Convection involves the transfer of heat by
the motion and mixing of "macroscopic" portions of
a fluid (that is, the flow of a
fluid past a solid boundary). The term natural convection is
used if this motion and
mixing is caused by density variations resulting from
temperature differences within
the fluid. The term forced convection is used if this motion
and mixing is caused by an outside
force, such as a pump. The transfer of heat from a hot water
radiator to a room is an example
of heat transfer by natural convection. The transfer of heat
from the surface of a heat exchanger
to the bulk of a fluid being pumped through the heat
exchanger is an example of forced
convection.
Heat transfer by convection is more difficult
to analyze than heat transfer by conduction because
no single property of the heat
transfer medium, such as thermal conductivity, can be defined
to describe the
mechanism. Heat transfer by convection varies from situation
to situation (upon the fluid
flow conditions), and it is frequently coupled with the mode
of fluid flow. In practice, analysis
of heat transfer by convection is treated empirically (by
direct observation).
Convection heat
transfer is treated empirically because of the factors that
affect the stagnant film thickness:
Convection involves the transfer of heat
between a surface at a given temperature (Ts)
and fluid at a bulk
temperature (Tb).
The exact definition of the bulk temperature (Tb)
varies depending on the
details of the situation. For flow adjacent to a hot or cold
surface, Tb is
the temperature of the
fluid "far" from the surface. For boiling or
condensation, Tb is
the saturation temperature of
the fluid. For flow in a pipe, Tb
is the average temperature measured at a particular
crosssection of the pipe.
The basic relationship for heat transfer by
convection has the same form as that for heat transfer
by conduction:
The convective heat transfer coefficient (h)
is dependent upon the physical properties of the fluid
and the physical situation.
Typically, the convective heat transfer coefficient for
laminar flow is
relatively low compared to the convective heat transfer
coefficient for turbulent flow. This is due
to turbulent flow having a thinner stagnant fluid film layer
on the heat transfer surface. Values
of h have been measured and tabulated for the commonly
encountered fluids and flow situations
occurring during heat transfer by convection.
Example:
A 22 foot uninsulated steam line crosses a
room. The outer diameter of the steam line is
18 in. and the outer surface temperature is 280oF.
The convective heat transfer coefficient
for the air is 18 Btu/hr-ft2-oF.
Calculate the
heat transfer rate from the pipe into
the room if the room temperature is 72oF.
Many applications involving convective heat
transfer take place within pipes, tubes, or some similar
cylindrical device. In such circumstances, the surface area
of heat transfer normally given in
the convection equation ( ) varies as heat passes through the
cylinder. In addition, 
the temperature difference existing between the inside and
the outside of the pipe, as well as the temperature
differences along the pipe, necessitates the use of some
average temperature value in
order to analyze the problem. This average temperature
difference is called the log mean temperature
difference (LMTD), described earlier.
It is the temperature difference at one end
of the heat exchanger minus the temperature difference
at the other end of the heat
exchanger, divided by the natural logarithm of the ratio of
these two temperature
differences. The above definition for LMTD involves two
important assumptions: (1)
the fluid specific heats do not vary significantly with
temperature, and (2) the convection heat transfer
coefficients are relatively constant throughout the heat
exchanger.
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