Learning Forum

[jmonteros23]

Hello community!

I'm starting a simulation involving the recirculation of water contained in a 3,500 m³ tank, along with the injection of sodium hypochlorite into the discharge pipe. The main objective is to analyze how the hypochlorite concentration inside the tank evolves over time.

The system has three lines:

An inlet corresponding to the tank filling.

An outlet representing consumption or emptying.

A recirculation line, which extracts water from the bottom of the tank and reinjects it through the top, with the injection of hypochlorite into said pipe.

My question is about how to correctly model this recirculation in Fluent. When generating the fluid volume from the 3D model, the recirculation pipe is directly connected to the tank domain, so I cannot assign inlet or outlet boundary conditions to it.

Could you guide me on the best way to realistically represent this recirculation? Any suggestions on how to separate the zones or set boundary conditions would be greatly appreciated.

Thanks in advance!

[NickFL]

For the circulation line at the tank exit you could just model it as a second outlet. At the recirculation inlet you would then have the same exiting mass flow rate with the new concentration. Basically you are removing the complexity of modeling the pipe length, pumping system, etc.

[Rob]

To add to Nick's comment, if you use a report of concentration on the loop outlet and then an expression of concentration on the loop inlet with more mass (and concentration) to account for the addition. That also accounts for the increasing concentration entering the loop. 

[jmonteros23]

Hi Nick, Rob!

Thank you both for your comments.

Rob, you’re right—it’s exactly as you described. However, my question is: how can I configure Fluent so that the concentration leaving through the suction is used as the concentration entering through the recirculation inlet?
Right now, I can only set the suction as an outlet, which means that the concentration leaving there is "lost" and not re-injected into the system.

[NickFL]

I would suggest you make a simple 2D model to debug this. Debugging your the set-up on that large model 3D model would be painful. Also I would suggest not running a transient in workbench, instead run Fluent in standalone. 

You have (or can get) the mass fraction as the recirulation exit, correct? Then at the recirculation inlet you can then add a small amount back onto this value to account for the increase in hypochlorite.

[Rob]

Read my comment more carefully. 

[Rob]

Look at surface reports and expressions. What goes out is lost but nothing says you can't put it back in again!