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how to determine the local volume fraction?

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Old   February 27, 2008, 21:27
Default how to determine the local volume fraction?
Soo MK
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Currently I am doing a research on Oil in water system in a horizontal pipe. I was given 60% water and 40% oil for the input materials of a 9.7 m length pipe with 25.4 mm internal diameter. My objective is to see the phase distribution / dispersion of oil in the system. My problem is i cant find the "local volume fraction/ in-situ oil fraction "variable in the post processing result. The variables that i can find is just the water volume fraction and oil volume fraction variables. These variables varies from 0.6 to 0.62 and 0.4 to 0.41 respectively.

Can somebody guide me on how can i see the dispersion of oil in the system?
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Old   February 28, 2008, 02:47
Default Re: how to determine the local volume fraction?
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For obtaining the dispersion of oil, you must derive the contour plot of oil or water, in post-processing. Also for local oil volmue fraction you must create the point or line and then use of chart in post-processing of CFX.
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Old   February 28, 2008, 05:38
Default Re: how to determine the local volume fraction?
Soo MK
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I had derived the contour plot for water volume fraction using the varialbe "water volume fraction". But the problem is that the values of the water volume fraction varies from 0.6 to 0.62. But this is not the result that i want. The result that i intend to get is some sort like when you cut the horizontal pipe vertically, you will be able to see how is the distribution of oil in the pipe at 1m from the pipe inlet, 5m from the pipe inlet, and 7m from the pipe inlet. How many percentage of oil dispersed near the pipe wall, how many percentage of oil dispersed at the middle of the pipe, how many percentage of oil dispersed at the top part and how many percentage of oil dispersed at the bottom part.

Please provide me some guidelines as I am a new user of ANSYS CFX 11.0. Also, i had lack of time as i need to pass up my survey project in 2 weeks time.

THanks in advance!!!
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Old   March 2, 2008, 09:28
Default Re: how to determine the local volume fraction?
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Ok, it sounds like you are viewing the results correctly. Therefore it seems your results really do show that the volume fractions do not change much from their initial values, and that therefore little or no phase separation is taking place in your simulation.....

You have got buoyancy turned on haven't you? - You need to specify gravitational accelaration yourself - it is not automatic!!

If you have, then questions you need to ask yourself are:

- Do I really expect phase separation in this geometry, with this flow rate, and these materials? The results you have *may* be correct for the fluids and flow rate you are using! E.g:

*For instance, a very high speed flow or a very turbulent flow will probably cause the oil and water to stay mixed.

*A very slow flow, laminar flow will probably allow separation.

- If you're not sure whether large scale separation should occur at the speed you are using, then:

*Try a test problem with very low speed flow where separation definitely should occur.

*Try a test problem with high speed flow where separation should not occur.

*Check your results give the expected behaviour in each test case!

*If the test cases don't work, then check your model carefully - are you using an appropriate multiphase model?

*If the test cases do work then go back and simulate your real case.

Good luck. Andy

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Old   March 4, 2008, 21:14
Default Re: how to determine the local volume fraction?
Soo MK
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Hi Andy,

Thanks for responding to my problem.

Indeed i had turned on my buoyancy as well as specifying the gravitational acceleration correctly.

I had tried out with low velocity (where the iterations will produce "F") and high velocity (no difference from the current velocity). Indeed, both gives almost same result.

I need to view the dispersion. I just found out that I assume both the oil and water were mixed at the initial state when entering the pipe. So is this "both liquids were mixed initially when entering the pipe?" a major problem to my current result?

cheers, Soo MK
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