# Defining Boundary conditions for heat transfer

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May 13, 2010, 09:03
Defining Boundary conditions for heat transfer
#1
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Sujay
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The domain is nearly reactangural box with inlet pipe submerged at top and outlet at the bottom. The top (rest of inlet pipe) is like opening BC with heat radiation wherras whole bottom of reactangle is outlet. Rest four sides are wall with heat flux conditions. Kindly guide me to set these boundary conditions.

As i choose thermal enegry in heat transfer and thermal radition, four wall also shows thermal radiation model with opaque showing emmssivity and diffue fraction as 1.

I have heat flux for these wall and donot want to add the radiation here.

For opening BC, it asks opening or static temperature. What is meaning of these temperature.

Kindly guide me
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 May 13, 2010, 10:41 #2 Senior Member     Attesz Join Date: Mar 2009 Posts: 355 Rep Power: 9 Only a question, Total Energy model isn't suitable in your case?

May 13, 2010, 19:18
#3
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Glenn Horrocks
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Quote:
 For opening BC, it asks opening or static temperature. What is meaning of these temperature.
Opening temperature means during outflow the temperature is set by the temperature of the fluid flowing out the opening, and during inflow the temperature is set by the specified temperature (usually a total temperature).

Static temperature means the opening is always set to have the specified static temperature, regardless of the flow direction.

The temperature boundary during outflow only makes a difference for the radiation model.

May 13, 2010, 23:53
#4
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Sujay
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As heat extraction is differnt from the differnt wall, domain needs to be defined with respective flux rather than total energy. So i think total energy model is not suitable.

I have no idea about the total energy model
Quote:
 Originally Posted by Attesz Only a question, Total Energy model isn't suitable in your case?

May 13, 2010, 23:59
#5
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Glenn Horrocks
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Quote:
 As heat extraction is differnt from the differnt wall, domain needs to be defined with respective flux rather than total energy. So i think total energy model is not suitable.
I have no idea what you are talking about. Are you saying you want to specify a heat flux at an opening? This would be very weird. The total energy model is to get CFX to solve the full compressible flow equation rather than incompressible equations so is a totally different thing.

May 14, 2010, 02:12
#6
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Sujay
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1. I want to give heat flux at the four side walls
2. Inlet temperature at inlet
3. radiation BC at the opening
Kindly guide me

Quote:
 Originally Posted by ghorrocks I have no idea what you are talking about. Are you saying you want to specify a heat flux at an opening? This would be very weird. The total energy model is to get CFX to solve the full compressible flow equation rather than incompressible equations so is a totally different thing.

 May 14, 2010, 04:41 #7 Senior Member     Attesz Join Date: Mar 2009 Posts: 355 Rep Power: 9 I'm not so experienced in it, but if you want to calculate with radiation, you can not define it at the opening. You can set there an energy source (as flux), to take into account the effect of the radiating environment. I hope I'm not wrong... Otherwise, if you use gas as fluid, the Total Energy model can give better results. Regards Last edited by Attesz; May 14, 2010 at 05:02.

May 14, 2010, 08:00
#8
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Glenn Horrocks
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Quote:
 I want to give heat flux at the four side walls
Then set the side walls with a specified heat flux. Isn't that obvious?

Quote:
 Inlet temperature at inlet
Then set the temperature at the inlet. Again, isn't that obvious?

Quote:
 radiation BC at the opening
It is not clear what you mean. Do you mean there is a radiative heat source outside the domain which is shining into the domain and heating it up? In that case you need to define a directional radiation source at this boundary, and specify its radiative properties.

Quote:
 Otherwise, if you use gas as fluid, the Total Energy model can give better results.
If buoyancy effects are not significant then incompressible flow should be fine. If the temperature range is small then the buosinessq incompressible buoyancy model is fine. Only if buoyancy effects are significant AND the temperature range is large would I consider a compressible model, that is the Total Energy model. It involves a lot more CPU time so only use it if you need to.

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