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Old   September 30, 2009, 05:46
Default Heat loss through walls
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Franz Roman
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Hi all,
I am quite an CFD beginner, and need some urgent help to simulate heat loss through the walls of my system. It is a simple geometry, a cubic space with thin metallic walls, one small horizontal inlet at one side, and one vertical, much wider outlet. The thing is, air enters the system at about 10 m/s and with 30 to 40 C above ambient (surrounding) temperature. What I want to see is the energy loss through the walls due to the lower surrounding temperature and how this affects the temperature distribution at the outlet. How should this be done? Do I need to include in the geometry the room where my actual system sits and define there the temperature of the surroundings? I am guessing not. In the Thermal tab of the Wall panel there appears different thermal conditions (heat flux, temperature, convection, etc) but I do not know which is appropriate, if any. So, to make it short, my problem is, how to simulate the loss of heat through the walls of my system.
I appreciate your help.
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Old   September 30, 2009, 10:28
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Tim Maguire
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I would just turn on the thermal model, and then specify a flux out of cube by specifying the boundary condition for the wall. You will just have to figure out what the value is for your material.
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Old   September 30, 2009, 21:16
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You need to understand where the heat goes after it loss through the cubic wall.
U will need to simulate the room if u expected the room will be one of the factor of heat loss of ur cube.

I would suggest you do some assumption on ur simulation. Assumption such as constant temperature on the cube wall, or constant heat loss on ur cube wall.
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Old   October 1, 2009, 07:57
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Franz Roman
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hi, thanks for the answers. herntan, yes, I understand that i need to know where the heat goes. actually the heat goes to the room where my system is, and this room is at ambient temperature, 20 C or so less than the inlet air to my system. the thing is that as I understand, if I assume a constant heat flux on the wall the same heat will be lost in all parts of all walls regardless of the actual air temperature flowing adyacent to each wall region. Isnt that right?
I see that there is five Thermal Thermal Conditions for Walls available in Fluent, heat flux and temperature as you say, but also Convective Heat transfer. It says for Convective heat transfer BC one inputs an external heat transfer coeff. and an external free heat-sink temperature. I was thinking this could be a good assumption. the problem is, although I can assume that the ambient temperature is 20C, I dont know if that would be the heat-sink temperature for my case, since the air close to the walls could at a higher temperature, especially because the is no wind to carry away the heat. Did I explain it ok?
Please, a little help. Thanks a lot
Franz
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Old   October 2, 2009, 03:20
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i cannot understand what u trying to say.
Some picture will be helpful.

Try to post with picture so people understand what u trying to say. I think most of the post here didn't include picture, and many of them didn't get reply.
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Old   October 2, 2009, 08:00
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Hi,

You would be better off running conjugate heat transfer problem. Here is how it works.

Mesh the fluid domain as well as solid walls (with thickness). Solve for momentum as well as energy. You do have inlet and outlet BCs. For solid wall facing ambient, define convection and try finding out convective heat transfer coefficient. There are empirical correlation that would provide HTC values. It is generally around 5 w/m2 k for a light breeze in the room.

I hope this helps.
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Old   March 29, 2010, 12:11
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Thierry Corbeil
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Hi Everyone

Like franzdrs, Im a beginner (brand new) with CF Design, Im trying to simulate heat lost (heat gain) from the surroundings, room air volume (in gray) to the small enclosure (part in black).
Ive started with boundary conditions of 0 psi and 80F for the top surface of the surrounding volume (room) and -5F for the air volume inside the black part

The simulation is not working, I can not see the temperature gradient in the wall (black part) and the heat lost cause by the surrounding

All this manipulation ans simulation is to get use with the software afterward I want to implement TEs module for cooling base on the capacities needed



Thank you
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