Simcenter STAR-CCM+ Best Practice for Fluid Film Evaporation/Condensation

This Best-Practice has improved many simulations but there are likely limitations.
Most tested applications came from home appliances with water/air systems.

Please contact your DSE if you find issues with these settings.

Symbols used in this document:

The fluid film model uses shell regions for the meshing. As shells are defined on a boundary the quality of the boundary mesh is important for good fluid film convergence.

If very bad quality boundary cells cannot be avoided one may use the remove invalid cells on the fluid region. This will automatically remove the shell cells adjacent to the bad quality volume cell.

In fluid film evaporation/condensation cases the heat transfer is always important and often CHT is considered. Thus, all best practices for CHT meshing apply for the best heat flux (and mass transfer) approximation. Low y+ meshes are preferred for this reason.

If simulation speed is of high importance - especially for long simulations (minutes, hours) - high y+ meshing can be a good way to go. The convergence can be better and cell count is reduced leading to faster run times while keeping reasonable results.

Fluid Film Models & Settings

Select Fluid Film Models as shown on the right.
The physics needs to use multi-component gas. Air and H2O (Steam) is used here but can be changed. The Fluid Film has to use multi-component liquid with H2O.

• 🗸 Enable Stabilized Film Thickness Equation.
• • Needed for high time step sizes or steady state.
  • No known downsides.
• 🗸 Set appropriate Maximum Film Thickness.
• • This is generally advised for stability.
  • Film Mass above this limit will be discarded WITHOUT warning.
  • Unlike an Outlet Edge you cannot track the outflow of mass directly.
  • Track the evaporation rate directly using the fluid film evaporation rate field function.

Phase Interaction Models & Settings

• 🗸 set Evaporation/condensation URF = [0.01 - 0.25] ~ 0.15
• • Needed for high time step sizes or steady state.
  • The condensation rate generally changes slowly.
  • Combination of high Energy URF and evap. URF may lead to high oscillations in the temperature field.
  • Considering overall convergence a low URF here is much preferred vs. reducing the energy URF, slowing down the solution.
• Increase Nucleation Density to 1e9 - 1e12 /m.
• • Only with present film OR high nucleation density condensation takes place.
  • 1e12 /m ~ same rate as wet surface
• Enable Thermal Limitation. (formerly known as Implicit Coupling).
• Disabling Condensation or Evaporation can stabilize the simulation further.
  • • Should not be needed and reduces physical fidelity.

Solver Settings

• 🗸 Keep Fluid Energy at 0.98-1 if possible.
• • It's advised to first reduce the evap/cond. URF up to [0.01 0.25]. (see above)
  • If this is not sufficient reduce the Fluid Energy URF up to 0.9.
  • Even lower values necessitate too many inner iterations normally to be worth it.
• 🗸 Set Solid Energy URF to 0.99999.
• • The standard URF of 0.99 is too conservative for many cases and limits time step convergence.
• 🗸 Set Species URF to the same value as Fluid Energy URF.
• • This is generally advised to ensure correct balances.

Material Data

• 🗸 Heat of Formation
• • Set H2O vapor (gas) heat of formation to zero
  • Set H2O in film (liquid) to the latent heat
  • This is the difference in heat of formation liquid - gas
  • for water = -2240 kJ/kg
• 🗸 Set saturation pressure to Antoine or Wagner equation
• • Default parameters are correct for Water.
  • Make Sure Minimum Temperature is lowered if Temperature may be below 0 C.

Outlet Edge

In transient simulations always check for convergence in each time step.

The relevant quantities on the film should converge asymptotically to one value. You could check Temperatures, Relative Humidities or the Evaporation Rate. In this example you can see how the evaporation rate is not perfectly asymptotic at 0.6 seconds, but looks good at 1 second.

Many of the above mentioned settings influence the time-step convergence. The most important ones are listed below.
Play around until you find a good compromise.

1. Time Step Size.
2. Fluid Energy Residual.
3. Evaporation Condensation Residual.
4. Inner Iterations count.

The main driver for evaporation and condensation is the relative humidity in the air. Above 100% relative humidity condensation should occur.
As there is no 'humid air' material in Simcenter STAR-CCM+ using fluid film, there is unfortunately no special built-in function for relative humidity.

The attached java macro creates partial pressure and relative humidity field functions for water - air. The Antoine coefficients are set up using global parameters, so the method can easily be adapted for different materials.

The units %, per cent, is also created. You can choose these units for any value/function with an empty dimension - changing 0.01 to 1 %.

As described above, the macro also sets up the 'Env RH Mole Fraction Water' parameter to be used in boundary conditions.

With the settings described in this document Time Step sizes of 0.2-2 seconds are possible.
Smaller time step sizes are expected to be needed, though. Especially when the evaporation rate changes a lot, a smaller time step is necessary. A reason could be changing thermal boundary conditions or simply the start of the simulation.