Extended Surfaces

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.