I am doing analysis using FLUENT wherein two fluid streams mix together as follows. In a straight channel there is another inlet at the midway where the second stream of fluid enters (same fluid) and then both the streams pass through as one.
My concern is whether the analysis can be performed using laminar assumption or do I need to consider any turbulence also?
The channel cross section is of the order of 200 microns and the Re number at both the inlets is well within the laminar regime (around 200-500).
P.S: The fluid is liquid under consideration.
Dear Deewaker,
In my opinion, the Reynolds numbers you're considering are quite
high. Assuming the fluid to be water at 20^C, the minimum and
maximum velocities in the microchannel would be:
>> nu=1.004*10^(-6); d=200*10^(-6); Re_min=200; U_min=(Re_min*nu)/d
U_min =
1.0040 m/sec
>> nu=1.004*10^(-6); d=200*10^(-6); Re_max=500; U_max=(Re_max*nu)/d
U_max =
2.5100 m/sec
This means that the fluid would travel between 5020 and 12550
diameters per second. It sounds unrealistic for microchannel
applications. I would expect Reynolds numbers of order 1-10.
I suspect that, because of the extremely high velocities,
vortices will form at the inlet of the second fluid and
will be convected downstream therefore mixing the two fluids
or, perhaps, a mixing layer forms between the two fluids,
a mixing layer that will become unstable. In these cases,
mixing would be chaotic. However, in my opinion, these
phenomena are artefacts produced by the unrealistically
high velocities of the fluids.
Two words about chaotic advection. Chaotic advection at
low Reynolds numbers doesn't form spontaneously. In general,
it must be induced by intelligently manipulating the mixture
by force fields, velocity fields or moving boundaries.
We recently published two studies on optimal mixing in
laminar regimes. These studies might interest you. They
are available on ResearchGate:
Towards the design of an optimal mixer
O. Gubanov, L. Cortelezzi
Journal of Fluid Mechanics, vol. 651, pp.27-53, 2010.
On the cost efficiency of mixing optimization
Oleg Gubanov, Luca Cortelezzi
Journal of Fluid Mechanics, vol. 692, pp. 112-136, 2012.
Best wishes,
Luca
----------------------------------------
Prof. Luca Cortelezzi
Department of Mechanical Engineering
McGill University
http://people.mcgill.ca/luca.cortelezzi/
I would suggest that you start the simulation as a laminar flow problem; then see the results of mixing.
The problem of microscale mixing is well developed and investigated. Please, Google the 'microscale mixing' topic and you can get a lot of literature.
For the present problem, (Re ~ 200-500), the laminar model can work quite well. However, you need to check for steady and unsteady simulations if some secondary motion appears.
Indeed, I would check both steady and unsteady models, as well as a few different turbulence equations. Do you have experimental data or flow visuals to compare with? If so, all the better.
Dear Deewaker,
In my opinion, the Reynolds numbers you're considering are quite
high. Assuming the fluid to be water at 20^C, the minimum and
maximum velocities in the microchannel would be:
>> nu=1.004*10^(-6); d=200*10^(-6); Re_min=200; U_min=(Re_min*nu)/d
U_min =
1.0040 m/sec
>> nu=1.004*10^(-6); d=200*10^(-6); Re_max=500; U_max=(Re_max*nu)/d
U_max =
2.5100 m/sec
This means that the fluid would travel between 5020 and 12550
diameters per second. It sounds unrealistic for microchannel
applications. I would expect Reynolds numbers of order 1-10.
I suspect that, because of the extremely high velocities,
vortices will form at the inlet of the second fluid and
will be convected downstream therefore mixing the two fluids
or, perhaps, a mixing layer forms between the two fluids,
a mixing layer that will become unstable. In these cases,
mixing would be chaotic. However, in my opinion, these
phenomena are artefacts produced by the unrealistically
high velocities of the fluids.
Two words about chaotic advection. Chaotic advection at
low Reynolds numbers doesn't form spontaneously. In general,
it must be induced by intelligently manipulating the mixture
by force fields, velocity fields or moving boundaries.
We recently published two studies on optimal mixing in
laminar regimes. These studies might interest you. They
are available on ResearchGate:
Towards the design of an optimal mixer
O. Gubanov, L. Cortelezzi
Journal of Fluid Mechanics, vol. 651, pp.27-53, 2010.
On the cost efficiency of mixing optimization
Oleg Gubanov, Luca Cortelezzi
Journal of Fluid Mechanics, vol. 692, pp. 112-136, 2012.
Best wishes,
Luca
----------------------------------------
Prof. Luca Cortelezzi
Department of Mechanical Engineering
McGill University
http://people.mcgill.ca/luca.cortelezzi/
Change your software to COMSOL and compare results it in both Laminar and Turb. regim.
In my opinion, Laminar simulation would be compatible and valid for microscale simulation.
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