Phase change is the process of mass transfer between the liquid and gas phases of a certain fluid species. The mass transfer from liquid to gas is termed evaporation if driven by elevated temperature, or cavitation if driven by reduced pressure. The mass transfer from gas to liquid is termed condensation.
In applications where liquid is the predominant working fluid, cavitation is encountered often and is hence a focus of study. Following Osterland et al. [66], vapor cavitation refers to the process of phase change from liquid to gas due to pressure drop; gas cavitation refers to the degassing process in which the gas dissolved in liquid is released to its free form; pseudo cavitation refers to the expansion of gas volume fraction in a two-phase fluid due to compressibility when pressure drops.
All these three types of cavitations can be simulated by Ansys Forte. Pseudo cavitation is a feature of compressible flow, and it is simulated by Forte’s compressible two-phase flow solver by default. It does not involve mass transfer. Both vapor cavitation and gas cavitation are mass-transfer processes. Vapor cavitation is a type of phase change, and gas cavitation is not. The present section on phase change covers the topic of vapor cavitation. Modeling Gas Cavitation covers gas cavitation.
Zwart-Gerber-Belamri Phase-change Model describes a phase-change model in more detail.
Since phase change is a local mass-transfer process, we model it using source terms in a
transport equation. Consider the species conservation equation (Equation 2–1) without combustion and sprays, and re-write it for the gas phase of species
as:
 | (11–8) |
Where 
is the mass fraction of species 
’s gas component in the two-phase mixture. It can be written as
, where 
is the mass fraction of the gas phase and 
is the mass fraction of species in the gas. 
is the mass fraction of species 
in all the species that participate in the phase change. 
and 
are the rates of evaporation (or vapor cavitation) and condensation,
respectively.
Correspondingly, the species conservation equation for the liquid phase of species
is
 | (11–9) |
Where 
is the mass fraction of species ’s liquid component in the two-phase mixture.
The phase change processes involve growth and collapse of bubbles in the liquid. When the
local pressure is lower than the vapor pressure of the fluid (denoted as 
), cavitation occurs and the bubbles grow. Conversely, when the local pressure
is greater than the vapor pressure, condensation occurs and bubbles collapse. Following the
model proposed by Zwart-Gerber-Belamri [107], the source terms are
calculated as:
When 
,
 | (11–10) |
When 
,
 | (11–11) |
in which 
is the gas phase volume fraction calculated as 
. The modeling parameters and their default values are as follows:
is the bubble radius;
is the nucleation site volume fraction;
is the evaporation coefficient;
is the condensation coefficient.
Simplification is applied to the model when multiple components of a fluid participate in
phase change. A vapor pressure of the mixture is estimated according to the mass-averaged vapor
pressure of the species. Equation 11–10 and Equation 11–11 are used to compute the overall rate of
evaporation or condensation, and they are distributed among the species by the mass fraction of
the species (
).
The second modeling option for the local phase-change processes is based on equilibrium
theories, requiring that the fluid in each cell must be in their most stable thermodynamic
states. In this approach, the known quantities are the fluid mixture's density (
), specific internal energy (
), and species mass fraction (
), which are determined by the flow transport processes. A Gibbs free energy
minimization solver is called to calculate the phase concentrations in each species, and the
pressure and temperature at phase equilibrium. This equilibrium status is enforced in each CFD
cell and in each time step, so the time to reach equilibrium is assumed to be infinitely fast.
This is different from the Zwart-Gerber-Belamri model, in which the rate of phase change is
finite.