Introduction of Fins:

Fins
are the extended surfaces designed to Increase heat transfer rate for a fixed
surface temperature, or lower surface temperature for a fixed heat transfer
rate.
Types of Fins:
Types Of Fins
Types Of Fins 
HEAT TRANSFER FROM FINNED SURFACES

The
rate of heat transfer from a surface at a temperature Ts to the
surrounding medium at Tinfinity is given by Newton’s law of cooling as,
Equation of fins
Equation of fins 
where.
As is
the heat transfer surface area and
h is
the convection heat transfer coefficient
temperatures Ts
and Tinfinity are fixed by design considerations,
There
are two ways to increase the rate of heat transfer: to increase the convection
heat transfer coefficient h
or to increase the surface area As. Increasing
h may require the installation of a pump or fan, or replacing the
existing one with a larger one, but this approach may or may not be practical.
Besides, it may not be adequate.
The
alternative is to increase the surface area by attaching to the surface extended
surfaces
called fins made of highly conductive materials such as
aluminum. Finned surfaces are manufactured by extruding, welding, or wrapping a
thin metal sheet on a surface. Fins enhance heat transfer from a surface by
exposing a larger surface area to convection and radiation.

Finned
surfaces are commonly used in practice to enhance heat transfer, and they often
increase the rate of heat transfer from a surface several fold. The car
radiator shown in Fig. below is an example of a finned surface. The closely
packed thin metal sheets attached to the hot water tubes increase the surface
area for convection and thus the rate of convection heat transfer from the
tubes to the air many times. There are a variety of innovative fin designs
available in the market, and they seem to be limited only by imagination.
innovation in fins
Innovation In Fins 
In
the analysis of fins, we consider steady operation with no heat
generation
in the fin, and we assume the thermal conductivity k of
the material to remain constant.
We
also assume the convection heat transfer coefficient h to be constant
and uniform over the entire surface of the fin for convenience in
the analysis.
We
recognize that the convection heat transfer coefficient h, in general,
varies along the fin as well as its circumference, and its value at a point
is a strong function of the fluid motion at that point.
 The value of h is usually much
lower at the fin base than it is at the fin tip because the fluid
is surrounded by solid surfaces near the base, which seriously disrupt
its motion to the point of “suffocating” it, while the fluid near the fin tip
has little contact with a solid surface and thus encounters little
resistance to flow.

Therefore,
adding too many fins on a surface may actually decrease the overall heat
transfer when the decrease in h offsets any gain resulting from the
increase in the surface area.
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