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Eldoret October 11, 2003 10:41

Particles in a stirred tank
 
Hi all, Are there people out there working with a 3 phase system in a stirred tank (CFX5.6)? If you have any ideas please help. How do I introduce particles into a liquid-gas domain such that I have specific volume fraction for all the 3 phases.?? Tut 9 is not applicable in my case because the solids must stay in the reactor, only gas comes out thro' the degassing boundary.

regards Eldoret

Maseno October 11, 2003 11:41

Re: Particles in a stirred tank
 
I too have a similar problem. Does CFX5.6 have Eularian -Eularian 3 phase option, where gas and solids are the dispersed phase with volume fractions?

Maseno

Robin October 14, 2003 08:53

Re: Particles in a stirred tank
 
Hi Maseno,

Any number of phases can be selected. CFX-5 provides n-phase, coupled Eularian-Eularian multiphase.

Regards, Robin

Maseno October 14, 2003 10:08

Re: Particles in a stirred tank
 
Hi Robin, I have failed to have both solid particles and bubbles plus the liquid on the Eulerian-Eulerian scheme.

Where Am I going wrong? Regards Maseno

Robin October 14, 2003 13:15

Re: Particles in a stirred tank
 
Are your solid particles Lagrangian or Eularian?

-Robin

Maseno October 15, 2003 04:06

Re: Particles in a stirred tank
 
My Particles are Eulerian. This is because i want to specify particles volume fraction.

Thanks Maseno

Robin October 15, 2003 11:52

Re: Particles in a stirred tank
 
How is it failing? You can certainly set it up, but the boundary conditions and initialization of n-phase problems are a little more difficult. Try writing out a backup file at zero to see your initial guess (there is an expert parameter to do this) and write out backup files at each of the first 10 iterations, or a suitable number before the solver blows, to see what is happening (there is also an expert parameter to prevent the deletion of backup files).

This should give you some clues at to how to fix it.

Regards, Robin

Maseno October 16, 2003 05:16

Re: Particles in a stirred tank
 
Hi Robin, Thanks for your kind response. You are right, the initial conditions is a problem as i noticed this when i was setting up the Eulerian-Lagrangian Model. I finally managed to get some meaningful data with E-L. But now......

Problem 1. This is what I have done: I went to the pre-processor to set up the physics etc.Domain--->General--->Fluid list or Particle tracking. I can not see the list of particles at all unless I activate Particle Tracking. However, I was thinking that if I activate particle tracking after speciying the Liquid/Gas in the fluid list then I am invoking Eulerian-Lagrangian modelling. Isn't this so?

Problem 2. If I activate the Particle tracking then I do not see where to specify the volume fraction. In any case i thought that Particle tracking solves transport equation for (an) individual particle(s). As such, I did not expect to specify volume fraction there any way. Please do correct me where I am wrong. Where is the n-phase specified in CFX5.6? I cant see anything like that in the Expert Parameters!

Thanks for your time Maseno


Robin October 16, 2003 15:10

Re: Particles in a stirred tank
 
Hi Maseno,

There is no "n-Phase" option, just as there is no "Multiphase" option. It is simply implied when you select more than one fluid (i.e. select multiple fluid, and solid materials from the list). It's that simple.

You are right about Lagrangian solving a transport equation for each particle. In the eularian case, you must define at least one fluid as the continuous phase, for the others you will need to specify particle sizes, volume fractions, etc.

Regards, Robin

Maseno October 17, 2003 04:01

Re: Particles in a stirred tank
 
Hi Robin,

Thanks for your comments. I am very sorry if my problem is NOT clear yet. Do we have particles on Eularian model in CFX5.6? If so, where do you specify this? I am aware, and I have done this successfully, with Eulerian-Lagrangian option, you specify particle size and and maximum packing, I have no problem with that.

My problem is here; my domain has this: 20% gas (dispersed phase) 10% solid (dispersed phase) 70% liquid (continuous phase) This to me is an obvious Eulerian-Eulerian case (or not?) , How do you set up this please?

Thanks. Maseno

Robin October 17, 2003 09:51

Re: Particles in a stirred tank
 
Hi Maseno,

I think you are misunderstanding the meaning of Eularian-Eularian multiphase. In an inhomogenous multiphase simulation, the dispersed phases are particles with an independant momentum field. Particle interaction with the continuous phase is acheived through inter-phase momentum transfer models (which there are variety to choose from). The coupled momentum equations of all phases are then solved simultanously.

The Solver Modelling: Multiphase Flow Modelling section of the documentation explains these models in detail.

Regards, Robin

Maseno October 17, 2003 11:47

Re: Particles in a stirred tank
 
Hi Robin, Thanks for your response. I am not an expert in multiphase modelling, however, I do not think that i do not understand those basics. What I do not understand and what I am asking someone to help understand is the application of these basic concepts to CFX5.6 modelling.

This is my Eulerian-Eulerian formulation for each phase (what you called "independant momentum field"):

Continuity eq. d[FiDi]/dt + d[FiDiUi]/dxi =0

Momentum eq. d[FiDiUi]/dt+ d[FiDiUiUi]/dxi= -Fi.dP/dx -diRi/dxi+IM +So

where Fi= volume fraction of phase i, Ri =Stress

Di= density of phase i, P =pressure

Ui= velocity of phase i

So= other sources

IM= Interphase momentum transfer

Do you agree with my SIMPLIFIED formulation? If you do, you will see that there is a volume fraction (Fi) in my formulation and I need to specify this subject to the constraint that Sum[Yi]=1, every where in my domain. I, therefore, need to give initial values subject to this constraint.

Simple question, how do i specify the initial volume fractions [especially, FOR THE {SOLID} PARTICLE phase] in CFX5.6?

We are all learners in this world, I stand to be corrected. If there is something i am doing wrong, I am willing to learn with great pleasure.

Thank you again Robin

Regards Maseno

Robin October 18, 2003 10:39

Re: Particles in a stirred tank
 
Hi Maseno,

OK, I see. I didn't mean to offend you with my previous post.

The initial guess for volume fraction can be specified in the initial guess form (globally) or on the initialization tab for each domain (per domain). In addition, volume fraction can be specified in inlet and opening boundary conditions.

If you do not want a constant volume fraction, simply specify an expression for the initial spatial variation.

Regards, Robin

Maseno October 19, 2003 11:57

Re: Particles in a stirred tank
 
Hi Robin, Thanks for your response. No, I am not offended.

We had agreed that if I do not want Lagrangian, I should not go the Particle Tracking route. Therefore, i must go the "FLUID LIST ROUTE" (or which route?)

This is what I do: Create domain---> General option----> Domain Type--->Fluid domain---> Fluid list (particles are not in the fluid list????) so.....??????

How then do you specify their volume fraction in the Global initialization etc?? HOW DO YOU INCLUDE THE PARTICLES IN THE EULERIAN DOMAIN, in the first place??

I guess I am not supposed to select the domain type as solid.

regads Maseno



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