[visitor]

I am trying to figure out an expression to monitor the mole fraction of liquid and vapour methanol at a boundary.

May need two expressions, one for liquid and one for vapour.

Any suggestions? Thanks in advance. Using ANSYS academic software.

I have tried this, it didn't work "Bad expression value 'MethanolM detected in parameter Expression Value' in object ....

molf(MethanolM)@out

MethanolM is the liquid which is being boiled, at the outlet there should be some methanol liquid and some vapour.

[Opaque]

What is the name of the material used in the software, i.e. MATERIAL: <name>? MetanolM or something else?

[visitor]

I have named it as MethanolM. It's just Methanol. Trying to find out mole fractions of vapour and liquid methanol.

Also the liquid methanol is going to boil, I am looking for TASCfliw RGP. Or any other method which can simulate methanol liquid to vapour phase change.

Thanks.

[visitor]

This is another issue I am dealing with. Methanol material properties. I am looking for Zero Pressure Coefficients for the Fourth Order Polynomial. As an alternative to NASA inputs.

Any tips, links? Please.

[Opaque]

There are several ways to access Molar Fraction

1 - Create a monitor point at a position of interest, and select MethanolM.Molar Fraction

2 - Create a monitor point expression, and set the expression to ave(MethanolM.Molar Fraction)@BoundaryOfInterest, or
ave(MethanolM.Molar Fraction)@VolumeOfInterest

Hope the above helps,

[visitor]

Thanks, Opaque 👍.

In the attached data sheet; I am trying to figure which are the Upper interval coefficients, and Lower interval coefficients. For methanol, any tips which are the Upper a1 ....a7, lower a1 ....a7, from the attached data sheet?

Again thanks in advance.

[visitor]

I have entered the following expression;

ave(MethanolM.Molar Fraction)@BoundaryOfInterest

Got the following message in the monitor tab, while trying to link this expression.
"
The variable 'Molar Fraction' referenced by parameter 'Expression Value' in object'/Flow:Flow Analysis ....is not allowed to have a phase prefix.
"

It sounds like i need to go back to the materials property tab. Find a suitable materials property for methanol, which can be boiled and evaporated.

[Gert-Jan]

you at least should use:

areaAve(MethanolM.Molar Fraction)@BoundaryOfInterest

or

massFlowAve(MethanolM.Molar Fraction)@BoundaryOfInterest

or

volumeAve(MethanolM.Molar Fraction)@VolumeOfInterest

[visitor]

Thanks Gert.

Will try it out.

It's interesting that when I try the conventional way by selecting the coordinates point (Output Control>monitor), could not see Molar Fraction. For now i have selected MethanolM.Volume Fraction.

Still don't know what to enter for material properties. Need something to take care of phase change, boiling. I have selected General Material for methanol and: -
Molar mass 32.94 [kg kmol^-1]
Density 791 [kg m^-3]
Specific heat capacity 2500 [J kg^-1 K^-1]

Q1:: Reference state ??
Default settings are for Specified Point ;

Q2:: Ref. Temperature 25 C

Q3:: Reference Specific Enthalpy >Ref. Spec. Enthalpy 0 J/kg

Q4:: Reference Specific Entropy>Ref. Spec. Entropy 0 J/kg/K

Buoyancy Properties
Q5:: Value>Thermal Expansivity 11.9e-4 [K^-1]

Basically methanol named MethanolM is present in water, and set to boil. Water selected IAPWS Library, this takes care if water phase change. Not to sure with methanol material properties setup.

I have this link with materials property on methanol. What can be extracted from the link below to fill Q1 to Q5?
Deleted link***

[visitor]

Don't know if this suffices for a fourth order polynomial for Methanol, and zero pressure coefficients. If someone can tell me this is correct, I can progress with it and share it with others.

Please see attached pdf.

[Gert-Jan]

If you go to Materials you need to go to Import Library Data to import Methanol. Go to Gas Phase Combustion and select CH3OHvl. This is a vapour Liquid pair for methanol. When using these, you can include evaporation since the liquid and gas are linked automatically.
You need the same for water: H2Ovl

Be carefull, the CFD does not use any activity coefficients, azeotrope or whatsoever. If you need to include that as well, you indeed need your own materials. But then I'm out.

Regarding the monitor points, if you have your mouse on the monitoring point definition in Pre, use your RMB and select: "Edit in command editor". There you can change the Volume fraction in Mole fraction using your keyboard. Hope it works.

[visitor]

Thanks Gert. Will try out your suggestions.

Just for sharing, i did get vapour with default methanol material properties. See attached screenshot for inputs. It's just that I am not sure if the "Reference State" inputs are correct. With Ref. Temperature 25 C, and Reference Pressure 1 atm. For Reference Specific Enthalpy and Reference Specific Entropy, ?? I don't and don't if needed for vaporising.

Thanks

[Opaque]

Originally Posted by visitor

I have entered the following expression;

ave(MethanolM.Molar Fraction)@BoundaryOfInterest

Got the following message in the monitor tab, while trying to link this expression.
"
The variable 'Molar Fraction' referenced by parameter 'Expression Value' in object'/Flow:Flow Analysis ....is not allowed to have a phase prefix.
"

I think you have not described your setup accurately here. A phase and a material are different concepts in ANSYS CFX.

Do you have an entry such MATERIAL: MethanolM anywhere? Based on the message you do not. You cannot compute the Molar Fraction of a mixture, but you can compute the Molar Fraction of a component in the mixture.

Q: which materials is your mixture made of?

[visitor]

The liquid mixture is 0.9 water and 0.1 methanol in volume fractions. I do get vapour which is then condensed. Specific enthalpy was set at -727e3 J/kg at reference temperature 25 C.

Now with gas phase combustion materials, CH3OHvl, I got this error message in Fluid Specific Models;
"When using a homogeneous binary mixture one equilibrium constraint and one equilibrium fraction must be specified".
Question?
Which is which for CH3OH & CH3OHl? Equilibrium constraint/fraction?

Similar for water, H2O & H2Ol?

The other error message
"
Equilibrium phase change in one phase in a multiphase run is not supported by the solver for this combination of thermodynamic states.
"

Thanks

[Gert-Jan]

Either gas (CH3OHv) or liquid (CH3OHl) is calculated. The other is the constraint: 1-calculated fraction. You pick one, I would pick the one from which you have the most.


The second message says you cannot have CH3OHvl combination simultaneaously with H2Ovl. It is one or the other. This limits the options you have when modeling the combined evaporation of water and methanol simultaneously.
If you really need it, you need to go back to your initial approach. Or combine it somehow. Contact ANSYS.

[visitor]

Thanks Gert & Opaque. I can include you in the aknowledgement section of paper, when I get round to writing up. If you accept, please give me your email, thanks.

I am just interested in methanol, mole fraction at the condenser outlet. Not interested in water. I assume if I ignore this message, everything will be fine, will try it out.
"
Equilibrium phase change in one phase in a multiphase run is not supported by the solver for this combination of thermodynamic states.
"

Thanks

[visitor]

The equilibrium/constraint issue resolved. Though I am not too sure, if for example the liquid phase is constrained. This should show once validation against experimental data is done. The liquid phase, will change in fractions, as boiling happen.

Regarding below, I have suppressed message and run processing. Output for water and methanol volume fractions are similar. Looking for a solution for this, obviously I will need equilibrium phase change in the two phases, water and methanol.
"
Equilibrium phase change in one phase in a multiphase run is not supported by the solver for this combination of thermodynamic states.
"
Thanks.

[visitor]

After suppressing the warning message
"
Equilibrium phase change ....
"

I got a result, methanol is twice (about 2.4) the amount of water in volume fraction at the condenser outlet.

Now I am trying to measure the liquid vapour equilibrium, as a check. Liquid mole evaporation must equal vapour mole in equilibrium condition.

Measured mole concentration CH3OH gave 36.38 mol/m3, and CH3OH l gave 24540.6 mol/m3. It doesn't make sense.

[visitor]

Thanks for all participants.

I think we have covered a few interesting points. It will be good if more database is entered. Unless I am wrong.

[Gert-Jan]

Take 1 m3 of methanol with a density of 792 kg/m3 and a molar weight of 32 g/mol makes 24750 mol/m3. Looks correct to me.

However, I don't know:
- what your intention is;
- what question you exactly have;
- how you determined these numbers;
- in which phase they are present (if any);
- how your geometry looks like,

In other words: don't expect us to be the Wizard of CFX to help you without any background/knowlegde/pictures/etc.