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Vapor formation during single droplet condensation problem

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Old   May 6, 2020, 12:28
Default Vapor formation during single droplet condensation problem
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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
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Old   May 6, 2020, 16:36
Default Evaporation-Condensation Model
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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.
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