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how to define a heat flux field?

Posted May 29, 2012, 1:23 p.m. EDT Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD) Version 4.2a 4 Replies

freebody
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Hi,
I am using the "Conjugate Heat Transfer (turbulent)" module to solve a problem.
It seems the "Heat Flux" boundary condition only allows me to define the normal inward heat flux.
The complicated geometry of my model makes it impossible for me to define a normal inward heat flux.
I wish to define a heat flux field that only has z-component.
Is there a way to do it?
Thanks a lot!!
4 Replies Last Post May 30, 2012, 12:57 p.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago May 29, 2012, 2:52 p.m. EDT
Hi

but what kind of heat flux are you considering ?

If it's a heat flux value that varies with the height Z its easy to define a function of z and apply that to your boundary

But if you have a heat flux onto a boundary, for each elements ds or dx*dy of your edge/surface the heat will propagate into the material with a normal direction (to the boundary).

It is only when considering EM waves you have a directional effect with the index of refraction of the material that changes the incident direction

--
Good luck
Ivar
Hi but what kind of heat flux are you considering ? If it's a heat flux value that varies with the height Z its easy to define a function of z and apply that to your boundary But if you have a heat flux onto a boundary, for each elements ds or dx*dy of your edge/surface the heat will propagate into the material with a normal direction (to the boundary). It is only when considering EM waves you have a directional effect with the index of refraction of the material that changes the incident direction -- Good luck Ivar
freebody
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Posted: 1 decade ago May 29, 2012, 4:04 p.m. EDT
Hi Ivar,
Thanks a lot for your reply.
By "z-component" I mean defining the heat flux as something like 0 x_hat + 0 y_hat + P/(2*3.14159*sigma^2)*exp(-x^2/(2*sigma^2))*exp(-y^2/(2*sigma^2)) z_hat. (a 2-D Gaussian)
The heat flux varies with x and y but is only in the z direction.
It models the heat generated from a beam hitting a metallic surface.
In my case, the surface is not trivial.
According to the user manual, the value I enter for "General inward heat flux" is the normal inward heat flux.
Should I calculate the normal inward heat flux with
P/(2*3.14159*sigma^2)*exp(-x^2/(2*sigma^2))*exp(-y^2/(2*sigma^2)) * nz ??

Hi Ivar, Thanks a lot for your reply. By "z-component" I mean defining the heat flux as something like 0 x_hat + 0 y_hat + P/(2*3.14159*sigma^2)*exp(-x^2/(2*sigma^2))*exp(-y^2/(2*sigma^2)) z_hat. (a 2-D Gaussian) The heat flux varies with x and y but is only in the z direction. It models the heat generated from a beam hitting a metallic surface. In my case, the surface is not trivial. According to the user manual, the value I enter for "General inward heat flux" is the normal inward heat flux. Should I calculate the normal inward heat flux with P/(2*3.14159*sigma^2)*exp(-x^2/(2*sigma^2))*exp(-y^2/(2*sigma^2)) * nz ??
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago May 30, 2012, 1:50 a.m. EDT
Hi

from what I understand I believe so, but the best you can do is to validate it on a simple cylinder (with an axis along x or y if I got your correct ;)

such a case you would see immediatly it it's right or wrong, no ?

Note that must entries in COMSOL are expected as fields (over the selected entities) with an implicit dependence on the spatial coordinates and on time. But mostly we enter "just" constant values. This is often overlooked and misunderstood by many users

--
Good luck
Ivar
Hi from what I understand I believe so, but the best you can do is to validate it on a simple cylinder (with an axis along x or y if I got your correct ;) such a case you would see immediatly it it's right or wrong, no ? Note that must entries in COMSOL are expected as fields (over the selected entities) with an implicit dependence on the spatial coordinates and on time. But mostly we enter "just" constant values. This is often overlooked and misunderstood by many users -- Good luck Ivar
freebody
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Posted: 1 decade ago May 30, 2012, 12:57 p.m. EDT
Hi Ivar,
Thank you so much for your help!
Hi Ivar, Thank you so much for your help!

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