## Convection

Convection is defined as that mode energy transfer in which
heat is transferred between a solid surface and the adjacent liquid or gas that
is in motion, and it involves the combined effects of conduction and fluid
motion. The faster the motion of the fluid, the more heat is transferred, and
hence the more the convection heat transfer. When there is no bulk fluid
motion, heart transfer between a solid surface and the adjacent fluid is by
pure conduction. When bulk motion of fluid is present, the heat transfer
increases between the solid surface and the fluid, but determination of heat
transfer rates is complicated.

Convection is called forced convection if
fluid is forced to flow over the surface by external means such as pump, fan,
etc. Convection is called as natural (or free) convection if the fluid motion
is caused by buoyancy forces that are induced by density differences due to the
variation of temperature in the fluid. Those heat transfer processes that
involve change of phase of a fluid are considered to be convection because of
the motion of fluid which is induced during the process, such as the rise of
vapor bubbles during boiling or the fall of liquid droplets during
condensation.
The rate of convection heat
transfer is directly proportional to the temperature difference and is
conveniently expressed by Newton's law of cooling as

Where

*h*is known as the convection heat transfer coefficient and it is expressed in w/m2.k, is the surface area through which convection heat transfer takes place, Ts is the surface temperature, and T infinity is the temperature of the fluid sufficiently far from the surface. In conduction and convection heat transfer material medium is present. In convection, fluid motion is present. Heat transfer through a solid surface is always by conduction. Heat transfer through a liquid, or gas is by conduction or convection depending on whether bulk fluid motion is present or not.
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