Numerical Simulation Of Arc Dynamics In A D.C. Non-transferred Plasma Torch

Electrical arcs and, more generally thermal plasma has been widely used in the fields of spraying, cutting, welding and chemistry engineering due to its characteristics of high temperature, high speed and high energy density. Because the fluctuation and instability of electric arc would affect the life of electrode and coating quality, the study of physical processes in the plasma torch is extremely important, and a better understanding of arc dynamics inside the torch would contribute to the design optimization of plasma torches. A three-dimensional transient local thermodynamic equilibrium (LTE) magneto-hydrodynamic model of non-transfer D.C. plasma torch is presented. The mass, momentum, energy conservation equations and Maxwell’s equations are solved simultaneously to predict the electric arc dynamics and the effect of arc behavior on the characteristics of plasmas inside the plasma torch. The results show that the signal of arc voltage presented restrike mode which has large voltage fluctuations with a saw-tooth shape profile and the arc attachment on the anode surface present jump distribution. The plasma flow and heat transfer inside the plasma torch is transient. The plasma temperature declines along the central axis, and the maximum point is around the cathode tip inside the torch. While the plasma velocity increases firstly and then decreases along the central axis. Affected by the fluctuation of arc root inside the plasma torch, the distributions of temperature and velocity at the torch exit are similar as Gaussian distribution.Based on the model given above, a three-dimensional transient model with swirl flow is presented. The influence of gas injection methods on Ar-H2 electric arc instabilities and plasma characteristics are investigated under certain gas flow and electric current. The results show that the streamlines of plasma are characterized by spiral distribution and the amplitude and frequency of voltage fluctuation are higher when operated with swirl flow. The arc attachment can also be observed with a small circumferential movement. Compared with straight flow, the average maximum temperature is lower around the cathode tip with swirl flow. The effect of both gas injection methods on the plasma velocity inside the torch is not obvious. The plasma temperature distributions at the torch exit with both ways are similar, while the arc operated with swirl flow has a higher average maximum velocity.Based on the first part of work, the effect of gas flow rate and arc current on the Ar-H2 arc behavior and plasma characteristics inside a non-transfer D.C. plasma torch is further discussed when the arc operated with swirl flow. The results show that the fluctuation of arc length inside the plasma torch increases with the increase of gas flow rate, but the effect of argon flow rate is greater than the hydrogen flow rate. With the increase of arc current, the fluctuation of arc length inside the plasma torch presents a first increases and then decreases trend. Compared with the gas flow rate, the effects of arc current is less. The average arc voltage also increases with the increase of gas flow rate, while the effect of argon flow rate is less than the hydrogen flow rate. With the increase of arc current, the average arc voltage inside the plasma torch also presents a first increases and then decreases trend. It means that the peak voltage in the volt-ampere characteristics curve decrease with arc current increase, and then increase with arc current continue to increase. The gas flow rate and arc current have different effects on the plasma temperature and velocity inside non-transfer D.C. plasma torch. With the comparative study on the effect of gas flow and arc current, this paper finds that the one most influential to the plasma temperature is arc current, it is moreover the hydrogen flow rate, influence the minimum one is the argon flow rate. While the one most influential to the plasma velocity is the argon flow rate, it is moreover the hydrogen flow rate, influence the minimum one is arc current.