Page 186 - Mechanical Engineers' Handbook (Volume 4)

2 Convection Heat Transfer 175

Figure 13 Free convection heat-transfer correlation
for heated vertical plates and cylinders. (Adapted from
Ref. 14. Used with permission of McGraw-Hill Book
Company.)

Free Convection from Spheres
For free convection from spheres, the following correlation has been developed:
Nu 2 0.43(Gr Pr) 0.25 for 1 Gr 10 5
D
D
D
Although this expression was designed primarily for gases, Pr 1, it may be used to
approximate the values for liquids as well. 15
Free Convection in Enclosed Spaces
Heat transfer in an enclosure occurs in a number of different situations and with a variety
of configurations. Then a temperature difference is imposed on two opposing walls that
enclose a space filled with a fluid, convective heat transfer will occur. For small values of
the Rayleigh number, the heat transfer may be dominated by conduction, but as the Rayleigh
number increases, the contribution made by free convection will increase. Following are a

Figure 14 Free convection heat-transfer cor-
relation from heated horizontal cylinders.
(Adapted from Ref. 14. Used with permission
of McGraw-Hill Book Company.)