## Fluids

#### Erroneous O2 mass transfer from the air into the water

• Biborka Boga
Subscriber

Hi!

I am working on the simulation of O2 mass transfer from the air into the water.I have used the Mixture multiphase model.

Based on my calculations (Henry's law), the equilibrium concentration of O2 in H2O (at 25 C) would be 2.55*10^(-4) M.
Taking into account this value, I have defined 1.62e-10 as initial mole fraction for O2 in H2O (which corresponds to 9e-9 M O2 in H2O, please find below)

However, when I am visualizing the plot of O2 concentration in H2O along the channel, the concentration of O2 is above the equilibrium also at the inlet and at the outlet (please find below).

Can you please offer me some hints why this happens? How can I obtain  a correct profile?

The settings were for O2 mass transfer:

The liquid-side mass transfer coefficient used was calculated based on  a certain model from the literature.

I have also performed the simulation specifying a constant value fro molar fraction as Henry coefficient option, or ther Van't Hoff correlation, however unfortunately the same result has obtained.

• Rob
Ansys Employee

What temperature are you running at? Turn off boundary diffusion too. Why constant rate over 2-resistance?

• Biborka Boga
Subscriber

First of all, thanks for your questions and hints.

The temperature is set to 25 C.

Can you please let me offer some further explanations,what do you mean by 'Turn off boundary diffusion too'? Thank you very much in advance.

Until now  Species Model --> Options --> Full Multicomponent diffusion has been enabled, but right now I have disabled. After disabling the Full Multicomponent diffusion, the same concentration profile (with the same values) has been obtained for O2 as previously.

As a first approach I have thought to consider constant liquid-side mass transfer coefficient (calculated based on emprical rel.) and zero resistance for the gas-phase, and only after this to modify to be calculated considering a certain correlation.
Regarding to the Henry coefficient options, I have also performed the simulation considering the Van't Hoff correlation, but unfortunately the same result has been obtained as the first case presented (when Cinitial O2 bulk=4e-4 M, which is higher than the equilibrium concentration)

Can you pelase offer some further himts how can I solve this problem? Thank you very much in  advance!

• Rob
Ansys Employee

/define/models/species/inlet-diffusion?    Default is yes, if the flow rate is very low diffusion sources can override what's going on in the domain if you're not careful with the settings.

The theory is covered here  https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_th/InterfaceSpeciesMassTransfer-3A72B96D.html   What are the operating and domain pressures?

• Biborka Boga
Subscriber

Thank you very much your response and the explanation regarding to the inlet diffusion (theory).

I have checked whether this option was enabled or not by  typing in the console: ''define/models/species/inlet-diffusion?'' And the ''response'' to this was Include diffusion at inlets [no], so it seems that it was disabled. Unfortunately I have obtained the same concentration profile of O2.
The operating pressure 1 atm (Fluid Cell Zone).

Please find also below the absolute pressure of the mixture.

Can you please offer me some other hints whIch could be other potential  cause of this erroneous O2 conc profile? Thank you very much in advance!

• Rob
Ansys Employee

Can you plot velocity, temperature & phase contours?

• Biborka Boga
Subscriber

• Rob
Ansys Employee

That looks OK. Can you plot the calculated O2 concentration with the saturation values?

• Biborka Boga
Subscriber

First, thank you for your feedback regarding to the other plot.

I am really sorry but I am not sure if I have understood exactly your hint.

I have constructed the plot of O2 conc in H2O along the reactor length  (pls find above, the second figure), however the problem is that as far as I know the saturation concentration of O2 in H2O at 25C would be 2.55e-4 M, but unfortunately I have obtained higher conc values based on the simulation.

• Rob
Ansys Employee

Concentration will be higher than inlet & outlet as the oxygen is coming from the gas phase. Looking at the models in Fluent, I’ll have to see where Psat is defined.

You did set the gas as the To Phase didn't you? 23.2.10  https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_sec_multiphase_setup.html%23flu_ug_sec_mphase_massxfr

• Biborka Boga
Subscriber

Thank you very much your hint and the link to access the website.

Although I have specified in  a correct manner the To and From Phases within the ''Species Mass Transfer Model'' window  (To phase: gas-side mass transfer coefficient--> infinit, From Phase: liquid side coeffIcient) as I have shared previously the third figure, one step before I have inversed the two phases.

Thank you very-very much for reminding me to this from to phases issue! :)

Right now it looks great: Please find below.

Thank you again for all!