Simulation Study Of Transient Discharge Characteristics And Flow Field Of Microwave Plasma Torch

Microwave plasma technology is widely used in the fields of material surface treatment,gas catalytic conversion and chip etching.With its advantages of high energy density,many active particles,and controllable temperature and shape,it brings a lot of opportunities and challenges to high and new technology research fields.Taking the microwave plasma torch as the research object,a multi-physics field coupling model is established to investigate the transient discharge and flow field characteristics of the microwave plasma torch.To gain an insight view of the transient discharge characteristics of the microwave plasma torch,a multi-physics model including Navior-Stokes equations,electromagnetic equations,chemical reaction model and gas transport model is established.The chemical reaction model and the gas transport model are modelled externally,and imported into COMSOL software for iterative solution.The complex physical phenomena such as transient microwave discharges,ohmic heating and skinning effects are successfully reproduced in simulation.The evolution of parameters such as temperature,magnetic field,electric field and Joule heating rate inside the coaxial microwave torch with time is obtained,the chemical composition distribution along the central axis of the microwave torch is calculated.The research show that a strong electric field appears near the sharp ends of the inner conductor at the early stage of the plasma formation.The magnetic field strength is the highest along the surface of the inner conductor due to the coaxial design of the plasma torch.After the plasma is stabilised,a triangular zone of strong magneticfield is generated in the plasma region.The Joule heating rate is distributed in a needle-like pattern above the inner conductor.In the core region of the plasma,O and C atoms are the predominant chemicals.For the interaction mechanism between the electromagnetic field and the flow field of the microwave plasma torch,the effect of different working parameters(microwave power,microwave frequency and CO2 volume flow rate)and structural parameters(inner conductor length and outer conductor radius)on the flow field characteristics of the microwave torch is investigated in this paper.The results show that the optimum microwave power is 220W.Increasing the microwave power leads to an increase in the temperature and volume of the plasma,but a decrease in the Joule heating rate,thus the efficiency of microwave coupling is reduced.Microwave frequency has less effect on the plasma,and the power loss of microwaves in the resonant cavity increases when the frequency is increased,so the most suitable frequency for decomposing CO2 is 1.4GHz.When the volume flow rate is small,the electric field strength and Joule heating rate are low,and the microwave energy efficiency is reduced.The CO2 decomposition rate is reduced when the volume flow rate is larger,so a volume flow rate of 12L/min is more conducive to the CO2 conversion.The length reduction of the inner conductor and the radius increase of the outer conductor can expand the range of coupling between the microwave and the flow field.The plasma volume increases,and the temperature decrease in the axis of the microwave torch becomes slower.More microwave energy is used to ionise charged particles,and the main chemicals present in the plasma region are changed.The results of this study provide theoretical guidance for improving the CO2 conversion efficiency and optimising the structure of the next generation coaxial microwave torch.