Learning Forum

[Nitin_Bansal]

I am trying to simulate the outflow of liquid nitrogen from a pressurized container, Using 2-d axisymmetry geometry and initially container is at 2 atm pressure and heat is provided through the wall of container so that evaporation of liquid nitrogen can take place. now I want mass outflow rate of liquid nitrogen and also want to see that how it comes out from the container. To see this I create a air domain of 1atm pressure at outside of liquid nitrogen container. I am using VOF with realizable k-e turbulent model and my fluent shows floating point exception in the beginning itself. i am attaching some images here related to my problem. I want suggestions regarding this.

thanks

[Nitin_Bansal]

In this all pressure of air domain as mentioned above suddenly reaches to infinite and inside the container shows negative pressuren

[YasserSelima]

No need for air domain nor VOF... make the boundary at the end of the pipe as outlet pressuren

[Nitin_Bansal]

thanks for your responsenwithout air domain how I can see the condition after my pressure outlet boundary. Instead of VOF what would you suggest because I am doing multiphase modeling so I have to choose one of the multiphase models.n

[YasserSelima]

yes, you are right ... if you want to study the jet of N2 after it leaves, you have to use multiphase model. VOF should work .. but you need to set all the boundaries surrounding air as outlet pressure to keep the air pressure at 1 bar.n

[Rob]

Do you include the N2 flashing to gas in your model? n

[Nitin_Bansal]

Thanks for the replynwhat do you mean by N2 flashing?n As I understand flashing means sudden evaporation and fumes formation due to temperature and pressure difference. I don't wish to consider that. my sole purpose in drawing the external air domain is to see that how liquid nitrogen comes out from the container and what happens to it while coming in contact with the external air domain.nI have observed one thing that while I am giving outflow boundary condition instead of pressure outlet, my simulation doesn't show floating-point exception instantly but results given by this change are also not acceptable. I need your valuable suggestions. If you want some more details, I am happy to share them with you.nn

[Rob]

Don't use outflow condition, it's old and shouldn't be in the code any more. nOK, so you patch the liquid phase at 2bar and the compressed N2 zone at 2 bar, air at ambient? What is the mesh resolution like and what time step did you use?n

[Nitin_Bansal]

yes...exactlynMy mesh resolution is 5 and initially, the time step size was 1*e-3, but as the error show, I reduce the time step to 1*e-5. Still, divergence occurs.nthat's why I want to know that I have made mistake in inputs or I am with the wrong model for my problem.n

[DrAmine]

Use VOF but avoid outflow condition. Are you expecting evaporation in the pipe? Or cavitation of n2 liquid? Please be careful with backflow quantities at the outlet especially the volume fraction.n

[Nitin_Bansal]

nI am expecting that liquid comes out from the pipe and after that, it will evaporate in the air domain. According to you what condition I should give at the outlet so that backflow can be accounted for. Actually, the pressure outlet is not working for me, that's why I am going with the outflow condition.nAs you can see my system, any other suggestions you want to give regarding schemes, models, and operating conditions. I am taking N2 vapor as compressible ideal gas because it is pressurized inside the container.n

[Aitor]

If you know ambient pressure, pressure-outlet is definitely the best (more natural) option. nWhy are you using power law for the spatial discretization? Are you considering steady or transient simulations? Moreover, could you show a picture of the pipe mesh?n

[DrAmine]

So your N2 liquid will flash in the ambient air plenum and evaporate to N2 vapor? So you basically require a phase model to handle that.n

[Nitin_Bansal]

nWith second order upwind my solution is diversing that's why power law. i am considering transient simulation.nn

[Nitin_Bansal]

nI am avoiding flashing here that's why I am using multiphase VOF.n

[YasserSelima]

VOF by itself, has no phase change ... the liquid will remain a liquid. You need to set a mass transfer mechanism from Liquid N2 to Gas N2 ... flashing, seems to be the best mechanism describing the physics here.n

[Nitin_Bansal]

nI am using the lee model for mass interaction between liquid and gas by mechanism evaporation and condensation. By which model we can account flashing?n

[YasserSelima]

I think cavitation model would be better as you have sudden pressure changen

[Rob]

Flashing is complicated. It's not so much the model as all we need is a mass transfer mechanism as the speed it happens at. n

[YasserSelima]

so, you think it is better to use Lee model and tweak the evaporation coefficient to 0.5 or so?n

[DrAmine]

You can use cavitation model here but you need to tune the coefficients. Same with Lee if thermal effects are taken into account. From my experience a custom mass transfer law will be required here.n

[Rob]

Probably a smaller time scale that that:flashing is a fairly explosive process! n

[DrAmine]

We ran successfully in the past hydraulic flip with three phases: vapor, its liquid and non-condensable gas. Be as conservative as possible in these runs!n

[YasserSelima]

nDo you have experimental data for validation? If yes, this would be an interesting project.nTry using both mass transfer mechanisms suggested and tweak the parameters to validate the model.n