Convection: Heat transfer between a solid surface and a moving fluid is governed by the Newton’s cooling- law: q = hA(Ts -T .

**EXTENDED SURFACES**

Convection: Heat transfer between a solid surface
and a moving fluid is governed by the
Newton’s cooling- law: q
= hA(Ts -T . Therefore, to
increase the convective heat
transfer, one can Increase the
temperature difference (Ts - T ) between
the surace and the fluid.

Increase the convection
coefficient h. This can be accomplished by increasing the fluid flow over the
surface since h is a function of the flow velocity and the higher the velocity,
the higher the h. Example: a cooling fan.

Increase the contact
surface area A. Example: a heat sink with fin. Ac : the fin cross-sectional
area.

P: the fin perimeter.

Many times, when the
first option is not in our control and the second option (i.e. increasing *h*)
is already stretched to its limit, we are left with the only alternative of
increasing the effective surface area by using fins or extended surfaces. Fins
are protrusions from the base surface into the cooling fluid, so that the extra
surface of the protrusions is also in contact with the fluid. Most of you have
encountered cooling fins on air-cooled engines (motorcycles, portable
generators, etc.), electronic equipment (CPUs), automobile radiators, air
conditioning equipment (condensers) and elsewhere.

The fin efficiency is
defined as the ratio of the energy transferred through a real fin to that transferred
through an ideal fin. An ideal fin is thought to be one made of a perfect or
infinite conductor material. A perfect conductor has an infinite thermal
conductivity so that the entire fin is at the base material temperature.

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Mechanical : Heat and Mass Transfer : Conduction : Extended Surfaces |

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