[juni11]

Hello everyone,
I am running a 3D Transient simulation of a Bubble Column with 3-phases in CFX. Column is about 65% filled with water and 35 % air initially. Air is also being injected from the bottom of column with a third phase.
Normally I use water as Continuous Fluid, Air as Dispersed fluid and Third phase as Particle Transport Solid or Dispersed solid or Dispersed fluid too.
I have used a lot of models and their combinations but the problem i am getting everytime is that third phase always stays in the water. It never comes out of the water surface. Why is it so???
Few pics of volume fractions of all three phases at about 3 seconds are attached below

Thanks in advance for your help!!!

[Antanas]

Originally Posted by juni11

Hello everyone,
I am running a 3D Transient simulation of a Bubble Column with 3-phases in CFX. Column is about 65% filled with water and 35 % air initially. Air is also being injected from the bottom of column with a third phase.
Normally I use water as Continuous Fluid, Air as Dispersed fluid and Third phase as Particle Transport Solid or Dispersed solid or Dispersed fluid too.
I have used a lot of models and their combinations but the problem i am getting everytime is that third phase always stays in the water. It never comes out of the water surface. Why is it so???
Few pics of volume fractions of all three phases at about 3 seconds are attached below

Thanks in advance for your help!!!

What do you expect (in reality)?

[juni11]

Dear Antanas
I am expecting that there must be some fraction of third phase (let say dust or some gas) which must be thrown out of water.
Let me explain a little bit further; I am trying to study the removal of third phase from Air. Air, as i said earlier, is inlet from the bottom of Bubble Column along with third phase (dust or some gas etc). I have to calculate how much "Third Phase" is removed from the air and is mixed with water.
Obviously there MUST be some fraction of "Third Phase" which is not removed from air and hence MUST be thrown out of water surface.
Thanks again

[ghorrocks]

If the "third phase" is in the air, what form is it in? solid, liquid or gas? particles, droplets or molecularly mixed?

[juni11]

Originally Posted by ghorrocks

If the "third phase" is in the air, what form is it in? solid, liquid or gas? particles, droplets or molecularly mixed?

When the third phase is DUST (TiO2-Titanium DiOxide) I'm modelling it as "Dispersed solid" or "Particle Transport Solid". When the third phase is gas like chlorine or Iodine etc; I'm modelling the third phase as "Dispersed Fluid" or "Particle Transport Fluid". I have tried both Dispersed as well as Particle Transport approach.

Particle size of DUST is 1 micron, while mean diameter for Dispersed solids is 1mm. Same is the case for Gas as Third phase (1 micron for particle trasnport fluid and 1mm for dispersed fluid).
For defining DUST; I'm creating a new material and giving its molar mass and density along with thermodynamic state. Sometimes CFX asks for viscosity and I give it equivalent to air etc.

[ghorrocks]

When the "third phase" is not actually a third phase but is a gas then you better use a more appropriate model. So for the gas ones you should use a multicomponent fluid model for these, so you have the gas phase as air/chlorine/iodine multicomponent mixture, then multiphase with the liquid phase.

[Antanas]

Originally Posted by juni11

When the third phase is DUST (TiO2-Titanium DiOxide) I'm modelling it as "Dispersed solid" or "Particle Transport Solid". When the third phase is gas like chlorine or Iodine etc; I'm modelling the third phase as "Dispersed Fluid" or "Particle Transport Fluid". I have tried both Dispersed as well as Particle Transport approach.

Particle size of DUST is 1 micron, while mean diameter for Dispersed solids is 1mm. Same is the case for Gas as Third phase (1 micron for particle trasnport fluid and 1mm for dispersed fluid).
For defining DUST; I'm creating a new material and giving its molar mass and density along with thermodynamic state. Sometimes CFX asks for viscosity and I give it equivalent to air etc.

It seems to me that if you set your stuff as separate phases (fluids) then they are separated. I mean air won't be carrier of your third phase. They will be equivalent and get into your domain separated. But may be I'm totally wrong.

[ghorrocks]

It is good to refer to the CFX documentation for definitions of these things:

Multiphase flow refers to the situation where more than one fluid is present. Each fluid may possess its own flow field, or all fluids may share a common flow field. Unlike multicomponent flow, the fluids are not mixed on a microscopic scale in multiphase flow. Rather, they are mixed on a macroscopic scale, with a discernible interface between the fluids. ANSYS CFX includes a variety of multiphase models to allow the simulation of multiple fluid streams, bubbles, droplets, solid particles, and free surface flows.

It is the level of mixing which determines the model to use. If the mixing is at the molecular level then use the multicomponent model for that pair of materials. If the mixing is at the macroscopic scale with a distinct interface between the materials then you need the multiphase model.

[juni11]

Originally Posted by ghorrocks

When the "third phase" is not actually a third phase but is a gas then you better use a more appropriate model. So for the gas ones you should use a multicomponent fluid model for these, so you have the gas phase as air/chlorine/iodine multicomponent mixture, then multiphase with the liquid phase.

Ok but what for DUST???
And secondly is there a mass transfer or species transfer phenomenon in either DUST or GAS case??? I mean DUST or third phase GAS has to move from air to water. If there is such kind of phenomenon, then what should I mention in mass transfer options??? As i dont know how much mass has to be transferred. Actually this is what I have to find that How much mass of "third phase" is transferred from Air to water and how much left with outlet air.

[juni11]

Originally Posted by Antanas

It seems to me that if you set your stuff as separate phases (fluids) then they are separated. I mean air won't be carrier of your third phase. They will be equivalent and get into your domain separated. But may be I'm totally wrong.

Even if fluids/phases are separated, some fraction of third phase must exit from water surface. But it's not happening like this as CFX results show. Why is it not exiting?

[juni11]

Originally Posted by ghorrocks

It is good to refer to the CFX documentation for definitions of these things:



It is the level of mixing which determines the model to use. If the mixing is at the molecular level then use the multicomponent model for that pair of materials. If the mixing is at the macroscopic scale with a distinct interface between the materials then you need the multiphase model.

Nopes; phases are not that thoroughly mixed. That's why I am using Multi-phase but not Multi-component. Air contains Iodine in Sublime form just like Air contains DUST.

[ghorrocks]

Ok but what for DUST???

Dust clearly is a third phase.

And secondly is there a mass transfer or species transfer phenomenon in either DUST or GAS case???

If mass/species transfer occurs then you can look at model for that.

Air contains Iodine in Sublime form just like Air contains DUST.

Is the iodine solid,liquid or gas?

[juni11]

Originally Posted by ghorrocks

Is the iodine solid,liquid or gas?

Iodine is in Gas phase.

[ghorrocks]

So for the iodine models, both air and iodine are gases so isn't that a multicomponent mixture, not multiphase (just for that fluid pair).

[arjun]

I think you are using mixture multiphase model.

In this case flow comes from relative velocities and in air for third phase relative and drift velocities are 0, thus iodine would only get collected at air water interface.

This is one possible scenario for this type of behaviour. If you are using full multiphase then I guess it could be avoided.

[juni11]

Originally Posted by arjun

I think you are using mixture multiphase model.

In this case flow comes from relative velocities and in air for third phase relative and drift velocities are 0, thus iodine would only get collected at air water interface.

This is one possible scenario for this type of behaviour. If you are using full multiphase then I guess it could be avoided.

Thanks arjun but How do you know that I am using mixture multiphase model. I normally use Water as continuous (Eulerian); air as dispersed (Eulerian) and Third phase also as dispersed (Eulerian). I am using CFX and i think there is not such option in CFX to explicitly define whether the flow is Mixture multiphase or Full multiphase. If there is some option in CFX, can you please let me know as I dont know what is mixture multiphase or Full multiphase???


Secondly what i want is that Air bubbles must be the carriers of "Third Phase" while entering Inlet. And during the upward movement of bubbles in water, some bubbles can transfer "third phase particles" to water. In this way the air EXITING from Water Surface must be carrier of atleast some "Third Phase" if not all. i.e; some third phase must get mixed with water while some third phase must exit with air from water surface while staying inside the air bubbles.

But this is not happening. All the third phase in my CFX simulation gets mixed with water and moves with water but is never carried by air Bubble. Why is it so??? and how can I modify it???
Thanks again for your kind reply

[arjun]

To be frank I dont know what exact model you are using since I have not touched cfx for at least last 8 years.

But having implemented mixture multiphase model recently and now I am going to implement full eulerian model, I did study them.

Based on this, there are only two possibilities
(1) there is a bug in cfx. (very unlikely because model is used so much someone would have already reported)./

(2) the velocities for iodine transport is 0 so it can not transport as you expected. So now the question is why it could be 0, it could be 0 in mixture multiphase (cfx call it differently i guess) when the continuous phase is absent (volume fraction is 0 and thus relative and drift velocities are 0).
The way it could happen is that drag is defined relative to continuous phase and no drag interaction defined between iodine and air)

Originally Posted by juni11

Thanks arjun but How do you know that I am using mixture multiphase model. I normally use Water as continuous (Eulerian); air as dispersed (Eulerian) and Third phase also as dispersed (Eulerian). I am using CFX and i think there is not such option in CFX to explicitly define whether the flow is Mixture multiphase or Full multiphase. If there is some option in CFX, can you please let me know as I dont know what is mixture multiphase or Full multiphase???


Secondly what i want is that Air bubbles must be the carriers of "Third Phase" while entering Inlet. And during the upward movement of bubbles in water, some bubbles can transfer "third phase particles" to water. In this way the air EXITING from Water Surface must be carrier of atleast some "Third Phase" if not all. i.e; some third phase must get mixed with water while some third phase must exit with air from water surface while staying inside the air bubbles.

But this is not happening. All the third phase in my CFX simulation gets mixed with water and moves with water but is never carried by air Bubble. Why is it so??? and how can I modify it???
Thanks again for your kind reply

[juni11]

Can you please give me a one sentence difference between Full and Mixture Multiphase models???

Secondly what does "velocities for iodine transport" mean? How can you say it is 0? How can I define drag interaction between air and iodine? In short how should I modify my Models so as to include all what you said???

Originally Posted by arjun

To be frank I dont know what exact model you are using since I have not touched cfx for at least last 8 years.

But having implemented mixture multiphase model recently and now I am going to implement full eulerian model, I did study them.

Based on this, there are only two possibilities
(1) there is a bug in cfx. (very unlikely because model is used so much someone would have already reported)./

(2) the velocities for iodine transport is 0 so it can not transport as you expected. So now the question is why it could be 0, it could be 0 in mixture multiphase (cfx call it differently i guess) when the continuous phase is absent (volume fraction is 0 and thus relative and drift velocities are 0).
The way it could happen is that drag is defined relative to continuous phase and no drag interaction defined between iodine and air)

[arjun]

in mixture model 1 momentum equation is solved for the mixture (of phases) while in full eulerian multiphase model for each phase a momentum equation is solved.

In both cases 1 pressure equation is solved and pressure is assumed to shared between the phases (same pressure gradient).

So now in mixture case how do phases move, they move related to mixture velocity. (called drift velocity, v_drift_phase = v_phase - v_mixture).

This drift velocity is calculated from relative velocity
v_relative_phase = v_phase - v_continuous_phase ).

in your case
v_relative_iodine = v_phase - v_water

But this is calculated from drift law, which involve you to specify drag between phases.

Look into use or theory guide.