Computational study of atmospheric pressure low temperature plasma

Without the need of large and expensive vacuum equipment, atmospheric-pressure plas- mas have received growing attention in recent years for their potential economic and technical advantages. A myriad of applications, including displays, radiation sources, gas analysers, microreactors, aerodynamic flow control, material processing, and pollu- tant abatement, have been reported in the literature. Of particular interest is the recent development of low-temperature atmospheric-pressure discharges capable of operating close to room temperature. These have opened new opportunities for plasma technology that simply were not available in the past. Arguably, the most significant example is the use of low-temperature atmospheric-pressure plasmas for biomedical and therapeutic purposes, a new and rapidly growing field that is often referred to as plasma medicine. Despite the fast growth in the number of applications and the demonstrated po- tential of these discharges, the physico-chemical processes that govern their dynamics and chemistry remain not fully understood. This lack of understanding hampers the full exploitation of this technology and calls for further fundamental research in this area. On contrast with their design simplicity, diagnostics of low-temperature atmospheric- pressure discharges are very challenging due to their non-equilibrium character, large collisionality and often times reduced dimensions and short duration. Computational modelling can overcome these experimental difficulties and provide insightful informa- tion to help advance the current understanding of the physico-chemical processes at play in these discharges. The research presented in this thesis aims at making a contribution in this field by providing new insights into the dynamics and chemistry of low-temperature atmospheric- pressure plasmas. Novel insights obtained during the course of this thesis have already been published in 3 peer-reviewed journals and presented at 9 international conferences. Given the high collisionality in atmospheric-pressure discharges, fluid modelling offers an excellent compromise between accurate description of plasma physics/chemistry and computational efficiency. Thus, a fluid simulation scheme has been pursued in this thesis. Due to the high collisionality of atmospheric-pressure discharges, these plasmas rely heavily on their chemistry and therefore comprehensive chemical models with hundreds of reactions have been incorporated in the studies. A series of numerical techniques have been successfully implemented to address the computational demands of handling these large chemistry sets. Many applications require small scale plasmas. Therefore, in a first study we looked at microplasmas, which enable stable operation at atmospheric pressure and can be eas- ily integrated into portable devices. Microplasmas also provide interesting dynamics that differ from those in larger systems and which are difficult to unveil experimentally due to the reduced dimensions of the discharge. Modelling of both electropositive and elec- tronegative microdischarges has allowed opposing trends in electropositive experimental results to be reconciled, gaining a better understanding on the influence of the driving frequency on the discharge properties, and observation of double layer structures which result in the complete separation of electrons and anions in electronegative microdis- charges. Simulation results have also been used to validate experimental techniques often used to infer plasma density from impedance measurements and conditions where this technique works/fails have been identified. In a separate study, the generation of reactive oxygen species (ROS), such as ozone, atomic oxygen and singlet oxygen, which are of paramount significance for example in plasma medicine was investigated. Although oxygen is a well-established precursor for the production of ROS, water is considered here due to its ability to create complex chemistry and its presence in most open air operated discharges and moist biological materials. To date, the presence of water has been largely overlooked in the literature and this study indicates that not only water can be used as a single precursor for the generation of ROS but also it can be used in conjunction with oxygen to create new chemical cocktails with higher oxidation potential.