Thermal non-equilibrium in spray flash with vacuum vapor extraction

The spray flash is the core procedure in spray flash desalination, which is an intense phase-changing process with great potential in efficient low-temperature evaporation and high evaporation capacity. In a vacuuming spray flash with active vapor extraction and absence of external heating, the latent heat is self-supplied by the evaporating droplets, resulting in a temperature difference between continued cooling droplets and the hotter yield vapor along with the spray transport. Understanding such thermal non-equilibrium between vapor and droplets is curial and beneficial to desalination industry for improving evaporation efficiency and energy recovery. This paper aims to establish an integrated experiment-modeling-CFD methodology to study the spray flash and its thermal non-equilibrium under active vacuuming. In this work, various effects on the spray flash, including the polydispersion of atomized sprays, the heat conduction coupled evaporation of droplets, as well as the non-uniformity in vapor transport, are discussed. The proposed methodology is based on a Lagrangian-Eulerian modeling approach, which is calculated by a simplified process model and numerically solved via computational fluid dynamics (CFD) of ANSYS FLUENT with user-defined functions (UDF), respectively. Two experimental systems are developed: One is for calibrating droplets' velocity and size distribution; the other is a lab-scale vacuum spray flash system for measuring the evaporative characteristics. A dimensionless corrective method for the lumped heat capacity model is proposed in order to improve the accuracy of discrete phase modeling in CFD simulation. Theoretical predictions by the process model and CFD simulation meet good agreements with experiments. Case studies of the operation parametric effects on flash characteristics are demonstrated via both modeling predictions and experimental measurements. The results indicate that the temperature difference between the extracted vapor and the discharged water can be effectively generated during the process. Such a gap is positively impacted by the operating vacuum level and the initial spray temperature. The reported thermal non-equilibrium phenomenon indicates highly efficient evaporation of spray flash in an active-vacuuming environment, and such potential could be further utilized in desalination.