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Dejun Jing July 26, 2002 12:58

How can I prescribe my B.C.?

I am modeling fine powders flowing out of a hopper using the gas-solid multiphase model using CFX V5.5.1. Because I am a new user of CFX, I have a few confusion about boundary conditions for my model.

Initially the hopper is fully loaded with still powder, and the initial solid fraction (fs) is 0.55, and fg=0.45.

My question is:

(1) The top face:

No powders will enter the hopper, so the free surface of powders will decrease slowly. However, because the flow direction of air is unknown, I decide to use OPENING B.C. with zero relative static pressure. Also, in order to model zero powder flux here, I use fs=0 and fg=1.0.

Is this prescription correct?

(2) The bottom face

Although powders flow out through the bottom orifice, the flow direction is also unknown, so I continue to use OPENING B.C. with zero relative static pressure. My question is: I think the volume fraction at the exit orifice is unknown, they should be obtained from computation. However, fs and fg are needed for OPENING BC. So, I use the initial condition values, i.e. fs=0.55, fg=0.45.

Is this prescription correct?

(3) Side wall

I use non-slip BC for air. I think I should also use non-slip for powders, but the mannual said free-slip should be used for powders. However, I tried free-slip and got nonreasonable results. So, which kind of slip BC is correct for powders?

I use above B.C.s to carried out my simulation but it failed. It's difficult to converge. Especially, I found lots of bubble forms from the bottom orifice and extend upward, make powder flow unstable, leading to divergens and run termination. I am wondering my B.C.s are correct.

By the way, at present time I did not use kinetic approach, and only used the Gidaspow model implemented in CFX 5.5.

I ever tried INLET and OUTLET BCs, but I always got warning like this: " A wall has been placed at portion(s) of an INLET/OUTLET,...... If this situation persists, consider switching to an Opening type boundary condition instead. "

Can you please give me some suggestion for my model? Thank you very much.



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