boundary condition for heat transfer

PSander · January 17, 2020, 10:10

Hey,
I am trying to create a case to simulate 3D heat transfer from a pipe to the surrounding soil, using chtMultiRegionFoam . The mesh is working, now I`m trying to find the appropriate boundary conditions.
To the sites (walls) and bottom I decided for zeroGradient, so the heat can leave the system.

For the surface it becomes more complicated. Because of for example air or sun temperature, the surface should have a fixedValue, of lets say 280° K.
But the fluid in my pipe is much hotter, around 380°K. So in reality there should leave heat as well in direction surface, but in the same moment there is a influence of the surfacetemperatur. If i decide for a fixedValue, it`s not gonna happen.

So I need something between zeroGradient (heat can leave the system at the surface) and fixedValue (influence of the surface temperature caused by air or sun).

Could the bc "mixed" help me? And if yes, how do I understand the "valueFraction"?

Thanks a lot in advance!

simrego · January 20, 2020, 07:30

Hi!

If you set fixedValue, there will be still heat flow through the surface.

In mixed BC the valueFraction sets the fraction between the value, and the gradient. From the internalField, and the prescribed gradient, you can calculate the surface value, let me call it gradValue. But in the same time you prescribe the value on the surface as refValue. From these and the value fraction, you can calculate the actual surface value as:
surfaceValue = valueFraction * refValue + (1 - valueFraction) * gradValue
note that value fraction must be: 0 <= valueFraction <= 1. If it is 0, you get the fixedGradient BC, if it is 1, you get the fixedValue BC.

January 17, 2020, 10:10	boundary condition for heat transfer	#1
PSander New Member Join Date: Jan 2020 Posts: 26 Rep Power: 6	Hey, I am trying to create a case to simulate 3D heat transfer from a pipe to the surrounding soil, using chtMultiRegionFoam . The mesh is working, now I`m trying to find the appropriate boundary conditions. To the sites (walls) and bottom I decided for zeroGradient, so the heat can leave the system. For the surface it becomes more complicated. Because of for example air or sun temperature, the surface should have a fixedValue, of lets say 280° K. But the fluid in my pipe is much hotter, around 380°K. So in reality there should leave heat as well in direction surface, but in the same moment there is a influence of the surfacetemperatur. If i decide for a fixedValue, it`s not gonna happen. So I need something between zeroGradient (heat can leave the system at the surface) and fixedValue (influence of the surface temperature caused by air or sun). Could the bc "mixed" help me? And if yes, how do I understand the "valueFraction"? Thanks a lot in advance!

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January 20, 2020, 07:30		#2
simrego Senior Member anonymous Join Date: Jan 2016 Posts: 416 Rep Power: 14	Hi! If you set fixedValue, there will be still heat flow through the surface. In mixed BC the valueFraction sets the fraction between the value, and the gradient. From the internalField, and the prescribed gradient, you can calculate the surface value, let me call it gradValue. But in the same time you prescribe the value on the surface as refValue. From these and the value fraction, you can calculate the actual surface value as: surfaceValue = valueFraction * refValue + (1 - valueFraction) * gradValue note that value fraction must be: 0 <= valueFraction <= 1. If it is 0, you get the fixedGradient BC, if it is 1, you get the fixedValue BC.