Hi,
Thank you for your help.
Just to remind you, I am a PhD student.
I am working on a Solar Still, water evaporation-condensation.
1- Water in the tank evaporate due to solar radiation.
2- Water vapour condense on the cover due to the temperature difference
3- Water condensation gets collected into a discharge channel (the system productivity)
I need to know the condensation rate (mass flow rate)
I am using ANSYS 17.1-fluent multiphase VOF.
I got this behaviour;
It is converged just with a small time step 0.001s, which is unpractical cause it takes a long runtime!
I need to simulate 10 minutes for example, it may take weeks.
Please find attached photos.
Also, I have some precise questions;
1- Is VOF a successful tool? or mixture is a better choice?
2- Number of phases 2 or 3? (I used 3 including the air)
3- Explicit or implicit? (I used implicit)
4- Transient or steady? (I used Transient)
5- Sharp or dispersed interface? ( I used sharp)
6- Which Solar model? (I used Roseland)
7- Which viscous model? (I used k-epsilon-RNG)
8- Enable Eulerian wall film or not? ( I am not)
9- Enable surface tension or not? (I enabled with wall adhesion)
10- What is the SS of enthalpy for each phase? (I use 0 to water and 4e7 to vapour)
11- What is the primary Phase? ( I am air)
11- Solution method? (I used PISO)
13- URF values? (I used 0.2 to 0.5)
14- Standard or hybrid initialization? (I used hybrid)
15- adopt region to be patched, what properties should be defined and for which region(s)? (I defined just the volume fraction to water region as 1)
Please advise,
Many thanks,
Kind regards,
Alkaisi
The major parameters of importance after mass and heat transfer consideration are
hc=the convective heat transfer
Pw=water pressure
Pwg=water vapor pressure
see SOLAR DESALINATION, John H. Lienhard,1,¤ Mohamed A. Antar,2 Amy Bilton,1 Julian Blanco,3 & Guillermo Zaragoza4, ANNUAL REVIEW OF HEAT TRANSFER, 2012.
We used a simple approach which can be like rule of thumb:
Convective heat transfer at glass=condensation rate=m_dot . h_fg
mass flow rate can be approximated this way.
This can be modified by a correction factor to take into account other energy transfers.
The size time step strongly depends on the grid size. Perhaps your grid size is too small. In
Plus, are you sure it is a turbulent flow problem? Usually laminar flow will converge much faster than turbulent flow.
1. For multiphase flows in general, VOF is shown to be a very good method, so I would go with a VOF.
2. Since there are water, vapour and air, I would use 3 phases, since each of them have different material properties.
4. Transient
5. Sharp
6. Depends what are you trying to analyze and what is the optical thickness of the materials. Here, a P-1 would be good as well. Rosseland have a tendency to be bulk, that maybe could cause a convergence problem. Try to shut off the radiation completely to see if that is causing the convergence rate to be low.
13. URF values for energy equation are usually arroung 0.8-1.0 for heat transfer problem. Low URF values can cause the convergence rate to be low.
Also, as the previous comment, the grid can cause the convergence problems as well.
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