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urgent query regarding vof model plz rply

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Old   July 13, 2007, 02:20
Default urgent query regarding vof model plz rply
Garima Chaudhary
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I am facing some problems regarding the use of vof model for my project. First let me give you a brief idea about the project. Its on centrifugal contactor. The system consists of two concentric cylinders. The outer cylinder is stationary and the inner one is rotating. The feed is given to the system through the annular section from top and it travels down through the annular zone under the influence of gravity and centrifugal force. Further it travels through the gap between rotor and the outer cylinder under influence of centrifugal force. the outer cylinder under influence of centrifugal force. The inner rotor cylinder has an opening at the bottom to provide the fluid inlet into the rotor zone. When the fluid reaches the rotor zone it travels up into it under centrifugal force and gravity. The feed in the annular section consists of aqueous plus organic material which forms a dispersion. This dispersion travels into the rotor where separation occurs into lighter and heavier phases.

To ease the study of the system, we are doing single phase simulation separately for the rotor and the annular section. My work deals with rotor section.

(GEOMETRY 1) The geometry of the rotor which I am using is as follows. It consists of a cylinder of 8 cm ht. and 2 cm radius. It has an opening of .4 cm at the centre bottom for inlet to it . At the top outlet is there which is completely open to atmosphere. My materials consists of water and air.

(GEOMETRY 2) Simultaneously I am doing the same work on a differentgeometry which also resembles for the same conditions . It was employed to ease the system values at different boundaries. It consists of a cylinder of ht. 8 cm and radius 2 cm. At the bottom for the inlet to rotor a ring has been made of thickness .2 cm At the top for the outlet a ring has been made just adjacent to the periphery , rest part at the top is covered. Hence fluid comes from the bottom ring inlet and goes out through the top ring outlet. The bottom ring inlet's inner side is situated at the distance of .9 cm from the centre.

Now I am using VOF model for both geometries. Since in my actual system There is no flooding at the inlet , as soon as the fluid enters it moves outward due to centrifugal force,hence the parabolic shape which forms in the rotor at the centre has its shape in such a way that the inlet is exposed to almost atmosphere. This way water enters and it rises up along the walls and moves out.

For the first geometry, I made inlet as pressure-inlet and the outlet as pressure outlet. since at the pressure inlet boundary conditions asksfor the total pressure and not the static pressure which I want for system close to atmosphere since no liquid height build up occurs there in our system so kept total pressure values equal to 20 pascal. at the outlet I kept static pressure equal to 0 pascal. all these are gauge values. At the inlet I specified volume fraction of water as 1 and of air as 0, since no air enters or leaves at the inlet . I choose vof model. The rotation axis is X axis and positive direction of X axis is towards outlet of cylinder. I am using axisymmetric swirl case. I have defined custom field function as 100*y for swirl velocity to give better initial guess. Also I patched the ht of cylinder till 5cm approx with water as initial level of water. I have used euler-explicit scheme for vof to have stabilization of solution. Also I have used PISO AND PRESTO schemes in solver. But the problem what I am facing is that my residuals are diverging for both geometries. I have used the same conditions for both geometries.

Then even I tried with mass-inlet boundary conditions at the inlet. There also I am facing the same problem of divergence , which I am notable to resolve.

Can u please guide me how to set up the problem or model in fluent for this case of continuous flow.

Also I am not able to get whether I should use the open channel option or not and if yes then how to incorporate that. Is it that a open channel flow can be used only when the flow is perpendicular to the direction of gravity?

Can you help me regarding, 1. setting this problem in fluent 2. Resolving the divergence issue 3. Is there any Fluent simulation for the similar case apart from the VOF example of the rotating bowl. 4. Should I use the open channel option or not

I can send the case files for the geometries and also the schematic if you want and the geometry is not clear to you.

Regards Garima
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