Conduction and pressure question

If two plates, of equal thickness and surface area, one aluminum and the other steel (I'm trying to keep this simple for my question) are placed together, heat would move via conduction from one plate to the other. However, if I apply a force to the plates to press them together, how much of an increase of heat transfer can one expect? Just asking for ball park answers - but would it be 5%, 10%, etc more heat transfer from one plate to another or would it be much less or greater?

Second question: With and without pressure, if I placed one of the plates on the other - except being face to face, they are positioned "long" edge of one block to the face of the other block. Is the heatl transfer the same as the face to face described above - with or without pressure?

Thanks much,
Bill

 

[mduplessis]

Answering the first question, Pressure does have an effect on the thermal contact resistance of two nonsimilar solids. If the thermal resistance is given by 1/((hi)A), hi is around 3000-4500 W/m2K for Fe-Al interfaces and a marked decrease will be noted at the yield point of the Aluminum. How much this decrease will be is not well documented. I have details of an article that might help:
Veziroglu, T.N., "Correlation of thermal contact conductance experimental results". Prog. Astronaut. Aeronaut. 20, pp879-907, Academic Press, NY (1967)

The second question I do not entirely understand.

 

[SethN]

Bill
As mduplessis stated, 1/((hi)A will give you conductive thermal resistance, which does depend on contact area.

As for question 1, I can only confirm that pressure has a big influence on conductance, which is well known in lamination technology. The relationship of heat transfer and pressure is entirely empirical and dependent on the details of the system.

Good luck, I've been working on this puzzle for some time.
thanks to mduplessis for the citation.

Seth

 

[ecampos]

Mduplessis, SethN,
I believe that Bill asked whether "Pressure" had anything to do with the conduction. You both stated that Area is important. I agree but can you guys explain more. I am not sure what you are saying about the conductivity changing at the yield point? Why does contact "Pressure" between the two plates have anything to do with it?
Please elaborate.


Edson Campos
edsoncampos@earthlink.net

 

[mduplessis]

ecampos
Contact resistance between two solids is caused because surfaces are not perfectly smooth. The materials are in contact at relatively few places. The gaps are filled with air, sometimes liquid or vacuum. Applying pressure improves conductivity by reducing the contact resistance because the actual places were the materials are at contact increase. When one or both materials reaches its yield point it becomes more elastic and greatly increase contact points and therfor conductivity. Also sometimes in electronic cooling aplications interface materials are used that becomes elastic at high temperatures to fill the gaps and increase the conductivity between component and heat sink.

Hope that explains it correctly.

 

[ecampos]

AHA! I am with you know.
Thanks for the explanation.


Edson Campos
edsoncampos@earthlink.net

 

[rchook]

Actually the interactions of contacting surfaces depends on many factors (surface characteristics such as micro-hardness of the materials, finish, roughness, flatness etc, and contact factors such as mechanical pressure, joint mean temperature, ambient pressure, etc.) Certainly, one of the strongest dependencies is mechanical pressure at the contact. Madhusudana has a very recent book on thermal contact conductance that presents an overview of the most current theories and gives some empirical relations that can give you an indication of the pressure dependence of contact resistance.

 

[IRstuff]

That's what thermal grease is for ;-)

TTFN

 

[poetix99]

Thanks to all for the great discussion of gap thermal resistance.

Regarding Bill J's #2, various posts have alluded to an answer, but at the risk of stating the obvious: The RATE of conductive heat transfer (amount of heat transfer per unit contact area, BTU/hr-ft^2) will not be effected by the relative relationship of the blocks, but the total amount of heat transfer (BTU/hr) across the metal-metal interface will of course be otherwise proportional to the amount of contact area.

There might be other effects to consider. If this is a cooling problem, and if convective cooling (passive or forced) from the metal to the surrounding fluid (air?) might play a part, the orientation of the plates MIGHT be very significant in determining the total heat transfer from one of the plates to all of its surroundings - including the adjacent plate and the (air).

 

[IRstuff]

A slight addenda to the second question. The heat transfer per unit area would be the same IF the materials are isotropic. There are many materials whose thermal conductivity is a function of orientation relative to heat flow.

TTFN