Evaporation and Condensation Model

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diva posted this 05 November 2019

Hello,

I am trying to model a two-phase simulation with evaporation and condensation mass transfer process, To start the simulation I am trying to model a simple 2-d model. The geometry is like a bottle with a pressure outlet on the top, filled half with liquid and vapour phase of water. Heat flux is provided through the bottom of the bottle. The geometry is attached below.

 

I am running a transient simulation. My solution is diverging, but if I put the wall boundary condition instead of the outlet then my simulation works fine. So, according to my understanding, the pressure outlet is causing the divergence in the problem. I am unable to understand that when I specify my pressure outlet open to atmospheric pressure (0 Gauge pressure) it should work fine in principle but why it is unable to find the solution. Is there a theoretical mistake in describing the problem?

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abenhadj posted this 05 November 2019

Perhaps wrong backflow quantities and reversal flow is occurring. If you have an opening at the top then either via pressure boundary or a simplification could be a free slip wall. Please check the input you provide at the outlet.

Best regards,

Amine

diva posted this 05 November 2019

Thank You for your suggestion, There is only one input I can give for my pressure outlet which is the pressure value. I tried changing the pressure value as well but all in vain. I understand that it is happening because of the backflow quantities but I don't understand what boundary condition will work for this case if I don't want it to be a wall but an outlet instead open to the atmosphere.

 

Thanks

Regards

Divya

abenhadj posted this 05 November 2019

What about the back flow quantities for the secondary phase?

Best regards,

Amine

diva posted this 05 November 2019

I was trying to set the backflow quantities for my secondary phase, but it is only giving me an option to set the inputs for the phase "mixture".

 

Thanks

Regards

Divya

abenhadj posted this 05 November 2019

whenever you click on the boundary in the tree and expand it you can then provide particular conditions for phases.

 

I recommend that you go through some tutorials before starting with a high end application.

 

 

Best regards,

Amine

diva posted this 06 November 2019

Thank You for your response. My backflow quantities are as follows. I have set my backflow temperature equal to the saturation temperature of my fluid and backflow volume fraction as 1 for my vapour phase. I am using Eulerian multi-fluid VOF Model by Implicit formulation.

I am still getting the divergence.

With the following error. I don't understand what might be going wrong.

abenhadj posted this 06 November 2019

Try with superheated vapor phase. Also be conservative regarding backflow quantities for turbulence. Use zero operating density. You might also try using mixture model with interfacial area gradient and Lee midel it should be more stable. If Euler Euler use again interfacial area gradient.

Best regards,

Amine

diva posted this 5 weeks ago

Thank You for your response. I am using 0 as my operating density. The reason why I cannot model my system as a mixture model is that I want to use the Thermal Phase change model for evaporation and condensation which is only possible with the Eulerian model. I don't want to fix the rate of mass transfer between the two phases which is the case in Lee Model instead I want Fluent to calculate it based on my heat flux given to the system. Can you please elaborate what do you mean by being conservative with my turbulence quantities? 

Also, do you mean enabling Interfacial Anti-diffusion option for using interfacial area gradient?

I can use super-heated vapour phase with the wet steam model but that is possible when I am using density-based solver instead of pressure-based solver?

 

Thanks

Regards

Divya

abenhadj posted this 5 weeks ago

Increase temperature of vapor at outlet. Give it a try. I was not referring to wet steam.

Best regards,

Amine

diva posted this 4 weeks ago

Unfortunately, it is not working either. I tried increasing the backflow vapour temperature but my solution is still diverging.

abenhadj posted this 4 weeks ago

Sorry but I cannot help more here as I cannot look into your files. You might try ramp heat flux step by step.

Best regards,

Amine

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