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Subject: [CFX] Gas dissolution from liquid
Author: Roger Young
Date: 2008-04-22 02:49:02Hello all,
I am trying to model dissolution of CO2 from a dissolved liquid phase
to a mixed vapour-liquid phase as the confining pressure is reduced,
and/or the temperature is increased. What I am looking for is for
pointers on how to set up this problem. My main difficulty is that
I am unable to get the gas to exsolve in any decent quantity.
The background to the model is a geophysical one: hot fluid containing
dissolved gases (among them CO2) enters a lake through a vent at depth.
The fluid rises and there is an inequality something like
PTOT > PSAT(T) + PCO2
where PTOT is the confining pressure at depth, PSAT is the saturation
pressure for a given temperature, and PCO2 = K.XCO2 is the partial
pressure of (dissolved) CO2, related to the molar concentration XCO2
through Henry's constant K. When equality is attained, the CO2
comes out of the liquid phase, and there is a two-phase mixture of (mainly)
gaseous CO2 and liquid water (gas bubbles in the lake).
That is the background, but I have not succeeded in getting the model
to run satisfactorily using Ansys/CFX. There are many different modeling
possibilities, and I am probably choosing the wrong combination. To
begin with I have defined two mixtures, a liquid (lmix) comprising water
and aqueous CO2, and a gas (gmix) containing CO2 gas and air. There
is phase transfer between CO2(aq) and CO2(g) controlled by Henry's law.
The CO2(aq) molar concentration is about 25 mol/m3 and Henry's constant
is K=3000 Pa.m3/mol, generating to a CO2 partial pressure of about 0.75
bars. The inflow temperature is 80 deg.C corresponding to a saturation pressure
PSAT=0.5 bars. The lake is 10m deep (1 bar). I expect to see gas formation, but
I either get very low gas volume fractions (about 10^(-7)), or the simulation
simply breaks down with convergence errors.
Does anyone know of a working example for this problem, or a similar one?
Or can suggest what I need to do to get it working?
Many thanks,
Roger Young.
PS I can also mention that I have solved this problem using a geothermal
simulator (TOUGH2) assuming porous media flow with large permeabilities
and porosities. The only parameters required are Henry's constant together
with the saturation law PSAT(T). I might have thought that this would be
sufficient for CFX too, but it seems that I need other parameters as well
(eg an interface length scale (1 mm), and a mass transfer coefficient (2 m/s)).
This could mean that I am not modelling the right physics...?
--
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Dr. Roger Young
Industrial Research Ltd Telephone: 64-4-9313-247
Applied Mathematics Fax: 64-4-9313-003
P O Box 31-310, Lower Hutt Internet: R.Young_at_irl.cri.nz
New Zealand.
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