CFD Online Logo CFD Online URL
Home > Forums > CFX

How can I prescribe my B.C.?

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   July 26, 2002, 12:58
Default How can I prescribe my B.C.?
Dejun Jing
Posts: n/a

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.



  Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Confusion between 'non-slip' and 'slip' B.C jinwon park Main CFD Forum 7 March 6, 2008 05:56
how to set such B.C. in Fluent? Sherri FLUENT 1 January 12, 2006 13:32
Urgent! Help on UDF to set B.C. of 3rd type Ray Hong FLUENT 0 December 28, 2005 20:35
How to set B.C. of the 2nd or 3rd type in UDS? Ray Hong FLUENT 0 December 28, 2005 07:03
Non-Reflecting B.C. in NSC2KE Zou Chu Main CFD Forum 2 May 27, 1999 21:26

All times are GMT -4. The time now is 14:58.