Heat Transfer
CONVECTION HEAT TRANSFER
This results in a much simpler expression that is similar to the one developed for a flat plate heat
exchanger in Figure 6.
(2-11)
Uo
1
1
h1
Dr
k
1
h2
The convection heat transfer process is strongly dependent upon the properties of the fluid being
considered. Correspondingly, the convective heat transfer coefficient (h), the overall coefficient
(Uo), and the other fluid properties may vary substantially for the fluid if it experiences a large
temperature change during its path through the convective heat transfer device. This is especially
true if the fluid’s properties are strongly temperature dependent. Under such circumstances, the
temperature at which the properties are "looked-up" must be some type of average value, rather
than using either the inlet or outlet temperature value.
For internal flow, the bulk or average value of temperature is obtained analytically through the
use of conservation of energy. For external flow, an average film temperature is normally
calculated, which is an average of the free stream temperature and the solid surface temperature.
In any case, an average value of temperature is used to obtain the fluid properties to be used in
the heat transfer problem. The following example shows the use of such principles by solving
a convective heat transfer problem in which the bulk temperature is calculated.
Convection Heat Transfer
Example:
A flat wall is exposed to the environment. The wall is covered with a layer of insulation
1 in. thick whose thermal conductivity is 0.8 Btu/hr-ft-°F. The temperature of the wall
on the inside of the insulation is 600°F. The wall loses heat to the environment by
convection on the surface of the insulation. The average value of the convection heat
transfer coefficient on the insulation surface is 950 Btu/hr-ft2-°F. Compute the bulk
temperature of the environment (Tb) if the outer surface of the insulation does not exceed
105°F.
Rev. 0
Page 23
HT-02
