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Heat transfer, air-gap
Posted 27 ago 2015, 23:09 GMT-4 Heat Transfer & Phase Change, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 5.0, Version 5.1 1 Reply
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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!
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 ago 2015, 01:43 GMT-4
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Posted:
9 years ago
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
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