Numerical modelling of atmospheric pressure plasma jet discharges

Atmospheric-pressure plasma discharges (APPJs) have been one of the main active research topics of low temperature plasmas since they were firstly reported a decade ago. Their compactness, their ability to operate at ambient conditions (atmospheric pressure and room temperature), and their simplicity (no complex or expensive vacuum equipment are required) makes them very promising sources of active chemical species for a variety of applications, ranging from sterilisation of surfaces to space thrusters. Optimising APPJs to suit particular applications requires deep understanding of the plasma dynamics involved in their operation, which is an active field of research from experimental and numerical approaches. Because both approaches have practical limitations, the difference between the simulated conditions and the experimental conditions has become wide. One particular aspect of this difference is the time scales of the phenomena studied. In most numerical studies the time scale considered is in the order of hundreds of nanoseconds, while experiments are conducted under steady state conditions. In this work, a numerical model is built to study the behaviour of an APPJ discharge on relatively longer times compared to other numerical studies. The longer time scale in this work covers two consecutive periods of an applied pulsed DC waveform (up to 40 s), compared to only the pulse-on time for a single pulse in most other works. The study presented here considered two jet configurations, an open jet configuration and a surface configuration. The afterglow of the open jet configuration is studied, where it is shown that the absence of the applied potential causes the electrons to diffuse strongly from the plasma channel created in the pulse-on time, causing an increase by almost two orders of magnitude in the density of the negative ions. An increase in the density of the positive ions is also observed in the afterglow, which is attributed to Penning ionisation between the helium metastables and the molecules of air (O2 iv and N2). The study also shows that the characteristics of the discharge in the second period are noticeably influenced by the residuals from the first period. With respect to the surface configuration, the study presented in this work focuses on the fluxes of the active species to the surface. It is reported that the flux of the positive ions to the surface occurs mainly during the pulse-on time, with its maximum value coinciding with the location of the plasma bullet at a given time. The flux of the negative ions however occurs mainly during the pulse-off time at locations on the surface where no surface charge is deposited during the pulse-on time. In the second period, the deposited negative surface charge deposited in the previous period causes a decrease of the flux of negative ions to the surface. Whereas the residual plasma from the previous period causes an increase in the flux of positive ions to the surface where the residual plasma is in contact with it. The other topic of interest in this work is the induction of turbulence in APPJ due to the presence of the plasma, where it is shown that the plasma affects the background flow by electrohydrodynamic forces and by gas heating. It is shown than neither the EHD forces nor the gas heating by the plasma are capable of accelerating the flow to change the flow regime. Thus, a new explanation is proposed to explain the induction of turbulence by the plasma in APPJ