Page 406 - Mechanical Engineers' Handbook (Volume 4)

2 Heat-Transfer Correlations for Electronic Equipment Cooling 395

σ σ 2
1 TIM Material 2

Material 1

Bulk
resistance
2
k contact = k + k 2 k k TIM TIM
2
k TIM
2
k contact = k + k 1 k k TIM BLT
TIM
1
Temperature

Distance

Contact
resistance
Figure 11 Schematic showing temperature drop across an interface.

2.4 Forced Convection
External Flow on a Plane Surface
For an unheated starting length of the plane surface, x , in laminar flow, the local Nusselt
0
number can be expressed by
0.332Re 1/2 Pr 1 /3
Nu (54)
x 3/4 1 /3
[1 (x /x) ]
0
Where Re is the Reynolds number, Pr is the Prandtl number, and Nu is the Nusselt number.
For flow in the inlet zones of parallel plate channels and along isolated plates, the heat-
3
transfer coefficient varies with L, the distance from the leading edge. in the range Re
5
3 10 ,
h 0.664 0.5 0.33
k
fl
L Re Pr (55)
and for Re 3 10 5
h 0.036 0.8 0.33
k
fl
L Re Pr (56)
Cylinders in Cross-flow
For airflow around single cylinders at all but very low Reynolds numbers, Hilpert 23 has
proposed