[Luuuca94]

Hi everyone,


I am trying to simulate droplet condensation and coalescence on a flat surface. First of all I am trying to let a droplet grow which is already on the surface at t = 0 by water vapor condensation. The droplet and the surface beneath it have a temperature which is slightly lower than the water vapor saturation temperature. The droplet is surrounded by water vapor at saturation temperature of 373.15 K.
The problem I have is that after a few time-steps the liquid fraction inside the droplet starts to decrease until there is actual water vapor formation inside the droplet. Please see the image in the attachment to have a better idea of what is happening.


The water droplet has a temperature which is also below the saturation temperature, so what is happening seems quite unlogical to me.
I used the following operating conditions:
Operating conditions:

• Gravity perpendicular to surface where droplet sits
• Models: VOF (implicit body force ON), Energy ON, Laminar flow (Re = 4.14*10^-3)
• chosing water vapor and water liquid from fluent database (with given standard state enthalpies) and setting vapor as primary phase and water liquid as secondary phase.
• Mass transfer mechanisms: evaporation- condensation frequency of 500.
• Surface tension: Wall and Jump adhesion, coeff. = 0.0589 N/m (between liquid and vapor)
• Chosing copper as plate material, contact angle = 90°, 100x100x100 micrometers computational domain with the copper surface as bottom surface.
• Boundary conditions:
  
  • plate temperature 371.15K
  • interior fluid temp: 373.15 K
  • top-surface: pressure inlet (gauge pressure 0 bar, 373.15K inlet temp, volume fraction liquid = 0)
  • wall1-3 periodic boundary
  • wall2-4 periodic boundary
  • Operating conditions: set operating pressure at top-surface, specified operation density: 0.5542 kg/m3
  • Periodic conditions: pressure gradient 0 (no indication on this in reference 1)
  • User defined initialisation function: 1 droplet on the center of plate (radius = 20 micrometers), droplet temperature 371.15K.
• Solution methods: SIMPLE, Second Order Upwind for Energy and Momentum, Geo-reconstruct for volume fraction, default for rest.
• Convergence criteria: 10-3 for continuity, momentum and volume of fraction equation, 10-6 for energy equation
• Initialization: standard, patching done through udf

I would be happy of any help.
Thanks in advance

[vinerm]

You should use a lower temperature for the fluid or else evaporation can occur. A droplet at 373.15 will evaporate, so, use a slightly lower temperature. If the water is truly at 373.15, then, vapor pressure in surrounding should be equivalent to 100% humidity. Evaporation from the inside of droplet is unphysical and incorrect since ratio of liquid density to that vapor is 2000 approx., so, bubble will explode. Secondly, a contact angle of 90 is useless since it is equivalent to default condition of non-wall adhesion. Water on copper usually makes an angle of 70.

[EloNielsen]

Hi Luca
Did you get it to work?
I am working on the subject and agree that you should set your droplet to the wall temperatur because that is how Fluent will look at the liquid phase ant let it calculate the conduction though the droplet.
Regarding the used surface tension for water at ~373K you should consider incorporating the curvature of the droplet.