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

2024-06-18T07:38:44.000-0400
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Summary

Fluid Film condensation and evaporation simulations are prone to divergence or Floating Point Exception if the thermodynamic settings are not chosen properly. This article aims to help users identify and resolve the most common problems that can lead to such issues.


Details

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:

  • ✓ Always use this setting.
      • This setting has shown to benefit most fluid film evaporation/condensation cases.
  • Consider this setting.
      • This setting can be useful in some situations.

Mesh Considerations

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.


Physical Properties

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.

Domain and Boundary Conditions

Outlet Edge

  • 🗸 Always set at least one or more edges as 'Outlet'.
      • Otherwise film will accumulate at edges and cause instabilities.
      • If you need to track mass accumulation use mass flow reports and statistical reports to sum up

Humid Air Inlets/Open Boundaries

When setting up evaporation/condensation cases it is often necessary to define the relative humidity for the environment.
This can be calculated by hand and set using the mass or mole fraction setting on open boundaries.

  • For convenience the attached macro creates the 'Env RH Mole Fraction Water' parameter.
      • This parameter calculates the mole fraction based on the 'RH_env' and 'T_env' parameters.
      • Use this parameter together with the Temperature as T_env in any open boundaries.

Always Wet/Dry Wall Boundary

For some applications there are special walls that can be considered as always wet or dry. For these the film thickness should be neglected.

  • Not considering the film thickness increases numerical stability and makes steady state simulations possible.
  • To keep track of condensation/evaporation rates simply use a field sum monitor or surface integral reports.

Modeling of an always wet surface or water surface.

An 'always wet wall' can be any water surface like puddles, free water surfaces. This wall is considered never to dry out, hence it is not necessary to keep track of the film thickness.

  • Set low maximum Film Thickness for the film phase. (0.1 mm)
  • Initialize with max. Film Thickness.
  • Enable species source flux on the back boundary.
      • This is the wall boundary without an interface for simulations without a solid region.
      • Use the interface boundary connected to the solid in simulations with a solid region.
  • The mass flux value is not important - set to e.g. 1 g/s.
  • The mass will be discarded because of the max. Film Thickness.
  • Evaporation/Condensation rate is not influenced.

Condensating, mostly dry Wall Boundary

Many cooled walls for humidity removal are designed to let the film drain. Therefore, the wall remains mostly dry or only droplets remain on the wall. If a wall is always cooled below the dew point it can be beneficial to not consider the actual film thickness.

  • Initialize with 0 Film Thickness.
  • Set negative species flux on back wall to keep the wall dry (see always wet wall).
  • Apply appropriate thermal boundary condition.
  • For condensation on dry walls the Nucleation Density is of utmost importance.
      • Use appropriate value or set to 1e12 /m ~ same rate as wet surface

Analysis Controls

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.

Post-Processing

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.


Performance Considerations

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.


KB Article ID# KB000132505_EN_US

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