Numerical Simulation of Flows With Evaporation

Abstract

Multiphase flows are usually accompanied by thermodynamic effects. These effects are associated with gas-liquid phase transition which can occur in a single fluid system as well as in systems comprising more than one species. Appearance of the transition in a system has substantial thermal and mechanical consequences, such as transfer of mass, momentum as well as energy and change in the temperature field. Flows coupled with phase change occur abundantly in nature. They are responsible for atmospheric phenomena such as cloud formation, absorption of gases (including green house ones) by sea water and many other phenomena of a global or local scale, which influences everyday life. Multiphase flows are also often present in many industrial applications in which their physical features are advantageous or disadvantageous. Installations in the oil production industry and energy production plants are examples of installations in which multi-phase flows with phase transition appear. Phase transition is a desired phenomenon in vapor generation systems such as power plant boilers or water cooled nuclear reactors; as well as indirect or direct contact vapor condensers or mass transfer equipment used e.g. for humidification. Phase transition can also be an undesired phenomenon. It occurs in pumps and on ship propellers where because the pressure decreases considerably at the suction side of the impeller or propeller blade, cavitation appears. This sort of transition can cause oscillations and may threaten the structural integrity of the impeller or propeller. Two driving mechanisms for phase transition inside a fluid can be distinguished. The first is variation of the pressure leading to cavitation, whereas the second one is heat transfer (temperature) resulting in boiling and evaporation/condensation. Over the past decades researchers put much effort in the development of algorithms capable of numerically simulate multiphase flows with phase transition. The present study concerns the development of a method for the prediction of multiphase flow with temperature-driven phase transition for which the geometry of the gas-liquid interface is not known in advance. A single substance is considered consisting of incompressible phases. The gas-liquid interface in multiphase flows, with or without phase transition, involves a discontinuity in the physical properties of the flow at the interface. This leads to difficulties in preserving convergence in numerical algorithms for predicting single phase flows. The investigation of mixed convection heat and mass transfer on a vertical plate with film evaporation has been numerically examined. Results were obtained for mixed convection driven by combined thermal and mass buoyancy forces. The numerical results, including velocity, temperature and concentration distributions, Nusselt number as well as Sherwood number and evaporation rate are presented. The results show that below a certain temperature, water evaporation rate decreases as the humidity of air increases and above it this relation reverses. This temperature is named “inversion point temperature”. A numerical model using the finite difference method was developed and tested systematically.