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Degassing Condition.......Please Help

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Old   February 19, 2004, 08:38
Default Degassing Condition.......Please Help
Paresh Jain
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Dear Friends,

i m trying to simulate a gas-liquid reaction in a packed bed (its actually a 3 phase reaction but i m considering it as 2 phase). Initially, reactor contatins 0.7 volume fraction of liquidphase and 0.3 volume fraction gasphase. Now through inlet, i m entering some air. O2 reacts in liquidphase and produces some gas and liquid products. and also as per physics of the problem, liquid must not leave the system. (liquidphase=A+B+H2O, gasphase=O2+N2+CO2) reaction- A + B + 0.8 O2---> 2B + 1.1 H2O + CO2 (autocatalytic, exothermic rxn)

So as per my knowledge of CFX, when i dont want my liquid to go outside the system (system of gas dispersed in liquid), i should specify degassing condition at the outlet so that only gas will leave the system and i will have liquid fraction in the system as 0.7 (i.e. same as it was initially if same amount of liquid B is produced as liquid A consumed). But i have run a few simulations but output file shows that volume fraction of liquid is reducing with time and even reaching a value close to ZERO implying that liquid is leaving the system all the time.

What i think is, as i have gas also present initially (0.3 vol fraction) then when air is entering through the inlet; gas initially present in the domain must leave the domain and the liquid should not. As i m running a transient case with initial values of liquid vol fraction 0.7 so this condition has to be satisfied at very small time (t->0). and there will be no steady state as such for the problem at hand.

Can anybody comment on this why liquid is going out of the system and how it can be prevented? I m working on this problem since long. Please suggest me some way out. i will be highly obliged to you.

waiting sincerely for your comments and suggestions.

Thanks & Regards,

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Old   February 19, 2004, 12:19
Default Re: Degassing Condition.......Please Help
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A couple of things to check here:

- is your gas phase 'dispersed' ? The degassing boundary condition is only supposed to work with dispersed gas in continuous fluid.

- are you sure you're losing the liquid phase through the outlet, and not through 'some other means' ? (for example are your source terms for your reaction producing or depleting liquid ?)

- is it converging within each timestep ? Numerical errors may be causing the effects your are seeing.

You should be able to check sources and balances in the Solver Manager as the solution progresses.


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