Activity Coefficient Models Reference

In Fluid Film simulations with evaporation and condensation, Simcenter STAR-CCM+ provides two models to calculate the activity coefficient, which accounts for the interactions between the different components in a mixture.

Given that the vapor pressures at the liquid surface are the driving force for liquid evaporation, their accurate evaluation is essential for detailed modelling. The activity coefficient sets the method of calculating the vapor pressure for the interactions between the different components in a multi-component liquid mixture.

The activity coefficient models are available in a Film-Physics Continuum Interaction (for all other simulation types), and are accessible after selecting the Evaporation/Condensation optional model for phase interaction.

Raoult's Law (selected automatically) activity coefficient model should be used if all components of the multi-component liquid mixture are chemically identical. In simulations where components in a multi-component liquid are different in terms of molecular structure, such as for example, in fuel simulations with ethanol, you are advised to use the Modified UNIFAC correlative method for the activity coefficient computation.

Table 1. Activity Coefficient Models Overview
Model Names	Raoult's Law
	Modified UNIFAC
Theory	See Raoult's Law and Modified UNIFAC.
Provided By	[phase interaction] > Models > Evaporation/Condensation > Activity Coefficient
Example Node Path	Continua > Physics 1 > Models > Multiphase Interaction > Phase Interactions > [phase interaction] > Models > Raoult's Law or Continua > Physics 1 > Models > Multiphase Interaction > Phase Interactions > [phase interaction] > Models > Modified UNIFAC
Requires	Physics continuum selections: A single phase simulation with the Fluid Film model activated. Material: Multi-Component Gas in the physics continuum. A Film-Physics Continuum Interaction defined with the fluid film phase and the corresponding physics continuum. The Evaporation/Condensation Model should be selected for the phase interaction.
Properties	See Activity Coefficient Models Properties.
Activates	Model Controls (child nodes)	The following properties are available with the Modified UNIFAC model. Coordination Number See Coordination Number .
	Materials and Methods	Functional Group Composition Method See Materials and Methods.
	Field Functions	Activity Coefficient of [liquid component] See Activity Coefficient Models Field Functions.

Activity Coefficient Models Properties

Model	Corresponding Properties
Raoult's Law Use this method to obtain the vapor pressure at the liquid surface if you have a state of equilibrium between vapour and liquid at the liquid face. This is the default.	None.
Modified UNIFAC Use this method to obtain the vapor pressure at the liquid surface for complex mixtures, where the molecular structure of components is very different. This method categorizes a component/compound by its structural functional groups, and assumes that certain thermodynamic properties can be calculated by summing the group contributions.	Coordination Number Specifies the coordination number in Eqn. (2786). The default value is 10.

Materials and Methods

Functional Group Composition Method: When the Modified UNIFAC activity coefficient method is selected, the functional group composition becomes available as a material property for the multi-component liquid film. It is entered as a scalar profile for each component of the multi-component liquid.

Activity Coefficient Models Field Functions

The following primitive field function is made available for all components in the participating film phase, when any of the activity coefficient models is activated.

Activity Coefficient of [liquid component]: The film phase component activity coefficient in Eqn. (2778). This property is set to 1 for the Raoult's Law model.