Direct numerical simulation of phase change in the presence of non-condensable gases

This paper describes a new numerical method for the simulation of phase change phenomena between a liquid and a vapour in the presence of non-condensable gases. The method is based on an interface-tracking approach in the framework of single-fluid modelling. The principal innovative feature represented is the capability of simulating a mixture of the condensable gas (vapour) and non-condensable gases with different densities. The formulation and subsequent discretization of the governing equations for the species transport are discussed in detail. In particular, a volume-averaged velocity field is introduced into the species transport equation in combination with a mass-averaged velocity field approach for the momentum equations. The resulting algorithm has been implemented into the incompressible Navier-Stokes solver, PSI-BOIL, which features a finite-volume approach based on a fixed, rectangular, Cartesian grid. Several verification cases have been undertaken to ensure the code modifications have been correctly implemented. These include simulation of the Stefan problem, involving evaporation and condensation in a 1 D configuration, and an evaporating droplet under forced convective flow. In all cases, very good agreement has been obtained with analytical solution. A simulation of direct-contact condensation of a practical application is also presented, which serves to demonstrate the potential capability of the new approach to a wider range of engineering problems, including pressure suppression pools in nuclear reactors. (C) 2020 Elsevier Ltd. All rights reserved.