Heat loss through walls

franzdrs · September 30, 2009, 05:46

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.
Franz

timjm · September 30, 2009, 10:28

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.

herntan · September 30, 2009, 21:16

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.

franzdrs · October 1, 2009, 07:57

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 20°C, 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

herntan · October 2, 2009, 03:20

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.

owais70 · October 2, 2009, 08:00

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.

thierrycorbeil · March 29, 2010, 12:11

Hi Everyone

Like franzdrs, I’m a beginner (brand new) with CF Design, I’m trying to simulate heat lost (heat gain) from the surroundings, room air volume (in gray) to the small enclosure (part in black).
I’ve 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 TE’s module for cooling base on the capacities needed

Thank you

September 30, 2009, 05:46	Heat loss through walls	#1
franzdrs New Member Franz Roman Join Date: Jun 2009 Posts: 28 Rep Power: 16	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. Franz

September 30, 2009, 21:16		#3
herntan Senior Member YH Tan Join Date: Mar 2009 Location: Malaysia Posts: 119 Rep Power: 17	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. __________________ Herntan, Add Reputation if I am right

October 2, 2009, 03:20		#5
herntan Senior Member YH Tan Join Date: Mar 2009 Location: Malaysia Posts: 119 Rep Power: 17	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. __________________ Herntan, Add Reputation if I am right

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September 30, 2009, 10:28		#2
timjm New Member Tim Maguire Join Date: Sep 2009 Posts: 19 Rep Power: 16	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.

October 1, 2009, 07:57		#4
franzdrs New Member Franz Roman Join Date: Jun 2009 Posts: 28 Rep Power: 16	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 20°C, 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

October 2, 2009, 08:00		#6
owais70 New Member mangal Join Date: Oct 2009 Location: Canada Posts: 7 Rep Power: 16	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.