Fundamental Understanding of Plasma Discharge Formation in Liquid and Multiphase Configurations Through Multiphysics Modeling

During the last two decades, non-thermal plasma discharges in and in contact with liquids have received significant attention due to their wide range of applications including chemical analysis, medical, water treatment, fuel processing, etc. Despite the tremendous interest and advances attained in the experimental studies, modeling efforts providing a comprehensive understanding of the underlying physicochemical processes are limited. There is still no unified theory on plasma formation in dense medium and various theories have been proposed such as the presence of bubbles and tunneling which are topics of debate. In the first part of this study, a mathematical model is proposed to simulate the initiation and propagation of plasma in a liquid medium. An in-house numerical framework consisting of a compressible fluid solver together with the charged species conservation and Poisson’s equation solver is employed for the simulations. The effects of electrostatic, polarization and electrostrictive ponderomotive forces are studied on the initial stage of plasma discharge. The simulation results of the proposed model show that under the influence of nanosecond voltage rise, the liquid experiences the formation of negative pressure region near the vicinity of the powered electrode and surpasses the cavitation threshold pressure. The cavitation locations initiate as sub-micron regions and then extend up to a few microns. These sub-micron low-density regions act as plasma nucleation sites. The second part focuses in assessing the kinetic aspects of plasma formation in liquid water. A chemical kinetic model is proposed and utilized in the simulations. To solve the detailed chemistry problem, a novel operator-splitting scheme is developed. It has been shown that other than water ionization via Zener tunneling, electron detachment from negative OH ions is an important ionization pathway for liquid molecules. The electrons react with liquid water and generate aqueous ions and radicals. The kinetic analysis of plasma interaction with water shows that at lower voltages, OH ionization is the primary source of electrons while at elevated voltages almost all of electrons are generated via H2O ionization. Finally, formation of plasma in multi-liquid configuration is investigated. Compared to a single liquid, it has been found that plasma discharge can be achieved at lower voltages thus proving to be an energy efficient method for plasma generation. A multiphase fluid model is proposed to study the electrical forces and plasma discharge in multi-liquid configurations. The effect of interface location as well as the applied voltage profile on discharge probability is studied