Identifying heat transfer fins for analysis of problems

[Urmi Roy]

I've been getting mixed views on how to identify fins in different situations.

From my class I know that fins have a lot of convection around them, and the fin material in itself has high thermal conductivity, so there is negligible temperature gradient along its breadth.

However today we were analyzing a long structure, like a steel joist in a roof. It had insulation lining its bottom surface (towards the interior of house), but the TA said that its still a fin, though one surface is effectively adiabatic due to the insulation lining.

Could someone please tell me a full-proof way to identify a fin?

 

[sanka]

The way I've always defined a "fin" is that it is a geometrical extension/additional area to the primary surface. So as a classic example think of heat sink in computer. The main/primary surface is the heat sink base, which is fixed to the CPU/GPU, whatever the heat dissipating element is. Extended from this base are the fins. They are an extension of the primary surface. Maybe it is easier to think of a fin in terms of geometry instead of its thermal characteristics.

 

[Urmi Roy]

Yes, but not all extensions are fins. If the structure extending from the primary surface is embedded in insulation material, or is made of low conductivity material, its not a fin, right?

 

[256bits]

Yes, but not all extensions are fins. If the structure extending from the primary surface is embedded in insulation material, or is made of low conductivity material, its not a fin, right?

Fins would be used as a means to dissipate, or gather, heat from a high temperature reservoir, so high thermal conductivity would be a desirable feature. Embedding the fins in insulation would not serve any practical purpose that I can think of at the moment.

Fins are usually symmetrical. A large aspect ratio of length to thickness increases the surface area available for heat flow. There is a temperature gradient from base to tip; the gradient depending upon the thermal conductivity of the material, and rate of removal of heat. Being thin, one can consider the temperature at a section of a fin to be the same across its thickness. Heat flow from the surface of the tip can be considered negligible compared to the rest of the fin, due to the small surface area.

One finds fins in the radiators of cars, the heat exchangers atop Cpu's, as mentioned, or on power chips, the heating elements that warm your house, as well as many other loacations. They can be round like pins, flat protrusions from a surface, thin foils that surround a pipe, for example.

 

[Urmi Roy]

Thanks 265bits! Final question for you...you mentioned that a fin embedded in insulation doesn't make sense. However if it has insulation on only one surface, leaving the other for convection...and it is made of high conductivity material, it would still act as a fin, right?

 

[256bits]

Insulating one side would remove half of the available area of the fin to dissipate heat.

If this is in relation to the beam from your first post, I cannot really give a particular answer, as the criteria for the heat flow into the beam would be best described by the TA.

At first glance though, I would have thought that the top of the beam ( perhaps there is heat coming in from a hot sun-exposed roof which is in contact with the beam) could have been compared to the base of a fin, and the other insulated part of the beam considered as the tip, with the two sides of the beam being similar to the flat area of a fin convecting heat to the air space. Or, perhaps he means it is a fin attached to the wall and transferring heat to the air space along its length . But, as I have said, your TA would be best to describe which is which.