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heat flux

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Hi everybody, I need to modelize a vertical upward heat flux in a rectangle subdomain. I tried to put the condition "heat flux" at the superior bounary and a fixed temperature on the bottom boundary, but I obtain as a result that the heat flux is concentrated next to the upper boundary, and there is no heat flux inside the whole subdomain. Have you an idea of how could I do??
Please help me, it's very very important!!
thank you very much,bye

1 Reply Last Post Nov 14, 2009, 5:36 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 1 decade ago Nov 14, 2009, 5:36 a.m. EST
Hi

I repated what you wrote, with [htgh], apart perhaps that I split my rectangle a couple of times to have some interiour boundaries to calculate the heat flux over, then I put a constant teperature on the top surface, default insulated symmetry on the lateral sides and a negative heat flux on the bottom side (extraction of energy). Air in the middle.

I then solve, default settings and check the heat flux "ndflux_htgh" on the internal boundaries (split lines horizontal across my rectangle and normal to he expected flux), and I get the defined heat flux value correct to some 5-6 digits.

If I change the sign of the boundary heat flux I get the same, but opposed sign heat flux on the interiour (and external) boundaries in [W/m] where meter is the 2D depth

My temperature gradient is also nicely regular and horizontal. I must admit that I have not checked the calculated temperature values, but as I used the library Air for p=1[atm] I hope the difference of temperature for the two cases of +/-1[W/m] are correct, quite a large difference, but the physical air parameters are not that linear between -30 and +30 °C.

By the way I was NOT using radiative exchanges.

So i suppose you got something else wrong, many buttons in there I agree, it's easy to get something wrong

Good luck
Ivar
Hi I repated what you wrote, with [htgh], apart perhaps that I split my rectangle a couple of times to have some interiour boundaries to calculate the heat flux over, then I put a constant teperature on the top surface, default insulated symmetry on the lateral sides and a negative heat flux on the bottom side (extraction of energy). Air in the middle. I then solve, default settings and check the heat flux "ndflux_htgh" on the internal boundaries (split lines horizontal across my rectangle and normal to he expected flux), and I get the defined heat flux value correct to some 5-6 digits. If I change the sign of the boundary heat flux I get the same, but opposed sign heat flux on the interiour (and external) boundaries in [W/m] where meter is the 2D depth My temperature gradient is also nicely regular and horizontal. I must admit that I have not checked the calculated temperature values, but as I used the library Air for p=1[atm] I hope the difference of temperature for the two cases of +/-1[W/m] are correct, quite a large difference, but the physical air parameters are not that linear between -30 and +30 °C. By the way I was NOT using radiative exchanges. So i suppose you got something else wrong, many buttons in there I agree, it's easy to get something wrong Good luck Ivar

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