# Discussion Forum

## Problems with boundary condition

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María Josefa Myrus

December 6, 2012 10:13pm UTC

Problems with boundary condition

Hi!

We've tried to solve a problem. Here it is:

It has a system of two concentric pipes. An opaque liquid enters to the inner pipe with entrance velocity and temperature known. The space between the pipes is evacuated, but the inner mantle of the outer pipe acts as a heat source that is maintained at constant temperature.

We've represented the problem as a system of three rectangles, being the inner one the inside with the liquid, the middle one is the pipe wall, and the outer is the space between both pipes. The radiative condition is OK, but the convective one has been troubling us. When trying to model this condition, we cannot select the pipe's inner boundary. We thought separating rectangles with a little distance could be a solution, but then, the solver couldn't converge.

Any help will be welcome. Please, this is urgent!

Ivar Kjelberg

December 7, 2012 6:14am UTC in response to María Josefa Myrus

Re: Problems with boundary condition

Hi

My experience urgent issues tend to go wrong ;), OK first why rectangles and not circles ? for pipes, a minor issue but in the corners you have singularities, it takes often longer to solve. And circles sometimes allow you to go 2D-axi and the model runs much faster, useful when you are in a hurry

Now you haven't told us the model geometry size 3D, 2D or 2D axi ? and which physics NITF ?

is it so that the outer boundary is at constant T (which means that COMSOL indeed adds a fictitious source/sink to maintain the Temperature constant, and then you have surface to surface radiation exchange (easier in cylindrical symmetry as you can use know formulas and the view factor can be estimated well.

But then convection (there should not be any in vacuum ?, do you apply boance forces ? and the flow in which direction ?

Too many unknown for me at least ;)

--
Good luck
Ivar

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