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How to give temperature and heat generation B.C for a body inside a air domain?

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Old   July 17, 2021, 03:01
Default How to give temperature and heat generation B.C for a body inside a air domain?
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Rabi Pathak
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I have a computer in a room. The computer has surface temperature of 38 degree C and the heat generated inside the computer is 1000 w/m3. The problem is that when I make the geometry and input it in the cfx, there is a interface created between the computer wall and room air. Thus, not allowing me to put temperature B.C. at the computer surface which acts as a interface. Similarly if i convert the interface into the wall and give my B.C then the effect of convection is not seen, basically the two domains don't interact at all. How can i solve this problem? Please Help.
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Old   July 17, 2021, 06:17
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You cannot specify both temperature and heat flux on a boundary condition. One or the other.

And a boundary condition cannot also be an interface. One or the other.
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Old   July 17, 2021, 09:18
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Thank you for your reply. Can you please suggest me how can i model the above problem?
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Old   July 18, 2021, 02:34
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Then please describe:
* What you know about the scenario - you have already mentioned 38C computer temperature and 1000W/m3 heat generation, but please describe other relevant details such as details of the room, ventilation, what the computer is in the room etc
* An image of the setup would help

And, most importantly - why are you doing this analysis? What are you trying to learn from it? (please don't say "because my professor told me to") - this is most important because what you are trying to achieve will determine what we need to do.
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Old   July 18, 2021, 04:53
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There is a closed room without any inlet or outlet. The wall temperature of the room is 32 degree C. The room consists of air at 32 degree C as well. This room consists of 6 computers (modelled as just solid monitors) . The temperature at surface of computer is 38 degree C, initially then further heat generation starts about 1000 w/m3. I need to find out increase in temperature of surface of computer as well as the temperature distribution above the computer in the air domain.
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Old   July 18, 2021, 08:28
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The computer surface temperature of initially 38C then 1000 W/m3 only makes sense if the computer has thermal mass (ie m*c(p)). If you are not modelling the computer thermal mass then just use the 1000 W/m3 and let it go to whatever temperature that results in.

This model sounds like a simple heat transfer simulation. The computer(s) are just a heat source which slowly warms the room up, and themselves. Also a plume of hot air will rise above it.

This is all very basic stuff - so have a look at the CFX tutorials for how to set up basic simulations.
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Old   July 18, 2021, 08:41
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Thank you so much again. Yes the computer has a thermal mass. We have made a volume which has a given density and specific heat, thus there must be a thermal mass. How should I progress in such case where thermal mass is to be also taken into consideration and not just the heat generation?
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Old   July 18, 2021, 08:50
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Just because the source has a mass and specific heat does not mean that the thermal mass is significant. If the thermal mass causes the thermal time scale in the object to be very fast or very slow compared to the fluid time scales you can then simplify the simulation.

But if you say you need to model the thermal mass then this is a conjugate heat transfer (CHT) simulation. Again, there are examples of this in the CFX examples.
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Old   July 18, 2021, 12:51
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Yes it is a conjugate heat transfer problem. But I am not being allowed to give both heat generation source and temperature at the surface. I am allowed to choose between one among them only. If I choose to input heat generation then the surface acts as interface and I cannot put a B. C on it.. On other hand to put temperature as B.C, I need to remove the body as a whole so that I cannot input a heat generation. But is there a way to do both at the same time?
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Old   July 18, 2021, 14:15
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> But is there a way to do both at the same time?

It is physical nonsense. If you have a heat source you need accept the surface temperature it develops.
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Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)
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Old   July 18, 2021, 19:48
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It is physical nonsense and mathematically over specifying. It is like saying that x=2 and x=5 at the same time. They cannot both be true.
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Old   July 19, 2021, 05:35
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Completely agree and completely satisfied. Thank you so much everyone.
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