Learning Forum

[SR786]

Hi,

I have been attempting to model batch mixing with a tracer in a unbaffled stirred tank, where the free surface of the liquid deforms leading to a vortex.

To begin with I have modelled the hydrodynamics for the gas-liquid system (air-water) using the homogeneous VOF solver and SST K-Omega Curvature Correction model.

Once I have got a converged solution, I switched off all the flow & turbulence equations and activated the species transport equation, so the system is now gas-binary liquid (air-water tracer). I have defined the properties of the tracer liquid as the same as the bulk liquid (water) as I assume there is no significant change to the flow behaviour with the introduction of tracer material. I have defined the diffusivity between water and tracer as 1.473*10^-9 m^2/s as per literature reports. I have also approximated the volume of tracer by patching an appropriate number of cells in the mesh.

The injection position is the free surface of the liquid (0.2 cm from the wall) where the mixing is performed in batch (no inlets or outlets). An image of the initial conditions are provided below for context

What I am noticing is that throughout the course of the simulation, the tracer mass fraction contour appears to give a value of 0 in the domain:

Also my concentration plots don't make sense

I have ran the same simulation single phase (assuming flat liquid surface) which makes more sense:

I don't understand why applying the same methodology for mixing with a flat liquid surface has failed to work when considering a vortex. No one appears to have performed such a simulation before (all attempts ofr mixing have been with flat liquid surface). Are there any possible suggestions on where to proceed next?

[Rob]

In the VOF case look at the scale on the second image. In an unbaffled tank what is there to stop the tracer remaining unmixed and the "lump" of tracer just moving around the tank? Plotting on a single surface may not be the best way to find it! 

[SR786]

Hi,

Thank you for the response. I am unsure of what you mean? For the tracer I have injected it on the free surface so it is a bit different to the flat liquid surface which assumed all the tracer is in the liquid at time 0 whereas for the VOF simulation as it is injected on the free surface it is traversing the gas-liquid interface to begin with. The VOF equation is switched off in addition to all the flow and turbulence (leaving only species transport equation). Is this the correct method or approach. If the tracer is travelling through the interface do I need to consider mass transfer through the interface and should the VOF equation be switched on too with species transport?

[Rob]

You did inject it into the correct phase? I'd add it a little under the surface to make sure it's definitely in the right place! 

The method you're explaining is correct, and I've been using the single phase equivalent recently. Just check you've set the liquid phase as the species mixture. 

[SR786]

Hi,

Thank you for the reply. I have two phases, one for gas (air) & one for liquid mix (water-tracer) and the mixture properties are correct. I was attempting to replicate as close to the experiment as possible so the tracer was injected on the interface rather than below. However, if there are limitations in modelling this exactly then would it be better to model it below the interface as you suggest to ensure 100% of the tracer is patched?