Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Problems with boundary condition

MarĂ­a Josefa Myrus
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!

1 Reply Last Post Dec 7, 2012, 1:14 AM EST
Posted: 5 years ago Dec 7, 2012, 1:14 AM EST
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
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

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.