DEM/SQP, BC to allow only solids

MFiX How to
#1

I am trying out solids cooling, and would like to have particles to fall straight through the domain from the top, while crossflowing gas cools it down. Is this at all possible? I can only find advanced settings for BC’s where solids are not allowed to pass through, but not the fluid itself, which is the case now.

Appreciate any hints here :slightly_smiling_face:

#2

Can you please attach a diagram of what you want to accomplish (showing where the BCs are)? Are you trying to have solids exit an outlet, but not the gas phase?

#3

Yes exactly. If it is at all possible.

#4

This is not a well defined BC (maybe not even physical). You may be able to approximate as follows for the BC where you want particles out and zero gas flow:

  1. Instead of a Pressure outlet, define a Mass Inlet BC. Say this is BC#1
  2. Set the solids fraction to zero, the solids and gas velocity to zero
  3. In the Advanced pane, click the “+” sign on top of the keyword table. Start typing BC_MI_APPLY_TO_DES and select it when it appears in the list of keywords. Enter “1” for “BC” . Press Enter. Make sure the value is set to False. Click on the small icon next to “Update Keyword” and confirm you want to set the value. Press “Update keyword”. This will set BC_MI_APPLY_TO_DES(1) = .FALSE.
  4. Repeat step 3 to set BC_PO_APPLY_TO_DES(1) = .TRUE.

Alternatively you can edit the .mfx file and insert

BC_MI_APPLY_TO_DES(1) = .FALSE.
BC_PO_APPLY_TO_DES(1) = .TRUE.

What we have done is basically converted a MI into PO for the DES phase only. The gas phase is still technically an inlet but with a zero velocity, so there will be no flow across that plane.

#5

Thanks Jeff. This looks like a solution. Though I am open to look at alternatives if the procedure becomes very exotic. My intention is to look at the continuous cooling of steel particles, that’s why I would like the solids to exit the domain also. Would you say it is safer to let the solids exit along with the fluid through a shared outlet?

I would also like to add inlet BC with preferably only/mostly solids entering, in addition to an inlet with only the cooling fluid (air), but I consider these easier to set up.

#6

I would say it is physically difficult to only have particles and no gas leaving through an opening. In terms of CFD modeling, an inlet lets gas and solids in and an outlet lets gas and solids out. It becomes questionable when you mix inlets/outlets in the same BC, which is why you have to go through these “hidden” features!

#7

I will add a simple sketch now. This was my first idea. Air and solids sort of entering and exiting through separate BC’s. Not necessarily enforced, that would be the solid outlet BC in that case. But I have now evolved to merge the two outlets into one combined, to avoid the overly complicated setup. Perhaps I could also narrow the outlet and have the solids pile up on its way out (down), and become a natural blocking for the fluid.

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