Learning Forum

[SR786]

Hello,

I want to simulate mixing of a tracer in liquid (using a passive scalar) in a stirred tank reactor. The reactor is operated in batch (no inlets or outlets) with two phases (air & water) which form a distinct interface, known as a vortex, where mixing will only be performed in the liquid phase.

The tracer will be injected on the interface between the phases (for the experiment a plume was introduced, operated in batch mode).

Having read some online material, it has been suggested to activate a UDS and solve one scalar transport equation. However, for a UDS to work a boundary condition (BC) must be defined such as an inlet. In my domain, batch reactor, I only have BC's for the solid walls ("no-slip") and the top of the reactor ("free-slip"). How would I be able to use the UDS without an inlet BC in the tank to introduce the tracer?

[Rob]

Have a look at source terms and the Patch function. I'd probably look a species mixture over scalar to take advantage of some of the numerical tools and because you have a volume fraction between zero and one. 

[SR786]

Thank you for the reply! With species transport, can you set it up for a non-reactive system? Also what mixture template should I use as there are many available

[Rob]

Yes. You will need to alter the template to water and water-tracer (or whatever is in the tank). The only common error is to set the diffusivity to a liquid value as the default is for gases. 

[SR786]

No worries, for this problem I am assuming the tracer has the same properties as the primary liquid (water) so that the flow field is unaffected and I can switch off the flow & turbulence equations in the TUI using solve/set/equations.

I also notice the energy equation is switched on automatically with species transport. I do not need it as I am working with an isothermal system. When I try to disable it on the GUI it won't allow me to as it states 'Enabling energy equation as required by material method'. Is keeping the energy equation critical for the species transport model?

[Rob]

Check the material (species & mixture) density. Chances are something defaulted to something you don't want. 

Yes, if you just patch in the same material with a different name you just need to solve for species. 

[SR786]

Hi, I chose the 'mixture template' which gives water-vapour, oxygen & nitrogen as the default materials. This is a list of the mixture template properties which gives density, specific heat, thermal conductivity, viscosity & mass diffusivity. As I am working with an isothermal system I do not need the specific heat or thermal conductivity. 

[Rob]

You need to turn density back to constant or one of the mass/volume weighed mixture, and then you can turn off the energy equation. 

[SR786]

Thanks for the help, I just tried it and it worked. I first want to try mixing on a flat-liquid surface (without VOF), so just a water-tracer system. Then I will try with VOF with the vortex.