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Conjugate heat transfer pipe flow

Posted 1 nov 2012, 18:30 GMT-4 Fluid & Heat, Heat Transfer & Phase Change Version 4.3 5 Replies

Piero Caballero
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Hello,

I am trying to simulate a very simple laminar flow inside a pipe problem with a constant temperature at the walls. I have a cylinder made of water(liquid), and where one part is kept at a constant temperature at the wall, and rest of the wall changes from 293 to 393 for the remaining of the length.
My biggest issue is not so much the heat transfer part of the problem, but the velocity profile at various points of the pipe. I know I should get the same velocity profile everywhere and it should be a parabolic profile and since I am using a mean velocity of 0.02 m/s I know that my Vmax will be 0.04. However I only get this velocity profile at the inlet and not in the rest of the pipe. When I keep the walls temperature constant ( at 293 throughout) I get the correct velocity profile, it is only when I inflict the temperature gradient when the velocity profile changes, even though I am keeping my properties constant.
Any help will be appreciated. I am attaching my model

Thank you very much!
5 Replies Last Post 7 nov 2012, 14:29 GMT-5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 2 nov 2012, 03:07 GMT-4
Hi

A few things to observe, first if you have a symmetric system, its better to solve it in 2D-Axi as this runs far quicker, at least to get used to the different physics and their tweaking. One major reason to not do everything in 2D-axis is when you have a gravity body load that is not aligned along the axis of symmetry, then you must end up in 3D

Then what is essential is to have a correct mesh, with boundary layer mesh. The COMSOL default physics mesh is well suited (not the user defined you have currently) but it tend to be dense, which is adapted to the need of CFD, hence not always solvable on a "normal" PC/laptop

Then one should expect changes in the velocity profile as with the heat transfer you change also the proprieties of the fuild hence its homogeinity, it all depends on the relative velocities/diffusivities fluid/temperature, hence the importance of Reynolds, Nusselt, Knudsen ... numbers in calssical theory They remain of use to better tweak your model also for FEM etc

--
Good luck
Ivar
Hi A few things to observe, first if you have a symmetric system, its better to solve it in 2D-Axi as this runs far quicker, at least to get used to the different physics and their tweaking. One major reason to not do everything in 2D-axis is when you have a gravity body load that is not aligned along the axis of symmetry, then you must end up in 3D Then what is essential is to have a correct mesh, with boundary layer mesh. The COMSOL default physics mesh is well suited (not the user defined you have currently) but it tend to be dense, which is adapted to the need of CFD, hence not always solvable on a "normal" PC/laptop Then one should expect changes in the velocity profile as with the heat transfer you change also the proprieties of the fuild hence its homogeinity, it all depends on the relative velocities/diffusivities fluid/temperature, hence the importance of Reynolds, Nusselt, Knudsen ... numbers in calssical theory They remain of use to better tweak your model also for FEM etc -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 4 nov 2012, 12:44 GMT-5
It seems that the solution you are getting is not a converged solution. I am sure you can adjust some solver settings to reach convergence. I noticed however that the viscosity that you used is 1e6 times that of water. That very high viscosity could be affecting the solver settings. If I change the viscosity to that of water the results look much better (as much as I would expect given the course mesh density as Ivar mentioned).

Nagi Elabbasi
Veryst Engineering
It seems that the solution you are getting is not a converged solution. I am sure you can adjust some solver settings to reach convergence. I noticed however that the viscosity that you used is 1e6 times that of water. That very high viscosity could be affecting the solver settings. If I change the viscosity to that of water the results look much better (as much as I would expect given the course mesh density as Ivar mentioned). Nagi Elabbasi Veryst Engineering
Piero Caballero
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Posted: 1 decade ago 6 nov 2012, 21:45 GMT-5
Thank you very much
Thank you very much
Piero Caballero
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Posted: 1 decade ago 6 nov 2012, 21:46 GMT-5
thank you very much
thank you very much
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 7 nov 2012, 14:29 GMT-5
You're welcome!
You're welcome!

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