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Old   June 30, 2008, 00:19
Default Component Pairs
  #1
Karne de Boer
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Hi Everyone

I am in the process of developing a model (using CFX 11.0) for a mulitphase reaction mixture (two immiscible liquids). I am quite new to CFD modelling and am trying to understand what each of the settings relates to on the Fluid Pairs Tab.

So far I have been experimenting with the mass transfer options, however, I do not know where to start when it comes to the component pairs. I have read through the documentation, however, this provided little help in defining what each of the settings physically represents.

Can anyone direct me to similar examples, papers or further documentation that define the effects/relationship between settings like: - Two resistance - Interfacial Equilibrium model - Sherwood number etc...

I have trolled through the forum and gleaned some insights, but some specific advice would be greatly appreciated.

Have a good day Karne

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Old   June 30, 2008, 15:28
Default Re: Component Pairs
  #2
Sally
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Karne, these are all variations of the same thing. It's just how you want to define the mass transfer between phases. Tow resistance requires gas and liquid side resistances, IEM requires a rate constant and Sherwood calculates a rate constant.

I've thought about using these built-in models on several occasions, but I tend to shy away from them because they involve calculations that are not transparent.

I tend to define my own mass transfer equations using CEL (Rate =kla*(C'-C))and then insert Rate into the reaction sub-domain. It's much easier to keep track of what's going on in this way.

Hope that helps, Sally.
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Old   June 30, 2008, 19:32
Default Re: Component Pairs
  #3
Karne de Boer
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Thank you very much Sally

You hit the nail on the head as my biggest problem with these built in models is that they are not transparent and I am not sure what they are modifying. Forgive me for my ignorance but could you possibly explain what each term in your mass transfer expression refers to. Below is my best effort:

kla = Interfacial area?

C' = Concentration (mass or molar?) in one phase

C = Concentration (mass or molar?) in another phase

Do you by chance have a simple example of this (i.e.: a def file or something similar) so I can better understand how this is implemented.

I appreciate your insights

Have a great day

Karne

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Old   July 1, 2008, 13:53
Default Re: Component Pairs
  #4
Sally
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Graag dedan Karne,

I think you'll need to consult your chemical engineering handbook or library for a full explanation of how to model mass transfer rates. Check out the following link, which presents the equation I typed above in the proper format.

http://www.chemeng.queensu.ca/people...lications/Dave%20Final.pdf

Basically ...

kla (-/s) = kl (m/s) * a (-/m) = Liquid side mass transfer co-efficient (m/s) * Specific surface interfacial surface area (m2/m3)

C' = Equilibrium concentration (whatever units you wish, typically (mol/m3))

C = Instantaneous concentration (whatever units you wish, typically (mol/m3))

so

R (mol/ m3.s) = kla * (C'-C)

Regards, Sally.
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Old   July 1, 2008, 20:48
Default Re: Component Pairs
  #5
Karne de Boer
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Thank you very much Sally

I read through that paper and it provides me with a good starting point for a basic model. I have been hitting the books pretty hard and am starting to get a better grasp on this subject. I have also been in contact with the technical help at ANSYS and they given me further advice. It may be a while until I get the model working, however, once I make some progress (hopefully ) I'll post it so that others can hopefully use it.

Have a great day Karne

P.S. Don't let my name fool you, my dutch is not that good as I'm a third generation Australian.

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