NUMERICAL STUDY OF VACUUM-AIDED SPRAY FLASH DESALINATION

Spray flash desalination is an advanced distillation method by space flash evaporation of over-saturated multi-component droplets and/or ligaments to generate the distilled condensable vapor. Vacuum-aided spray flash is spraying the superheated and atomized droplets into a depressurized chamber whose low pressure is maintained by a continued vacuum extraction of vapors, a process of high thermal non-equilibrium and of high non-uniformity in phase changes and transport. This study presents a CFD numerical simulation to quantify some of the non-equilibrium and non-uniformity characteristics in vacuum-aided spray flash distillation, especially on the temperature difference and the thermal re-equilibrium tendency between vapor and liquid residue. Since ANSYS Fluent lacks the mathematic model of self-heat supplied evaporation in discrete phase modeling (DPM), a pressure-driven flash evaporation model is proposed and implemented into CFD via User-defined functions (UDF). Preliminary results meet a good agreement with experiments. It revealed the non-uniformed characteristics of the vapor phase (i.e., distributions of temperature and velocity), as well as the thermal non-equilibrium between the two phases. It also suggests that the salt fraction within droplets will result in negative impacts on evaporating efficiency and evaporating rate during the process.