Learning Forum

[majam]

Hello,

I am simulating O2 dissolution in water in a mixing tank. In the simulation of mass transfer, I have some doubts about some parts that might have sth to do with the model not working properly.

1- I am having two mixtures, one is (O2, N2) as gas and the other one is (O2, water) as liquid phase. For creating the mixtures I used the same O2 that is in the gas phase in my liquid mixture. (I found out this is the right way instead of using liquid O2). So now my questions are:

-Do I have to change the density of the O2 in the liquid phase? or leave it as it is?

-Which model do I use for the density of the mixture, I am using Compresible-volume-weighted-mixing-law, is that proper model to use?

2-In mass transfer modeling I am choosing species-mass-transfer model from O2 gas to O2 in liquid, and then I am choosing Henry's law for equilibrium.

-I am not sure what to enter for Molar fraction correlation (Pa) ??

Then I choose overall mass transfer coefficient and compile a mass transfer UDF based on higbies penetration theory .

-the Udf I am using gives Kla*density since the unit is m/s, is that correct?

I would appreciate it if anyone can help me figure out if my steps are correct or what can I do differently.

Thanks,

[majam]

-the Udf I am using gives Kla*density
Kl*cell volume since the unit is m/s
is that correct?

[majam]

Can anyone help me with this?

[Rob]

If you're using the UDF approach, I assume you've read this? https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v221/en/flu_udf/flu_udf_sec_define_mass_transfer.html Given the amount of oxygen that can dissolve is fairly minimal either leave the water density as fixed or do a literature study to find the correct values for the concentrations.

[majam]

Hi

Thank you for your response, yes I have read that.

I have another question. For modeling mass transfer of O2 dissolution in water, is it better to model the mass transfer after a few seconds of O2 injection (I mean after the gas phase has already been distributed in the liquid phase) or at the same time of starting the injection of gas flow?

Thanks Maryam

[Rob]

It often makes sense to build up model complexity. So running with no mass transfer to start with means you can trouble shoot more easily. The only draw back to running without mass transfer is if the gas is completely dissolved in the system, in that case you may find the results to be very different.

[majam]

Hello
Thank you again for your response.
For some reason, my simulation does not work. I just want to check the settings to find the problem.
1-I use Define mass transfer UDF and since kL unit is 1/m2S but the unit for the setting is m/s , in udf I multiply it by cell volume, is that correct?
m_lg = ((0.4)*pow(D_O2,0.5)*pow(C_D(c,liq)*C_R(c,liq)/C_MU_EFF(c,liq), 0.25))*C_VOLUME(c,thread);

2-Do I need to use Henry's law or I should go straight to Mechanism--->user-defined?

3-also here are the settings for the species transport model
4-Do I need to activate/deactivate any of the options?

Thanks

[DrAmine]

Henry s law can be used for dissolution. Usually Higbie is used to provide the nass transfer coefficient on the liquid side but you csn first rely on any of the built-in sherwood correlations. Species panrk: do not alter anything.

[majam]

Hello In my simulation which now works, the oxygen concentration exceeds the saturation value (8mg/lit). Can you please help me figure out why this happens?
the mass transfer coefficient is based on higgbi's theory which is used many times in literature, I use henry's law and define a constant molar concentration of 74.68x 10+6 Pa/kmol/m3.
thanks