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Radiation in conjugate heat transfer

Posted 21.01.2013, 16:24 GMT-5 Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD) Version 4.3 10 Replies

Josh Bate
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Hello all,

I have the CFD module and am using conjugate heat transfer. This means that I don't have surface to surface radiation but I do however have surface to ambient radiation.

I am modelling external natural convection in 2D using a rectangle of air with open boundaries. I am trying to add surface to ambient radiation on the boundary between the air and the solid part. COMSOL wont let me do this however. I don't know whether it is because it thinks that the air is opaque or because it wont let heat flux's be defined at boundaries between materials.

Would anyone be able to advise me on how to define this surface to ambient radiation flux at such a boundary.

Thank you all very much,
Josh
10 Replies Last Post 22.01.2013, 17:04 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 22.01.2013, 03:19 GMT-5
Hi

you can only apply surface to ambient for external boundaries, but you can turn on the "surface to surface radiation" option in the main NITF node (at least in my 4.3a it's so) and then you can apply surface to surface radiation at will

--
Good luck
Ivar
Hi you can only apply surface to ambient for external boundaries, but you can turn on the "surface to surface radiation" option in the main NITF node (at least in my 4.3a it's so) and then you can apply surface to surface radiation at will -- Good luck Ivar
Josh Bate
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Posted: 1 decade ago 22.01.2013, 03:23 GMT-5
Thanks Ivar,

I don't have the heat transfer module but do have the CFD module. This means that I can perform conjugate heat transfer but I can't use any of the radiation functions that come in the heat transfer module.

Cheers,
Josh
Thanks Ivar, I don't have the heat transfer module but do have the CFD module. This means that I can perform conjugate heat transfer but I can't use any of the radiation functions that come in the heat transfer module. Cheers, Josh
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 22.01.2013, 03:26 GMT-5
Hi

well as I have both that explains why I can turn them on, you have the choice, add another module or write out all the equations

--
Good luck
Ivar
Hi well as I have both that explains why I can turn them on, you have the choice, add another module or write out all the equations -- Good luck Ivar
Josh Bate
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Posted: 1 decade ago 22.01.2013, 03:29 GMT-5
I would like to write out the equations (they are simple for surface to ambient) but even so, you can't add it to the boundary because you can't add any sort of heat flux boundary between domains (or surfaces in 2D).

Can you think of any other way to get around this?

Thanks Ivar
I would like to write out the equations (they are simple for surface to ambient) but even so, you can't add it to the boundary because you can't add any sort of heat flux boundary between domains (or surfaces in 2D). Can you think of any other way to get around this? Thanks Ivar
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 22.01.2013, 03:55 GMT-5
Hi

But you can add a "boundary heat source" on an internal boundary, indeed the "Heat Flux" BC applies only to external surfaces again

--
Good luck
Ivar
Hi But you can add a "boundary heat source" on an internal boundary, indeed the "Heat Flux" BC applies only to external surfaces again -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 22.01.2013, 08:38 GMT-5
Through the Boundary Heat Source that Ivar mentioned you can emulate most of the other boundary conditions that are only applicable to external surfaces.
Through the Boundary Heat Source that Ivar mentioned you can emulate most of the other boundary conditions that are only applicable to external surfaces.
Josh Bate
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Posted: 1 decade ago 22.01.2013, 15:28 GMT-5
That is great! Thank you Ivar. You are such an asset to COMSOL. They should pay you (if they don't already)
That is great! Thank you Ivar. You are such an asset to COMSOL. They should pay you (if they don't already)
Josh Bate
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Posted: 1 decade ago 22.01.2013, 15:29 GMT-5
Thank you Nagi. That is very helpful.
Thank you Nagi. That is very helpful.
Josh Bate
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Posted: 1 decade ago 22.01.2013, 16:39 GMT-5
Hello again Ivar,

I am trying to write my own equation for surface to ambient radiation and am comparing it to another model in which I can define the default COMSOL version of surface to ambient. I am not getting the same results as the COMSOL version. I have defined my equation as -mat1.def.epsilon_rad*5.67e-8*(T^4-T_amb^4). Do you have any suggestions as to why this isn't working?

Cheers,
Josh
Hello again Ivar, I am trying to write my own equation for surface to ambient radiation and am comparing it to another model in which I can define the default COMSOL version of surface to ambient. I am not getting the same results as the COMSOL version. I have defined my equation as -mat1.def.epsilon_rad*5.67e-8*(T^4-T_amb^4). Do you have any suggestions as to why this isn't working? Cheers, Josh
Josh Bate
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Posted: 1 decade ago 22.01.2013, 17:04 GMT-5
Hello again,

My apologies for the request. It was because I had defined one of the adjacent material properties wrongly. This meant that it affected the built in function but not mine. The problem is now fixed. Thank you!
Hello again, My apologies for the request. It was because I had defined one of the adjacent material properties wrongly. This meant that it affected the built in function but not mine. The problem is now fixed. Thank you!

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