Particles are leaving from pressure outlet boundary_How the equations are balanced in such case?

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#1

I have modelled fluidised bed, where I have given high inlet velocity. I have observed that the particles are exiting from the pressure outlet boundary. I ran the software till all the particles got removed from the domain.
My question is how the equations are balanced if all the particles are getting removed from the domain?

#2

Hi @margidave007 - welcome to MFiX
What do you mean by “how the equations are balanced” - which equation in particular?

#3

I have modeled a fluidized bed where the bottom of the reactor mass inflow boundary condition and the top of the reactor pressure outflow boundary condition are given. At high superficial velocity almost two times Umf it shows that some particles are exiting out of the reactor, at the end of the simulation number of particles are not consistent. Please suggest how to apply boundary conditions at top of the reactor for a constant number of particles over all simulation time. No slip boundary condition at top bc_jj_ps =0 /1 is not working.

#4

If you have a high superficial velocity, it is possible some particles will be elutriated, especially if the outlet is located near the bed. This is not necessarily an indication of an ill posed boundary condition, nor that equations are not balanced.

#5

Thank you for your response. Is there any way to prevent particles exiting from the reactor? In my case, the particle’s superficial velocity is less than two times the minimum fluidization velocity and the reactor height is five times the packed bed height for the dense particle system.

#6

You can set the keyword bc_po_apply_to_des(BC_ID) = .false. , where BC_ID is the boundary condition ID of the pressure outlet. This will prevent the particles from leaving the domain. This is not a recommended setting so you will need to set it in the advanced pane:

  1. Go to the advanced pane
  2. In the search bar, type bc_po. You should see the keyword bc_po_apply_to_des on the left. Select it.
  3. Enter the BC iD corresponding to the pressure outlet BC. Press Enter.
  4. Uncheck the check box so the value is “false”. Press Update. If you have a lock icon next to the Update button. clicj it and confirm you want to edit the keyword.

You should see something like bc_po_apply_to_des(2) = False in the console. Save you file, Look at the .mfx file in the ditor and you should also see bc_po_apply_to_des(2) = .False. in the file.

You can also manually add this keyword in the .mfx file outside the GUI if you want.

I would only use this keyword if you have very few particles leaving. If you really have a lot of particles making their way to the outlet, it could create a plug.

#7

Thank you, I will try this one too