Hello everyone,
I am trying to simulate a laminar flow with heat transfer in a rectangular microchannel, but the problem is when I solve just the laminar flow everything (velocity, friction factor,...) is correct and matches with the analytical solutions. When I add the heat transfer it affects the laminar flow as well and changes the pressure and velocity in the microchannel. I entered all of the solid and fluid properties user defined and constant (independent of T), so the laminar flow results should not be changed.
I have a laminar flow module of comsol with P2+P1 discretization,water as working fluid, Inlet velocity (2.8932m/s calculated from Re=250) and 0pa at outlet.
Fluid and Solid (silicon) domains have been selected in Heat transfer module, boundary conditions are constant T at inlet of fluid domain, Outflow at outlet of fluid domain and constant heat flux at bottom of the solid part, and symmetry boundary condition has been used for both sides of channel for both laminar flow and heat transfer as you can see in the pic.
non-isothermal flow option is on in multiphysics and I am using Paradiso direct solver as fully coupled.
I have attached the comsol file (v 5.2).
I appreciate any help that can help me to solve this problem.
This forum can help you. Good luck.
https://www.comsol.com/community/
Dear Shiuh-Hwa,
I have entered all fluid properties user defined. My solution converges, but doesn't match with the analytical solutuion after I add heat transfer.
Best regards
Dear Siamak,
Normally heat transfer and fluid friction are related. so if you allow heat transfer the thermophysical properties should change (slightly for water, except for dynamic viscosity) and the results of friction coefficient and velocity profile are different from the case where the temperature of the fluid is constant (no heat transfer).
If you want to obtain the same results which is not very logic, but you already fixed the properties as constant independent of temperature , you can try not coupling the equations and use a segregated solver.
another option is to solve the simulation for flow alone, (you need to disable the energy solver) and when reaching the hydrodynamic convergence, you should stop the fluid solver (x-momentum....) and enable the energy solver. i think you should obtain the same results as without heat transfer.
Kind regards,
Kifah SARRAF
Dear Shiuh-Hwa and Sarraf,
I found out although my results match with analytic solutions they are not mesh independent (it changes from 5.2-5.315 with my meshes) and since I use P2+P1 discretization I cannot use finer meshes. again when I use p1+p1 I cannot get correct results with analytic solutions. do you have any idea what should I do to get correct results?
For your laminar only vs laminar+thermal, I don't see obvious difference.
Dear Shiuh-Hwa,
The results for velocity don't change much (around 0.2) but pressure has changed in the channel and I results for velocity doesn't match with the analytical solution.
I attach the excel file that I wrote from Henrik Bruus book for velocity profile for rectangular channels. Its input is pressure gradient and results match with comsol but when I add heat transfer pressure at same length of channel (7mm) increases and results of comsol and analytic doesn't match.
Kind regards
5.4 vs 5.2? To me, it's quite good enough. Does any other researcher predict much better than your prediction? Since you have a analytical theory for whole cross-section, why not compare comsol to theory for whole cros-section area?
Remember, analytical theory is always companion with assumptions. So, we don't expect our simulation to match with it exactly.
PS. In your thermal part, the velocity seems to be fixed while it should come from the laminar model.
Dear Rasul,
Regards for your answer. I am sure of my meshes, I have the mesh independency. I apply same meshes in heat transfer too. I tried to apply 0 heat flux in same file. I tried to have smaller residual in heat transfer. Again and again the results change. I don't see any reason for this change!? Results for momentum eqn. should be same.
Dear Siamak
If your fluid flow is correct, may be you inserted wrong parameters and heat equations in heat transfer section.
I suggest you at first you select a simple case study and validate the fluid flow and heat transfer on it, then extended it for your project.
You can find simple case study similar to your project by searching on Comsol's website.
Good Luck
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