Multiphase Systems

Two-phase liquid–vapor systems in non-equilibrium conditions are often found in nature and devices such as cooling systems of power plants, as sprays in the combustion chamber of internal combustion engines, and in boiling and industrial devices. The liquid–vapor interface has a thickness of a few nanometers and the fluid density has strong variation along the interface thickness. In numerical simulations, the way the interface position is determined plays a key role in modeling two-phase systems and we can classify the several methods that were developed for two-phase systems in two families. The first one is based on the singular interface approach when the interface is considered as a boundary separating the two phases. The second one is based on the diffuse interface approach when the interface is considered a diffused zone. These two families of methods are presented in this chapter. For illustrating purposes, the lattice-Boltzmann simulation results for the coalescence process between two liquid droplets and vapor condensation are presented at the end of the chapter.