Learning Forum

[annes]

Hi,

I am modelling a storage building in Fluent in which air with a certain CO2 mass fraction enters the building via an underground channel, then the air goes up through a porous zone (pile of produce) into the headspace and out of the building.

[Rob]

Check you actually have flow through the domain. The common error is to forget to share the topology of the parts so there's no flow path.

[annes]

Hi Rob,
Thanks for the tip, but the flow is going through the domain. This doesn't seem to be the problem. Any idea what else I can check?

[Rob]

Can you run a pathline (streamline in CFD Post) through too? Are you adding mass as well as CO2 in the source region? How does that compare to the inlet mass flow (total, not just CO2)?

[annes]

Yes, I am able to run a pathline through:

and I used both a mass and a CO2 source, is that wrong?
This is the overview of the mass flow rate balance:
Can anything abnormal be seen from this?

[Rob]

No, that's correct. If you only add a species source Fluent tends to convert mass rather than adding it.
Looking at the concentration graph stop the CO2 on the inlet and see what happens, I'm wondering if the result is masked by the scale. You've got a max change of about 2 in 6,300. Altering the scale may help too.

[annes]

Hi Rob,
I'm not sure if this is what you suggested: at t=0.0003s I stopped the solution, set the species inlet BC for CO2=0 and I patched CO2=0 in the ventilation channel that is underneath the porous zone. That way air without CO2 enters the porous zone. Then I started the solution again. These are the plots:
Area weighted average CO2 mass fraction in the porous zone:
Area weighted average mass fraction of CO2 in the headspace:
From the plot of the headspace I think that the CO2-production in the porous zone will eventually result in a rise of the CO2-mass fraction in the headspace, but that I just need a much longer period of time to see that result. Is that correct? Or can something else be concluded from these plots?

[Rob]

OK, that explains the issue. Simply, if it's a room scale domain it'll take several seconds for the CO2 from the source to mix with the rest of the flow. Similarly the CO2 will be diluted from below as fresh(er) air enters the space.
A simple check for how long you need to run is a few times the plug flow time for the domain. Ie in an ideal situation how long does the supply flow take to fill the room, then run for a few times that.

[annes]

Thank you for all the help, Rob! I'll follow your advice!