[cwl]

Good day everyone!

Let's assume a sample simulation of a multiphase flow of gas and multi-component liquid (liquid₁ and liquid₂).

Geometry would be simple - just a pipe with an inlet (only gas enter the region) and outlet.

And what we want - is to "inject" (simply just out of nowhere) multi-component liquid (m₁ kg/s of liquid₁ and m₂ kg/s of liquid₂) in some area of the flow volume, set by Cell Set or Threshold - whatever.

Surely, the injection of multi-component liquid should be accompanied by Energy Source, that's ok.

And so - there are things to be set in the Region physics - for multi-component liquid:
- Species Sources;
- Volume Fraction Source.

The question is - in order to "inject" the liquid:
- is it correct to set only Species Sources for liquid₁ and liquid₂?
- or is the setting of Volume Fraction Source (for multi-component liquid) is also required?
- and if the setting of Volume Fraction Source is required - is it like it should be set by somehow calculating it depending on mass (Species) sources and densities of liquid₁ and liquid₂?

It looks like added species mass is "taken into consideration" by the Volume Fraction Solver according to the very last term:



But is it that simple - I wonder ...

[profi_star-ccm+]

If the result of the calculation coincides with the practical data, then it is easy and simple, and when it does not match, then it is cumbersome and complicated.
Give up materialism, which the Universe did not do at its birth.

[bluebase]

Originally Posted by cwl

The question is - in order to "inject" the liquid:
- is it correct to set only Species Sources for liquid₁ and liquid₂?
- or is the setting of Volume Fraction Source (for multi-component liquid) is also required?
- and if the setting of Volume Fraction Source is required - is it like it should be set by somehow calculating it depending on mass (Species) sources and densities of liquid₁ and liquid₂?

Hi cwl,

i believe, by injecting only one liquid into the composition you might change up to three properties, the total mass, the volume fractions and phase densities . I'll try to explain why i think this way.

As an analogy, I'll imagine some kind of solids (not modeled) decompose and inject mass(gas) into the domain. The gas/mass added changes the total mass and therefore the user-defined phase mass source is not zero.
Now, if you are not changing the volume fractions, then one way to add gas, is by compressing it or it will have to go somewhere else...

In terms of the continuity equation, let's assume the first term rhs is zero (no phases are converted into one another). So the lhs of the continuity equation would need to adapt. The remaining variables left are the volume fractions, phase densities and the velocity through the faces of a cell.
If you do not change the volume fractions... two remain. Injecting a liquid likely means density is also constant, therefore the interfacing velocity has to change.

I guess, instantaneously adding a constant-density fluid, while keeping the fraction constant will likely mess up the solving process; since you are not running a coupled solver, extra iterations in the pressure correction will be necessary to compensate the additional mass flux... only if the solver maintains stable enough...

The saver route for injecting a fluid is in my opinion to also change the volume fraction---additionally to the added mass. So i would agree with your third suggestion:

- and if the setting of Volume Fraction Source is required - is it like it should be set by somehow calculating it depending on mass (Species) sources and densities of liquid₁ and liquid₂?

For a compressible gas, it would likely be possible to compensate the added mass by increasing its density (and consequently local pressure).