar
ow inside a circular pipe at uniform surface temperature can be determined from [5]:
Nu
D
D
D
DL
DL
=+
+
366
0065
1004
2
.
.(/)Re Pr
.[(/)RePr]
//
.
3
(11.75)
Again, it is seen that as L → ∞, the average Nusselt number approaches to its fully
developed value of 3.66.
Example 11.1
Consider an electronic enclosure shaped as a duct with a cross section of
350 mm × 50 mm and length of 400 mm as shown in Figure 11.7. Let’s assume ...