The modeling of phase transitional flows is of major interest to industry because of its relevance with regards to improving the efficiency of many processes such as cooling systems in nuclear power plants, micro cooling systems in electronic devices and fuel injection in combustion engines. Phase transitions also occur in heat exchangers, boilers and geophysical processes such as raindrop formation. When modeling phase transitional flows it is crucial to accurately capture the complex phenomena occurring at the liquid-vapor interface.
The interface between two fluid phases has attracted extensive investigation in the past and still presently. The idea of a diffuse interface (a rapid but smooth transition between the physical properties of the bulk phases) and the use of a capillary stress tensor to model the behavior of the interface is the central notion in the derivation of the numerical model developed in the present study. In this method the width of the interface is determined by relating the Helmholtz free energy of the fluid to the mass density gradient distribution at the liquid-vapor interface. The advantage of this approach is that the phase-change process and coalescence and break-up of the interface are all naturally embedded in the governing equations and do not require any additional models.
A method is developed for simulations of liquid-vapor flows in three spatial dimensions under nonisothermal conditions using a Diffuse Interface Model which is based on the nonisothermal Navier-Stokes-Korteweg equations combined with the Van der Waals equation of state. Simulations of droplet collisions show the existence of multiple collision regimes for which the energy transfer and dissipation processes during the collisions are studied in detail. These results are compared with a different simulation method for multiphase flows, namely the Local Front Reconstruction Method. An overall good agreement is seen between the simulation results of both methods.
The Diffuse Interface Model has also been employed to model droplet impact on a heated solid surface. A solid wall boundary condition was especially constructed which enables simulations with different wetting conditions of the solid surface. The boundary condition is further extended to include the effects of surface roughness on the behavior of the contact line dynamics. The simulation results demonstrate the influence of the wetting properties of the solid, with a higher cooling rate for hydrophilic than for hydrophobic wetting conditions. Surface roughness of the solid surface increases the cooling rate of the solid by enhancing the heat transfer between solid and fluid.