Ground (soil) heat transfer bounderies and radiation

[Anonymous]

Hi,

I am working on Ground Tubing heat exchange system. The model could be described as a large rectangular box (the ground), a tube (pipe) and a cylinder (air) inside the tube which enter the box from the top and they get out from the side of the box. For boundary conditions, the cylinder inlet takes ( air flow, and temperature). cylinder outlet takes (presser). I don't know what is the best boundary for the ground? should I put the boundary of the top of the box as a surface temperature or should I put it the boundary as a radiation so the ground temperature depth will vary depends on the location and the soil type.

Hope anyone can give me some suggestion.

Thanks,

malfadil

[Jon.Wilde]

What is acting on the ground in reality? Do you need to capture the heat load from the sun or just an ambient temperature?

Feel free to share images, always easier than words 🙂

[Anonymous]

Hi Jon,

I would like to capture the heating load from the sun through the ground. I know the ground temperature will change if we go deep. So for example, how do I know the ground temperature on 13 feet in Charlottesville VA with the soil of sand? do you know how to find the ground temperature in this depth?

Thanks

[Jon.Wilde]

I think you might need to calculate it, have a Google also though, I saw some useful pages.

We could do this with a heat flux on the upper surface, but you would need to be sure you have the correct properties for the ground - we have a 'soil' material but it might not be entirely representative.

Are you trying to design the best performing system though, or understand what depth it needs to be installed at? For the former, the soil/depth etc might not be so important.

[Anonymous]

I am trying to see the system performance in different depth. I put heat flux as a boundary condition for the top surface and temperature for the surface but it does not seem to change in depth.

thanks

[Jon.Wilde]

Hi,

OK, I think I understand 🙂

What temperature did you assign where?

Is the heat flux really on the outermost face? It looks like there is a condition assigned internally there?

[Anonymous]

Hi,

In reality, I put thermocouples sensors in the ground for deferent depth:

Air temperature on the surface (90f)

At 1.96 ft = 72f

At 4 ft = 71f

At 6.56ft = 67f

At 9.8ft = 64f

At 13.12ft = 62f

So I am not sure what is the right way to get the ground temperature as reality. Should I put the upper surface boundary condition as temperature 72 and the lower surface boundary condition as temperature of 62 (because the max depth of the geometry is 13.12ft)

Or

Should I put heat flux (0.2) as boundary condition for the upper surface with a temperature of 71f

Or

Should I put the upper surface boundary condition as a temperature and assign it as a non-constant temperature to add the temperature with different depth?

Also I don’t know if Autodesk CFD can predict the ground temperature if I know only the surface temperature and the soil type and properties.

Thank you very much

[Jon.Wilde]

If you know the temperature at the depth you have it, rather than have CFD calculate it, why not just apply it to the outside of the pipe and remove the soil?

[Anonymous]

because the temperature is very with depth ( the pipe is not horizontal), see the picture attached.

also, I don't know how to calculate the ground temperature on 20feet depth if I know only the soil type, properties and the temperature on the top surface. 

please if you can  explain what boundary that I will use and where should I sing it in each part I will be glad

Thanks

[Jon.Wilde]

Ha, very good point!

You could start with a heat flux on the top surface (depends where this is exactly/clouds/time of day) and a film coefficient of something like 10W/m2/K (at ambient temp) on the underside of the soil.

That ought to be a good starting point I would think, but you will need to figure out the right heat flux to apply 🙂