# Boundary condition for E-E multiphase model

Hi

I am simulating fluid-granular flow using Eulerian model.

I want to ask about the B.C at the inlet for the two phases:

The primary phase (fluid) has a defined velocity inlet, and the secondary phase has constant concentration (or volume fraction at the inlet, let say 4% (0.04)).

I used to specify the inlet fluid velocity and particle volume fraction while keeping particle inlet velocity at zero (because I thought that fluent will automatically calculate the particle velocity based on the volume fraction and the mass flowrate of the inlet fluid). However, i figured that this could be not correct and maybe the reason why I did not get reasonable results.

My question: Shall I specify the particle inlet velocity as the same as the fluid velocity or i need to calculate the actual velocity of particle phase (as mentioned earlier based on Vf and Vf) and then use that velocity as inlet condition.

Thanks in advance

## Comments

Your help is appreciated !

Just add the second phase at the same velocity and set the volume fraction to whatever you want it to be. If you set zero velocity the second phase won't enter the domain as there is no associated mass flow.

Thanks for your response Rob. However, I was wondering when I add the second phase at the same velocity of the primary phase, that means I am specifying similar volumetric flowrate (m3/s) for both phases (since they enter through a constant cross section area at the inlet). In that case, the volume concentration will be fixed at 0.5 for each phase. Please correct me if I am wrong.

No you are providing the true velocity here. What you are talking about is the volumetric flux which will then correspond to superficial velocity: the velocity of the phase if to flow alone in the pipe.

Thanks for the clarifications. Then I will use the same velocity values for both phases and see what else I am missing in this simulation.

Jepp

Hello Again

I just revised the case when I used similar velocities the the two phases, and I found that the volume flowrate (m3/s) at the inlet for the two phases is equal. I obtained these value from Report-surface integral- volume flowrate. If the inlet flowrate of the solid phase is equal to that of the liquid, how can the volume concentration be 0.04. I am confused?

Thanks

The velocity tells the solver how fast the material is being added to the model. The volume fraction is how much of it is added. Can you post screen shots of the boundary condition panel and the volume fraction & velocity contours in that area?

Volume fraction looks right. If you check the mass flux how does it look?

Mass flux looks ok. if you see the attached pictures, you will find mass of mixture is 0.785 kg/s, and that for the particle phase 0.075. When I translate theses value into volumetric flowrate, if found that mixture and particles have flowrates of 7.8e-3 and 2.96e-5 m3/s, which equals a volume fraction of 0.0376. However, I still don't understand why I got similar volume flowrate from surface integral report as shown in the above picture

One thing I was thinking about. If I specify the inlet v of particle at 0 but the volume fraction is 0.04, there will be no inlet mass flow of particles So what is the point of the volume fraction here. Does that mean the inlet initially has particles (with vf 0.04) but without extra particles coming into the domain ?

Volumetric Rate is just velocity time area. So all is correct.

Volume fraction is telling the solver how much of the phase is coming into the domain. If you still do not feel comfortable with all just use mass flow boundary condition and play a bit in a dummy case so that you understand the correlation between all inputs