[Siba11]

Hello,
I am simulating a flow having two phases the lower(primary) phase being water and the upper(secondary) phase being air. There is an orifice at the bottom of the domain and is very small in comparison to the length of the side. I want to create a vacuum-like condition(low pressure) in the upper phase and maintain it throughout the simulation. Can someone please help me out with it?
Thanks.

[sbaffini]

The specific way to do this will tipically vary from software to software, but it is simply doable in general.

It is a two phase flow and you can initialize your phases just as you want. For the air this means that you can assign pressure and temperature, which will give a near vacuum condition for very low pressures.

Some caveats:

1) If this condition will hold during the simulation or not is not something you typically enforce, especially for enclosures (maybe with some bc at the orifice, in case air reaches it)

2) Pure vacuum is not possible and even very strong vacuum has no physical meaning in continuum mechanics. Also, in reality, I expect the water to evaporate and fill that vacuum.

3) Not all codes are capable to actually work near vacuum conditions. Some have difficulties on preserving positivity.

[Siba11]

Thank you for the information but it will be invaluable if you could be more specific. I am using Ansys 2019v3 for the simulation.
My goal is to capture the pattern of flow of gas bubbles through the continuous lower phase(water) to the upper phase (air) which has been patched at low pressure than the atmosphere. The problem is I am required to maintain the low-pressure condition in the upper phase throughout the simulation but am not able to do so. I am attaching a screenshot for your reference.

Thanks in advance.

[sbaffini]

I don't have Ansys so I can't be more specific in your case.

In order to have some help, you should at least mention all the settings and models you are using. And probably the Fluent forum would be a better place for this.

Still, why you expect the low pressure to remain so in your case and are you sure there is no mechanism in your simulation (say, as part of a model) thay will somehow counteract that?

[q__]

Originally Posted by Siba11

Thank you for the information but it will be invaluable if you could be more specific. I am using Ansys 2019v3 for the simulation.
My goal is to capture the pattern of flow of gas bubbles through the continuous lower phase(water) to the upper phase (air) which has been patched at low pressure than the atmosphere. The problem is I am required to maintain the low-pressure condition in the upper phase throughout the simulation but am not able to do so. I am attaching a screenshot for your reference.

Thanks in advance.

Hi,

In this case you will not be able physically to maintain prescribed pressure in a closed domain. You need to make an opening and apply appropriate BC.

BR

[Siba11]

Yes, I have taken an open domain and assigned a gauge pressure of -3000Pa at the top boundary(pressure outlet). Also while patching the above region I patch the upper region with -3000Pa but as soon as run the simulation the water seems to vaporize and create more vacuum at the interface(more than 3000Pa in magnitude). Can you please advise me if the procedure I have used can be refined more?
Thanks in advance.

[q__]

Originally Posted by Siba11

Yes, I have taken an open domain and assigned a gauge pressure of -3000Pa at the top boundary(pressure outlet). Also while patching the above region I patch the upper region with -3000Pa but as soon as run the simulation the water seems to vaporize and create more vacuum at the interface(more than 3000Pa in magnitude). Can you please advise me if the procedure I have used can be refined more?
Thanks in advance.

First of all, your domain must be large enough, so the Knudsen number is small. Second of all you are modelling two phase flow with evaporation, so I assume you set up correct models and provided correct properties like e.g. surface tension, latent heat of evaporation etc. Finally, did you set the operating pressure?


PS

One more thing, if you really want to obtain "pattern of bubbles" the substance must boil, in case of water the gauge pressure should be close to -980 mbar at 25 degrees Celsius.

[Siba11]

Thanks for the reply,
I am facing problem in setting the operating pressure. Using Facesplit in Designmodeler I had separated the domain into two parts and meshed them separately, the upper part is where the air is patched later on. Now in fluent when I am trying to set the operating pressure of the upper part to 6000Pa the whole domain is assigned operating pressure of 6000 Pa and not just the upper part. Can you please let me know how to proceed?

[arjun]

While I can not be help on Ansys front but since I am currently working on 2 phase model that should handle this type of thing I am curious as to what is the equation of state you are using. ( I am working on Noble Abel Equation of state).

[q__]

Originally Posted by Siba11

Thanks for the reply,
I am facing problem in setting the operating pressure. Using Facesplit in Designmodeler I had separated the domain into two parts and meshed them separately, the upper part is where the air is patched later on. Now in fluent when I am trying to set the operating pressure of the upper part to 6000Pa the whole domain is assigned operating pressure of 6000 Pa and not just the upper part. Can you please let me know how to proceed?

You don't need to separate your geometry. In Fluent you can define a region which is to be patched with a gaseous phase. Operating pressure you set in Fluent, let it be normal pressure. Then you apply negative gauge pressure at the upper boundary. It needs to be less than saturation pressure of water.