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Old   April 10, 2020, 04:23
Unhappy Check for mass conservation
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Hello all,

I am doing a transient 2d multiphase simulation with evaporation-condensation. There are no inlets/outlets, the initial volume of water is patched into the region. My primary phase is water and my secondary phase is mixture-template (air + water vapor). I am reporting the mass from the reports>volume-integrals>mass option.

At t=0,
Total Mass = 1.6632497117e+00 kg
Mass of water = 1.6609126049e+00 kg
Mass of mixture-template= 2.3371067885e-03 kg

At t=0.62s,
Total Mass = 1.3104626319e+00 kg
Mass of water = 1.3081499147e+00 kg
Mass of mixture-template= 2.3127171514e-03 kg

Please let me know if I'm checking it correctly, and why it is incorrect, and how can I improve my simulation.
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Old   April 10, 2020, 04:40
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If the domain is closed, you need to ensure that the density of mixture of air and water-vapor is governed by either ideal or real gas. Constant density mixture cannot exist in a closed domain.
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Old   April 10, 2020, 06:07
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Hi @vinerm

I can confirm that I am using the incompressible-ideal-gas method. Any other thoughts of what I can check?

Btw, appreciate your reply!
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Old   April 10, 2020, 06:16
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It has to be ideal gas and not incompressible ideal gas.
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Old   April 10, 2020, 11:03
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Hi @vinerm,

Thank you. That solved my issue.

Please allow me to extend my question as this continues to baffle me -

I am reporting the mass fraction data as follows:

At t=0,
Mass Fraction of Air = 0.8131
Mass Fraction of Vapour = 0
Total Mass Fraction = 0.8131

At t=1,
Mass Fraction of Air = 0.3021
Mass Fraction of Vapour = 0.2111
Total Mass Fraction = 0.5132

Since the mass is already shown to conserve, is there any reason the total mass fraction continues to decrease with time?
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Old   April 10, 2020, 11:10
Default Mass Fraction
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Total mass fraction is always conserved and equal to 1. It cannot change. Probably, you are reporting something else, may be mass.
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Old   April 10, 2020, 11:55
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Hi @vinerm,

I am reporting the mass fraction from surface report>area-weighted average>species>mass fraction of air, phase>mixture-template

My workflow is as below-

I initialized the simulation with volume fraction of 1 for mixture-template. Then, I patched a volume fraction of 0 in the region where water will be located. Hence, the initial mass fraction of 0.8131 is logical to me and I would expect the max value to be capped at 0.8131.

Please advice, thank you.
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Old   April 10, 2020, 12:03
Default Mass Fraction
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Area-Weighted Averaging is used for boundaries, not for cell zones. Use volume average or mass average for cell zones.

It does not matter whether the domain is completely filled with mixture or partially filled, mass fraction will always be 1 because of the way it is defined.
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Old   April 10, 2020, 13:41
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Hi @vinerm,

Once again, thanks for your help! I would never have caught these details myself. It's always the basics that give me such a headache.

Please just clarify my statements below for my understanding.

1. In order to report values from the interior, I have to pick volume average and the cell zones. This applies to 2D and 3D simulations. (I have been using area average and the "interior" boundary)

2. For values such as heat flux, which is to be reported from the wall "boundary", it is okay to pick area average and the respective walls.

3. What about the contour plots? Are the values defined by the 'volume' or the 'area'? Would it be accurate to portray the mass fraction with contour plots and the respective animations?

Thank you
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Old   April 10, 2020, 13:48
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1. For volumetric averages, always use volume or mass averaged.

2. For boundaries, you can always use either area or mass weighted. Mass weighted cannot be used for boundaries that have no mass crossing them, such as walls. So, for walls, always use area-weighted.

3. Contours are not averages.
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