heat transfer through air or some gas

[packo]

when we have 2 materials and between them is film of air or some gas(in my case nitrogen under pressure),how to calculate heat transfer between those materials?what about thickness of that film,is it important?i suppose it is

 

[Latexman]

It depends on the amount of heat transferred and the temperature levels involved. Some more information on the application and the temperatures involved would help us a lot.

Good luck,
Latexman

 

[chicopee]

What you attempt to do in that film of air is provide a heat transfer mechanism,primarily, by conduction since gases have low "k" values and minimize convective heat transfer.

 

[Tunalover]

packo-
When you say "film" it sounds like the gap is .25mm (.01") or less and convection is reduced to only conduction. The only other way for heat to cross the gap is by radiation.

The conduction exchange will simple by Qc=-k*A*(T1-T2)/G where T1 is the absolute temperature of Surface 1 and T2 is the absolute temperature of Surface 2, G is the gap thickness, k is the thermal conductivity of the gas evaluated at temperature (T1+T2)/2, and A is the surface area. For the infrared band (heat with negligible visible light), surface absorptance is equal to the emittance for most materials. Assuming the areas facing each other are the same shape, the radiation exchange will be Qr=sigma*A(eps1*T1^4-eps2*T2^4) where sigma is the Stephan-Boltman constant, eps1 is the emittance of Surface 1, and eps2 is the emittance of Surface 2.

The total heat Q will be Q = Qr + Qc.

If the surfaces are touching each other then "the plot thickens"!

I hope this helps.



Tunalover

 

[packo]

nice answers,tnx all

tunalover
what about if gap is bigger and convection comes in the game?

in my case there is nitrogen under pressure,i suppose it's similar to air except maybe harder finding conductivity and convection constants

 

[Tunalover]

packo-
If the gap is bigger then much depends on the orientation of the gap. If the parallel surfaces are vertical then you have a chimney effect. If they are horizontal I would say that the gap would have to be pretty wide to make much of a difference.

What references have you looked in?


Tunalover

 

[packo]

one more question
if i have double tank with gap of nitrogen between tanks,sholud i separately calculate heat transfer through first tank with area of that tank,than through gap with area of gap,than second tank with area of second t.,and then sum all,or calculate with some mean measure of area between first and second tank ?because area through wich heat transfer changes from first to second tank

 

[packo]

tunalover
surfaces in gap are paralel,vertical
when i find exactly measure of gap i'll tell you,but for now maybe i just calculate transfer with conduction,and then add convection and radiation in the game

 

[ko99]

I recall a conference where this was briefly discussed. The speaker had characterized "fluid conduction" as a dimensionless number (ie, below that value, you can assume no convection).

Unfortunately, I don't recall the details, but the speaker was Prof Robert Moffat at

http://me.stanford.edu/faculty/facultydir/moffat.html

ko (

http://www.ecooling.biz)

 

[Tunalover]

packo-
You haven't convinced me that you have made a decent attempt on your own to research the subject.


Tunalover

 

[corus]

Depending on the gap size you may have to consider natural convection. Conduction is usually negligible but radiation would have to be considered as this is usually dominant. I refer to McAdams book on heat transfer which has methods to determine if the gap size is significant enough to include natural convection.

corus