Example of Volume of Fluid (VOF) evaporation
2023-11-02T05:11:59.000+0100
Summary
This article illustrates how to set up evaporation in the context of the volume of fluid ( VoF) model in STAR-CCM+
Details
The example case illustrated here demonstrates how to set up evaporation of water within the context of the volume of fluid (VOF) model in STAR-CCM+. The geometry under consideration consists of a basin containing water which is being heated by heating element. The water in the basin evaporates as a result. Since evaporation phenomena is being modeled, a multi-component gas species model within the Eulerian multi-phase model will be used to account for air saturated with water vapor.
A step-by-step procedure to turn on the evaporation model within VoF is shown below. Ensure that the following physics is selected.
A step-by-step procedure to turn on the evaporation model within VoF is shown below. Ensure that the following physics is selected.
- Create a first Eulerian Multiphase phase named Air and Water Vapor (G). Ensure that the following models are selected.
Under Multi-Component Gas, select H2O and Air as individual gas components. In the current example, it is assumed that the air entering the domain is saturated with water vapor.
- Enter Initial Conditions for the species mass fraction of the H2O and Air phases.
In the current example, the species mass fraction values are calculated based on percentage relative humidity. The vapor pressure is calculated according to the Antoine equation.
- Create a second Eulerian Multiphase phase named Water (L). Ensure that the following models are selected.
Under Multi-Component Liquid, select water as the liquid phase.
- Navigate to Multiphase Interaction node under Physics models. Select VOF-VOF Phase interaction and assign the primary phase to be Water(L) and secondary phase to be Air and Water Vapor (G) as shown below
- Additionally select optional models Evaporation/Condensation and Interface Momentum Dissipation
- Ensure that the components are mapped correctly.
- Setting up Evaporation/Condensation model will activate the field Saturation Pressure under Eulerian Multiphase > Water > Multi-Component Liquid
The Saturation Pressure method may be changed to Antoine equation or user defined field-function as needed.
- Navigate to the VOF waves node under Physics models and create a new Flat VoF wave. Assign Point on water level which will set the mean water level in the domain.
Note that this step is a convenient way to set the initial level of the liquid. Additionally, the initial conditions for volume fraction of heavy fluid and light fluid (water and air in this case) in a Eulerian VoF multiphase can be set up this way. This model also automatically accounts for the hydrostatic pressure of the heavier and the lighter fluids.
- Set the initial conditions for Pressure, Velocity as follows. Set the Volume Fraction method to composite. Subsequently for both water and air change the method to field function. Choose the field functions as follows:
- Under boundary conditions, set volume fraction at the inlet to method composite and set the field functions for the different species as follows:
- At the inlet and outlet boundaries, to set the vapor pressure for saturated air, change the following:
Set Inlet > Phase conditions > Air and Water Vapour (G) > Physics Values > Species Mass Fraction to method: Composite. Calculate vapor pressure at inlet and outlet based on percentage of relative humidity in the air at the inlet and outlet based on Antoine equations, as per guidance given in this how-to article.
- To monitor the mass of water evaporating, create a field function $Volume*${VolumeFractionWater (L)}*${DensityWater (L)}.
- Finally, run this simulation using the Double Precision version of STAR-CCM+. One of the characteristics of VoF evaporation simulations is that you are dealing with significantly small evaporating rates. The double precision version of STAR-CCM+ is generally intended for cases that involve physical phenomena of vastly differing scales. Examples where you would expect double precision to make a difference are natural convection, multiphase flows, aeroacoustics, electrodynamics etc.
- The volume of water in the pan is visualized below as per the guidance given in this advanced rendering tutorial
See also in STAR-CCM+ Documentation: