Hello everybody.
I am modelling micromixers and want to check their efficiencies.
For that purpose, I first used the Incompressible Navier-Stokes model, to calculate the flow inside my device. I set it up that way:
- Pressure, no viscous stress, 20 Pa at my inlets
- Pressure, no viscous stress, 0 Pa at my outlet.
- everything else is "Wall".
I simulate this with default microfluidics settings.
Then I use the Convection/Diffusion model, with these settings:
- Diffusion coef: 1e-9
- x-y-z velocity : u-v-w
- Concentration : 10 at one of my inlet, 0 at the other.
- Boundary condition at outlet: Convective flux.
The problem is: the resuts are absurd. First, I have negative concentrations and >10 concentrations, which is physically impossible.
Then, when I observe the outlet domain, the concentration is very homogeneous (to the hundredth), but not around 5, as it should be. On some of my designs, it is as low as 2 (or as high as 8 if I inverse the concentration at the inlets). It seems there is no mass/matter conservation.
Therefore I tried the "Conservative Equation Form" (in the Properties of the Convection/Diffusion model - Non conservative is default). I am not sure it is a good idea. The results are completely different, and now, I don't know which model I should trust.
Can someone help me with this ? I've already read the KnowledgeBase about negative concentration (www.comsol.com/support/knowledgebase/952/) and tried the "Refine Mesh" solution, with no success (except making my computer freeze!). I would like to try the "smoothing function" but cannot figure out what to do or where (I am new to Comsol AND microfluidics, so I don't know/understand what/where every feature is). And it is still no solution to my mass/matter conservation issue.
Thanks a lot in advance for any advice.
I am modelling micromixers and want to check their efficiencies.
For that purpose, I first used the Incompressible Navier-Stokes model, to calculate the flow inside my device. I set it up that way:
- Pressure, no viscous stress, 20 Pa at my inlets
- Pressure, no viscous stress, 0 Pa at my outlet.
- everything else is "Wall".
I simulate this with default microfluidics settings.
Then I use the Convection/Diffusion model, with these settings:
- Diffusion coef: 1e-9
- x-y-z velocity : u-v-w
- Concentration : 10 at one of my inlet, 0 at the other.
- Boundary condition at outlet: Convective flux.
The problem is: the resuts are absurd. First, I have negative concentrations and >10 concentrations, which is physically impossible.
Then, when I observe the outlet domain, the concentration is very homogeneous (to the hundredth), but not around 5, as it should be. On some of my designs, it is as low as 2 (or as high as 8 if I inverse the concentration at the inlets). It seems there is no mass/matter conservation.
Therefore I tried the "Conservative Equation Form" (in the Properties of the Convection/Diffusion model - Non conservative is default). I am not sure it is a good idea. The results are completely different, and now, I don't know which model I should trust.
Can someone help me with this ? I've already read the KnowledgeBase about negative concentration (www.comsol.com/support/knowledgebase/952/) and tried the "Refine Mesh" solution, with no success (except making my computer freeze!). I would like to try the "smoothing function" but cannot figure out what to do or where (I am new to Comsol AND microfluidics, so I don't know/understand what/where every feature is). And it is still no solution to my mass/matter conservation issue.
Thanks a lot in advance for any advice.