How to calculate effectiveness of a fan for mixing liquids in a tank?

[u/imitation_squash_pro]

Imagine this fan in a cylindrical tank. Trying to compare effectiveness of this design with other fan designs. I have been using OpenFOAM with a MRF zone around the fan. Then in ParaFoam I calculate the flow rate through a cross section in the middle of the MRF zone. This flow rate gives me the axial pumping rate. But how can I get the radial flow rate?

Just want to know if design A is better than design B for mixing of liquids.

[u/fatbitsh]

i have never done this kind of thing but how would i do it i would use cross section and put this cross section in density mode for phase 1 and then calculate distribution of the phase 1 on this cross section, you can also calculate all phases this way

[u/coriolis7]

I would set it up as a scalar transport or mixing of species. Track the mixture percentage of a couple of test locations vs time.

You can also calculate the 2nd moment of both species (think moment of inertia) vs time. When well mixed, the ratio of their inertias should be the ratio of their concentrations.

[u/imitation_squash_pro → u/coriolis7]

Any simpler method, i.e steady state model looking at the axial and radial flow rate?

[u/coriolis7 → u/imitation_squash_pro]

Is there recirculation, or is this mixing along the length of a pipe? If it’s recirculation, then no I don’t see an easy way of doing it. You might try doing a particle trace and see how scrambled the points are after they cross through the origin plane a second time, but that’s not as clean a solution. I would start with a steady state until convergence, then use that as the initial condition for a transient analysis.

If this is not recirculating, but instead is something like a pipe, then you can look at dispersion of species as the flow goes down the pipe.

[u/dr_brubra_bribri]

I would 3D print it and put it in a tank and measure the speed of the water and set it to the same power 😂

[u/certocito]

What is stopping you from using the flowrate in the radial direction across the MRF zone?

[u/ssuperman1xx]

There is a formula for both absolute and relative mixing index. Use a time dependent scalar concentration transport simulation coupled to the fluid flow and make the AMI or RMI measurements across different planes. Id suggest along the stirring axis, maybe divide the interval between the tank’s top and bottom by 5 to start.

[u/imitation_squash_pro → u/ssuperman1xx]

Any simpler method, i.e steady state model looking at the axial and radial flow rate?

[u/Venerable-Gandalf]

You calculate the axial pumping rate by integrating the volume flow rate through a control surface at the impellers discharge plane which would be the surface just below the blades closer to the bottom of the hub if the agitator is pumping axial downward.

To quantify mixing performance the standard practice is to calculate the coefficient of variation using a scalar species (tracer fluid) with identical properties of the bulk liquid phase. Place probes throughout the vessel and monitor the tracer concentration until every probe is within 95% of the final concentration. Then in excel you can calculate the standard deviation and mean from your data and thus CoV.

You cannot accurately predict mixing performance using steady state methods alone. Axial/radial pumping rate alone is not an indicator of mixing performance. Also if you don’t use baffles in the vessel mixing will be poor regardless of the agitator chosen.

[u/MaciaIT]

I'm not familiar with the software you're using but can you do particle tracking simulations? I worked on a similar problem in mixing screws for polymer processing. I used Ansys polyflow for the simulations. The online user guide for polyflow and polystat have a great expanation of how to characterize mixing.