CONVECTION
Convection Heat Transfer-Requirements
The heat transfer by
convection requires a solid-fluid interface, a temperature difference between
the solid surface and the surrounding fluid and a motion of the fluid. The
process of heat transfer by convection would occur when there is a movement of
macro-particles of the fluid in space from a region of higher temperature to
lower temperature.
Convection Heat Transfer Mechanism
Let
us imagine a heated solid surface, say a plane wall at a temperature Tw
placed in an atmosphere at temperature T ¥,
Fig. 2.1 Since all real fluids are viscous, the fluid particles adjacent to the
solid surface will stick to the surface. The fluid particle at A, which is at a
lower temperature, will receive heat energy from the plate by conduction. The
internal energy of the particle would Increase and when the particle moves away
from the solid surface (wall or plate) and collides with another fluid particle
at B which is at the ambient temperature, it will transfer a part of its stored
energy to B. And, the temperature of the fluid particle at B would increase.
This way the heat energy is transferred from the heated plate to the
surrounding fluid. Therefore the process of heat transfer by convection involves
a combined action of heat conduction, energy storage and transfer of energy by
mixing motion of fluid particles.
Fig.
2.1 Principle of heat transfer by convection
Free and Forced Convection
When the mixing motion
of the fluid particles is the result of the density difference caused by a
temperature gradient, the process of heat transfer is called natural or free
convection. When the mixing motion is created by an artificial means (by some
external agent), the process of heat transfer is called forced convection Since
the effectiveness of heat transfer by convection depends largely on the mixing
motion of the fluid particles, it is essential to have a knowledge of the
characteristics of fluid flow.
Basic Difference between Laminar and
Turbulent Flow
In laminar or
streamline flow, the fluid particles move in layers such that each fluid p
article follows a smooth and continuous path. There is no macroscopic mixing of
fluid particles between successive layers, and the order is maintained even
when there is a turn around a comer or an obstacle is to be crossed. If a lime
dependent fluctuating motion is observed indirections which are parallel and
transverse to t he main flow, i.e., there is a radom macroscopic mixing of fluid
particles across successive layers of fluid flow, the motion of the fluid is
called' turbulent flow'. The path of a fluid particle would then be zigzag and
irregular, but on a statistical basis, the overall motion of the macroparticles
would be regular and predictable.
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