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Thermodynamics Heat Transfer and Fluid Flow Volume 2 of 3 | Next
Convection |
CONVECTION HEAT TRANSFER
Heat Transfer
CONVECTION HEAT TRANSFER
Heat transfer by the motion and mixing of the molecules of a liquid or gas is
called convection.
EO 1.9
Given the formula for heat transfer and the operating
conditions of the system, CALCULATE the rate of heat
transfer by convection.
Convection
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:
Fluid velocity
Fluid viscosity
Heat flux
Surface roughness
Type of flow (single-phase/two-phase)
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 cross-
section of the pipe.
HT-02
Page 18
Rev. 0
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