Convection-induced heat transfer
is initiated by the movement of fluid on a macroscopic scale,
manifesting as eddies or circulating currents.
When these currents originate
from the heat transfer process itself, natural convection ensues, as
observed in the heating of a vessel containing liquid through a heat source
positioned beneath it. The liquid at the vessel's bottom undergoes heating,
expands, and ascends due to reduced density compared to the remaining liquid.
Cold liquid with higher density replaces it, establishing a circulating
current.
Forced convection, on the
other hand, involves the generation of circulating currents by an external
agent, such as an agitator in a reaction vessel or pumped turbulent flow in
a pipe. Generally, forced convection exhibits greater circulation
magnitude, resulting in higher rates of heat transfer compared to natural
convection.
In instances where convective
heat transfer occurs from a surface to a fluid, circulating currents typically
diminish in the immediate proximity of the surface. Instead, a turbulence-free
fluid film covers the surface, facilitating heat transfer through thermal
conduction. Since the thermal conductivity of most fluids is low, the primary
resistance to transfer lies in this film. Therefore, an escalation in fluid
velocity over the surface enhances heat transfer by reducing the film
thickness. Notably, the film coefficient increases with fluid velocity.
The rate of heat transfer in
the convection phenomenon is directly proportional to the product of the
mixing or movement area between hot and cold fluid bodies and the temperature
difference between these bodies. This relationship is expressed as
Q = h.A.ΔT,
where
h represents the heat transfer
coefficient for the film (with the reciprocal being the corresponding thermal
resistance),
A denotes the mixing or movement
area, and
ΔT is the temperature difference
between the hotter and colder points of the fluid.
It is observed that the heat
transfer rate per unit area (q) relies on physical properties
influencing flow pattern (viscosity μ and density ρ), thermal properties of the
fluid (specific heat capacity Cp and thermal conductivity k), surface linear
dimension (l), fluid flow velocity (u) over the surface, temperature difference
(ΔT), and a factor determining the natural circulation effect caused by fluid
expansion upon heating (the product of the coefficient of volumetric expansion
β and acceleration due to gravity g).
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