Three Dimensional Transient Modeling Of Arc Dynamics And Plasma Jet In A DC Plasma Torch

Based on the MHD (Magnetohydrodynamics) equations, a three-dimensional, transient, turbulent, LTE (Local Thermodynamic Equilibrium) model was developed to study the anode arc root reattachment process, the interaction between the arc and the gas flow, and the effects of arc fluctuations on plasma temperature and velocity distributions inside an argon-hydrogen plasma torch using computational fluid dynamics software ANSYS CFX. The effects of process parameters and the plasma torch nozzle structure and dimension on the arc fluctuations, plasma temperature and velocity distributions at the torch exit were also discussed to explore the approach of controlling the arc fluctuations by optimizing process parameters and nozzle structure and dimension.Simulation results show that as the arc is dragged by the cold incoming gas, the anode arc root is pushed downstream and the arc length increases. Therefore, the arc voltage and the local electric field strength increase. When the local electric field strength is high enough, the arc breakdowns and a new arc root is formed at the opposite side of the original attachment. Once a new attachment is formed, the arc is dragged downstream by the flow until the arc breakdowns again. The above process repeats, hence the arc voltage changes periodically with the time. The arc voltage fluctuations inside the plasma torch lead to the fluctuations of plasma temperature and velocity at the torch exit. The plasma flow and heat transfer inside the plasma torch have strong three-dimensional effects, moreover, the plasma velocity distribution has stronger three-dimensional effect than temperature distribution. When the arc current, the argon flow rate and the hydrogen flow rate are fixed at 500A,50slm and 9slm respectively, the predicted average arc voltage is 53.4V, and the arc fluctuation frequency is about 9.25kHz. Besides, the average maximum temperature of about 20622K and velocity of about 1680m/s are obtained at the torch exit.Every process parameter has different sensitivities for arc voltages and plasma temperature and velocity. The hydrogen flow rate affects the arc fluctuations most, the argon flow rate does less, and the arc current does the least. But the arc current affects the plasma temperature and velocity most, the hydrogen flow rate does less, and the argon flow rate does the least. When the arc current increases in the range of low arc current, the arc connecting column is shorter and so the average arc voltage decreases and the fluctuation frequency increases. When the arc current increases to about 600A, the arc current has little influence on the arc fluctuations. Besides, the plasma temperature and velocity increase significantly with increasing the arc current. The increase of the argon flow rate or hydrogen flow rate can increase the arc length and the average arc voltage, and decrease the fluctuation frequency. With the increase of argon flow rate, the plasma temperature has a small increase and the velocity increases obviously, but the plasma temperature and velocity both increase significantly with increasing the hydrogen flow rate.The structure and dimension of plasma torch nozzle affect the arc voltages and plasma temperature and velocity in various degrees. The nozzle channel length has little influence on the arc fluctuations, the compressed angle affects the arc fluctuations most, the expanded angle does less, and the channel aperture has smaller influence on the arc voltages than the expanded angle. But the order of these factors to the plasma temperature and velocity are channel aperture, expanded angle, compressed angle and the nozzle channel length. The average arc voltage has a small raise and the fluctuation frequency increases with increasing the nozzle channel length. The plasma temperature and velocity increase first and then decrease with increasing nozzle channel length, and the maximum values are obtained when the channel length is 9.1mm. If the channel aperture is increased, the arc column is shorter and so the average arc voltage and its fluctuation frequency decrease, moreover, the plasma temperature and velocity both reduce with increasing the channel aperture. The increase of the expanded angle favors the arc root attachment upstream. Hence, the average arc voltage decreases, the fluctuation frequency increases and the plasma temperature and velocity decrease. Moreover, the compressed angle increase has the same effect on the arc fluctuations and plasma temperature and velocity than that of the expanded angle.