Page 181 - Mechanical Engineers' Handbook (Volume 4)

170 Heat-Transfer Fundamentals

0.5
Nu hx/k 0.332Re Pr 1 / 3 for Pr 0.6
x
x
Nu hL/k 0.664Re Pr 1 / 3
0.5
L
L
Nu 0.565(Re Pr) 0.5 for Pr 0.6
x
x
Nu 1.13(Re Pr) 0.5
L
L
Here, all of the fluid properties are evaluated at the mean or average film temperature.
Turbulent Flow on a Flat Plate
When the flow over a flat plate is turbulent from the leading edge, expressions for the local
Nusselt number can be written as
0.8
Nu 0.0292Re Pr 1 / 3
x
x
Nu 0.036Re Pr 1 / 3 .
0.8
L
L
5
where the fluid properties are all based on the mean film temperature and 5 10 Re x
and Re 10 and 0.6 Pr 60.
8
L
The Average Nusselt Number between x 0 and x L with Transition
For situations where transition occurs immediately once the critical Reynolds number Re c
has been reached 7
Nu 0.036Pr 1 / 3 [Re 0.8 Re 0.8 18.44Re ]
0.5
L
c
c
L
8
provided that 5 10 Re 10 and 0.6 Pr 60. Specialized cases exist for this
5
L
situation i.e.,
Nu 0.036Pr 1/3 (Re 0.8 18,700)
L
L
5
for Re 4 10 ,or
c
Nu 0.036Pr 1/3 (Re 0.8 23,000)
L L
5
for Re 5 10 . Again, all fluid properties are evaluated at the mean film temperature.
c
Circular Cylinders in Cross-Flow
For circular cylinders in cross-flow, the Nusselt number is based upon the diameter and can
be expressed as
0.4
Nu (0.4Re 0.5 0.06Re 2 / 3 )Pr ( / ) 0.25

D
D
s
5
for 0.67 Pr 300, 10 Re 10 , and 0.25 5.2. Here, the fluid properties are
D
evaluated at the free stream temperature except , which is evaluated at the surface tem-
s
perature. 8
Cylinders of Noncircular Cross Section in Cross-Flow of Gases
For noncircular cylinders in cross-flow, the Nusselt number is again based on the diameter,
but is expressed as
Nu C(Re)Pr 1/3
m
D
D
where C and m are listed in Table 13, and the fluid properties are evaluated at the mean film
temperature. 9