Study on water flash evaporation under reduced pressure

• Published in: International journal of heat and mass transfer 2019-03, Vol.131, p.31-40
• Main Authors: Wang, C., Xu, R., Chen, X., Jiang, P., Liu, B.
• Format: Article
• Language: English
• Subjects: Depressurization; Evaporation; Flash evaporation; Flashing; Gas composition; Ideal gas; Initial pressure; Mathematical models; Outflow; Pressure reduction; Thermal follow-up coefficient; Vacuum; Vaporization; Water
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2018.11.009

[Display omitted] •Water flash evaporation under reduced pressure is investigated.•A mathematical model for water flash evaporation has been developed.•Stefan's law is applied to simulate water vaporization before flash evaporation.•Ideal gas law is applied to calculate gas outflow and update gas components.•A thermal follow-up coefficient is proposed to capture the flash temperature. In this paper, we conducted experimental and theoretical studies for water flash evaporation. We presented a typical flashing experimental system as well as a method for dynamic measurement of flashing mass. On the basis of the experimental results, we developed a one-dimensional mathematical model of flash evaporation as well as a numerical solution method. The model systematically described four major physical processes: gas outflow resulting from depressurization, mild evaporation, flash evaporation, and updated flashing chamber pressure and gas components. We applied Stefan’s law to capture the characteristics of water vaporization during mild evaporation. We proposed a thermal follow-up coefficient to evaluate the transient superheat degree of the liquid water in the flashing process. Further analysis indicated that this coefficient had a significant functional relationship with the depressurization rate. We treated the gas in the flashing chamber as an ideal gas and then described the gas outflow and the updated pressure and gas composition with ideal gas thermodynamic equations. We validated the performance of the model in predicting water-flashing temperature and mass under reduced pressure against the experimental results. To better understand the mechanisms associated with the flash evaporation and develop controllable flash technology, we also conducted a parametric study in which we addressed the effects of the gas outlet diameter, the initial pressure, and temperature on the flashing process.