Three Dimensional And Transient Numerical Modeling Of Plasma Jet

A three-dimensional, transient, turbulent, local thermodynamic equilibrium (LTE) model of plasma jet was established to study plasma characteristics during plasma spray and the impact of temperature and velocity fields in plasma torch along with the entrainment of ambient atmosphere on jet patterns by using computational fluid dynamics software ANSYS CFX. The effects of process parameters on the arc fluctuations and plasma temperature and velocity distributions during the plasma spray were also discussed. The simulated jets were compared with experiment results observed by a high-speed camera.Simulation results show that the fluctuations of plasma jet are caused by both the anode arc reattachment process and the entrainment of ambient atmosphere. The simulated jets agree well with experiment results. 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 breaks down and a new arc root is formed. 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. When the argon flow rate, the hydrogen flow rate and the arc current were fixed at 45slm,9slm and 400A respectively, the predicted time-average arc voltage was 59.6 V, and the arc frequency was about 8.7 kHz. The maximum temperature and velocity at the torch exit were 17316 K and 1243 m/s respectively. As the jet developed, the temperature and velocity started greatly decreasing at about 27.4mm downside from the torch exit, because of the enhancing cold-air entrainment into the plasma jet as well as the increasing exchange of energy along with momentum between plasma jet and cold air. Compared with the temperature distribution, the plasma velocity distribution had stronger three-dimensional characteristics.Every process parameter has different sensitivities for arc fluctuations. As the arc current increases, the current density increases while average arc voltage increases. The argon flow rate has little influence on the arc fluctuations. The arc current density and average arc voltage increase with increasing hydrogen flow rate. The process parameters differently affect the average maximum temperature and velocity of plasma during the plasma spray. The plasma temperature and velocity increase with increasing arc current. The plasma temperature and velocity significantly increase with increasing hydrogen flow rate while velocity increases more than temperature. With the increase of argon flow rate, both the plasma temperature and velocity increase. With the increase of argon flow rate, the plasma temperature shows little change, while velocity significantly increases. The effects of process parameters on the average maximum temperature and velocity of plasma jet are similar within the plasma gun.