title: Physicochemical effects of turbulence and entrainment in atmospheric pressure plasma jets.
Creators: name: Morabit, Youssef
Publisher: name: University of Liverpool Repository
Year: 2021

description: Atmospheric pressure plasma jets (APPJ) are currently the focus of an intense international research effort due to their unique chemical and physical characteristics. A defining aspect of the plasma jet is the interplay between plasma physics, plasma chemistry, and fluid dynamics which ultimately dictate application efficacy. This project focuses on uncovering the link between the fluid dynamics of the emerging noble gas channel into quiescent ambient air and the chemical reactions induced by the reactive species produced in the discharge. Several, biochemically relevant species, reactive oxygen, and nitrogen species (RONS) are created such as O, OH, O3, H2O2, NO. These reactions occur with the presence of air, thus at the interface of the shear layer of the plasma jet and the medium. Making it a crucial parameter toward the generation of relevant reactive species, this interface is intrinsically linked to fluid dynamic characteristics of the jet. Particle Image Velocimetry provides novel time- and spatially-resolved quantitative velocity measurements of the jet with and without the influence of the plasma discharge. This influence is characterised; the induced-turbulence is generated by disturbances created in the shear layer of the jet by gas heating and electrohydrodynamics (EHD), moreover the observation and measurement of velocity fluctuations and turbulence in the laminar region of the jet provide novel insight on the fluid dynamics consequences of the plasma discharge. Those perturbations propagate along the jet and alter the structure with an intensity dependant on the fluid dynamic parameters and the plasma input parameters. These observations show the presence of instability on the plasma jet structure, particularly the edges, thus the results imply the presence of air entrainment in the laminar region of the jet which impact the plasma physics and chemistry. Furthermore, the plasma-induced velocity increase of the jet is characterised, and is below a 10% increase of the initial velocity. In contrary to past assumptions, the velocity effect of the plasma discharge is thus unable to solely alter the state of the jet. Rayleigh scattering and Laser-induced fluorescence provide insights on the induced air entrainment and mixing in the shear layer, and the spatial distribution of relevant chemical species (OH). The results show that plasma-induced perturbations decrease the laminar region length, increase the turbulence intensity, and increase the quenching rate of OH fluorescent state. This indicates the critical aspect of the shear layer instabilities on the physicochemical characteristics of the plasma jet. Ultimately, the plasma jet applications mostly directed downward impinging on a target, the electrical parameters of the generation of the plasma revealed to be able to control the structure of the jet; thus the gas mixing and the chemistry. The adaptability of the plasma jet configuration and input parameters allow a decisive control over the jet structure and the chemistry while being used at atmospheric pressure and near room temperature, it makes it a convenient non-obstructing technique for localised treatment. This study shows the ability to control the chemistry of the jet through the electrical parameters, for applications where the wanted chemistry is already in the jet, generating a laminar jet will oppose greatly the mixing with the environment. Whereas, applications where the mixing with ambient air is mandatory to obtain relevant reactive species, electrical parameters prone to perturb and provoke earlier transition to turbulence will be crucial. The electrical parameters in a pulse-driven APPJ, mainly the repetition frequency and the pulse width are crucial to controlling the jet structure and chemistry in a cost-efficient way. The chemistry issued from plasma jet generation has proven to be of great interest in several fields, such as material processing, wound healing, bacteria and. These efforts will pave the way for new understanding and applications of the plasma jet.