Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Heat transfer, air-gap

Please login with a confirmed email address before reporting spam

Hello,

i currently try to model the heat transfer through multi-layer wall structures. These structures are filled with an air gap, measuring about 50mm between the shells.
I already have the experimental data for the transmittance factor and it differs alot from the transmittance factor i get from the simulations (for example u_simulation=0.6 W/m²K and u_simulation=0.3 W/m²K).

The simulation is pretty simple:
Heat transfer through solids for the solid parts and heat transfer through fluids for the air gap.

I think radiation plays a minor role here.
Do you think i should include natural convection in the air? (How? Using "laminar flow" like the example "vacuum_flask.mph"? ) I was told, convection here is not important...
Or any other effects, i couldnt think of?

The values i get from the simulation for structures without an air gap are matching the experiment, but as soon as a air gap is introduced, the values differ a lot.

Thanks alot!

1 Reply Last Post 28.08.2015, 01:43 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

Please login with a confirmed email address before reporting spam

Posted: 9 years ago 28.08.2015, 01:43 GMT-4
Hi

I would say that for such large air gaps you will get rapidly quite some mass transport and convection exchange in the air layers, but this is rather tedious to model, and is very model/geometry dependent. that is why one often try to avoid the air flow by different means, using porous media around the air, or setting up arrays of filaments/fibres to cut the mass transport.

If you have some measurements you can try to match the results by adapting the heat transport factor in the air and solve an inverse problem.

Indeed if you have only a few degrees of temperature drop the radiation exchange is often negligible, it goes as sigma_boltzman*((T_2)^4 - (T_1)^4) with T in KELVIN!



--
Good luck
Ivar
Hi I would say that for such large air gaps you will get rapidly quite some mass transport and convection exchange in the air layers, but this is rather tedious to model, and is very model/geometry dependent. that is why one often try to avoid the air flow by different means, using porous media around the air, or setting up arrays of filaments/fibres to cut the mass transport. If you have some measurements you can try to match the results by adapting the heat transport factor in the air and solve an inverse problem. Indeed if you have only a few degrees of temperature drop the radiation exchange is often negligible, it goes as sigma_boltzman*((T_2)^4 - (T_1)^4) with T in KELVIN! -- Good luck Ivar

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.