In recent years there has been growing interest in high pressure non-thermal micro plasma discharge utilizing the Paschen's 'pd' scaling law. At high pressures these plasma discharges were found to be sufficiently "warmer" than the classical non-thermal discharge even though they maintain non-equilibrium characteristics, making it critical to resolve the neutral gas heating in the system. In this current work high pressure micro plasma discharge were simulated using a one dimensional hybrid multi-physics model. The model included charged and neutral species conservation with detailed gas phase chemistry, self-consistent solution of the electric field, electron and neutral gas temperature as well as an external circuit model. In addition, conjugate heat transfer in the electrode was also considered. Simulations were carried out for a DC micro plasma discharge over a broad range of pressure, inter-electrode separation and electrode material for a He-N 2 feed gas. Special attention was given on the conjugate heat transfer aspect and its effect on the gas temperature and the plasma characteristics predictions (i.e. electron density, voltage-current characteristics). Unlike the conventional and widely accepted isothermal wall boundary conditions, predictions with the conjugate heat transfer model indicated significantly different gas temperature and plasma characteristics. The gas temperature prediction from the conjugate heat transfer model was found to be in good agreement with experimental measurements.

Atmospheric pressure microplasma devices, have been the subject of considerable research during the last decade. Most of the operation regime of the discharges studied fall in the 'abnormal', 'normal' and 'corona' modes -increasing and a 'flat' voltage current characteristics. However, the Negative Differential Resistance (NDR) regime at atmospheric and high pressures has been less studied and possesses the unique characteristics that can be employed for novel applications. The NDR regime has been studied for low pressure systems and has been characterized to be associated to relaxation oscillation only. In this work we report a detailed study on the different modes of self oscillation in high pressure micro plasma discharges. Detailed 2D numerical simulation has been conducted with a validated model. Predictions and experimental measurements are found to be in favorable agreement. The different self-pulsing modes of oscillation have been identified as, relaxation oscillation having medium to low frequency oscillation at low discharge current and high frequency free running oscillation at comparatively high current condition. In the relaxation oscillation, the discharge switches between a dark and glow like discharge, whereas in the free running mode the transition is observed to occur within glow like modes. These two modes of oscillation are found to be more prevalent at higher pressure. Depending on pressure, the frequency of relaxation and free running oscillations are in the kHzMHz and MHz -GHz range respectively. External parameters influencing these self oscillations are studied.

In the present study, bubble growth and departure characteristics during saturated pool boiling were investigated numerically, and a comprehensive model was proposed and developed to study the heat transfer during growth and departure of a bubble as well as bubble growth rate and departure time. Two-phase characteristics of the boiling phenomena can be captured by well-known Volume of Fluid (VOF) method. However, the VOF method is susceptible to parasitic currents because of approximate interface curvature estimations. Thus, sharp surface formula (SSF) method was employed to effectively eliminate the presence of the parasitic currents. VOF method is a volume capturing method and hence, may be subject to interface diffusion, due to the fact that interface is smeared through some number of computational cells. Interface compression scheme is applied to prevent the plausible interface diffusion of the VOF method. To avoid unrealistic temperature profiles at the solid-liquid surface, a conjugate heat transfer model was used to calculate the heat flux going into the liquid region from the heater through the solution of conduction equation in solids. Phase change at the interface was incorporated based on Hardt and Wondra’s model in which source terms are derived from a physical relationship for the evaporation mass flux. Furthermore, effects of micro region heat transfer on the departure time of the bubble was investigated. Micro region heat transfer was included in the model by solving a temporal evolution equation and incorporating the resulting heat flux in the tri-phase contact line. In this study, OpenFOAM package was used to investigate the characteristics of the bubble growth and departure as well as the wall heat flux. The model was benchmarked by comparing the simulation results to available experimental and numerical literatures, as well as analytical solutions.

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