heat transfer through air or some gas
heat transfer through air or some gas
(OP)
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





RE: heat transfer through air or some gas
Good luck,
Latexman
RE: heat transfer through air or some gas
RE: heat transfer through air or some gas
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
RE: heat transfer through air or some gas
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
RE: heat transfer through air or some gas
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
RE: heat transfer through air or some gas
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
RE: heat transfer through air or some gas
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
RE: heat transfer through air or some gas
Unfortunately, I don't recall the details, but the speaker was Prof Robert Moffat at htt
ko (www.ecooling.biz)
RE: heat transfer through air or some gas
You haven't convinced me that you have made a decent attempt on your own to research the subject.
Tunalover
RE: heat transfer through air or some gas
corus