COMSOL 6.2 - Vapor–Liquid Interface

Vapor–Liquid Interface

Use this feature to account for evaporation or condensation at a vapor–liquid interface. The feature prescribes the vapor pressure as a function of the composition in the liquid and the temperature and pressure at the interface. Single species liquids as well as multi component liquid mixtures can be modeled. Local concentration gradients introduced by the vapor pressure result in a species transfer to or from the interface. A vapor pressure higher than the partial pressure in the surrounding domain will result in evaporation of the species in question. Correspondingly, when the vapor pressure is lower than the surrounding partial pressure, condensation occurs.

The feature can be used on exterior boundaries to solve for mass transfer due to evaporation or condensation. The resulting mass flow and latent heat of evaporation are defined and announced and available for use in physics interfaces for fluid flow and heat transfer. The Vapor–Liquid Interface feature solves for mass transfer to or from a gas phase domain. To also model the resulting composition changes in an adjacent liquid phase, use the Vapor–Liquid-Mixture Interface.

To solve for the motion of the vapor–liquid interface, resulting from the phase transfer, use a fluid flow interface including a Fluid-Fluid Interface feature (described in the CFD Module User’s Guide). When solving for heat transfer apply the latent heat of evaporation using a Boundary Heat Source feature (described in the Heat Transfer Module User’s Guide).

The Vapor–Liquid Interface feature can also be used on stationary boundaries. This is for example efficient when the mass transfer is low and would only lead to insignificant variations of the interface position.

boundary selection

Select the boundaries that represent an interface separating a vapor and liquid phase domain. The boundaries should be exterior to the domains where vapor transport modeled.

vapor equilibrium

Use this section to select how the equilibrium vapor pressure is defined. The vapor pressure is applied on the vapor side of the interface and drives the condensation or evaporation.

Vapor pressure

In the list, select to prescribe the corresponding quantity at the vapor–liquid interface. It is also possible to instead prescribe the vapor phase Mass Fraction.

The composition in the liquid phase is controlled from the list:

•

Use to model condensation or evaporation to or from liquid water. In this case predefined expressions for the saturated vapor pressure and heat of evaporation are used.

•

can be selected in models that contains a coupled to a interface. In this case all thermodynamic properties needed at the interface (vapor pressure or fugacity, heat of evaporation, liquid phase density) are automatically added to the .

•

Selecting from the list, the equilibrium vapor pressure, Pvap,i, and heat of vaporization Hvap,i, for evaporating or condensing species can be manually defined.

Below the settings are detailed for the respective entries in the list.

•

When is selected, use the list to identify the species solved for that corresponds to water vapor. The saturated water vapor pressure is prescribed to the species selected.

•

For also select an interface from the list. In order for a interface to be available, it needs fulfill the following criteria:

The interface should include at least one species.

The interface should be set to consider a concentrated mixture. This is applied by selecting in the list at the top of the section (in the settings window of the interface).

The interface should be coupled to a . In the section, select and chose an appropriate entry from the list.

The selected system needs to include both a vapor and a liquid phase.

Gas phase mixture properties should be produced by selecting in the list of the section.

The interface cannot have enabled in the section.

Evaporating/Condensing Species

When a is used, select the evaporating or condensing species using the check boxes next to the respective species. When a species is correctly coupled, the mass fraction variable as well as the chemical formula and compound name is indicated, for example as “wA, C3H6O (acetone)”. If the species is not correctly coupled, the corresponding label is “wA, Not coupled”. To inspect the coupling, use the button to navigate to the interface. In the section make sure that each species is coupled both to a mass fraction solved for as well as to a species .

Liquid Phase

Use this section to specify the liquid phase composition when using a .

Select a — , , or . Use the table to specify the corresponding concentration of each species in the liquid.

Mass Fraction

To define the vapor side condition directly, select from the list. For each of the evaporating or condensing species, specify the equilibrium mass fraction, ω0,i, and heat of vaporization Hvap,i.

Transient initialization

When prescribing boundary conditions in time-dependent problems, and in particular vapor pressure conditions, it is important not to introduce a discontinuity with respect to the conditions in the domain. When this happens the solver may struggle to converge and resort to small time steps. When using transient initialization a smooth step function, fs, transitions the boundary condition, from the domain initial value, ω0, to the condition to be applied in the boundary, ωeq, over a small time span Δt. The smoothed value applied on the boundary, ωbnd, is defined as

where

(3-138)

When is set to the initialization time is automatically defined as a fraction of the time between the initial and the last output

Select to manually define the duration of the transient initialization Δt.