Numerical Simulation Of The Dc Arc Plasma Torch

Electrical arcs and, more generally thermal plasmas, are widely used in many industrial applications) such as cutting, welding, spraying, metallurgy, ultrafine particles synthesis and waste treatment. The understanding or the improvement of the corresponding processes or systems often requires precise modelling of the plasma. The thermal plasmas characteristics are first investigated with different turbulence models, i.e. the Reynolds Stress Model (RSM), the k-εmodel and its variants, the ReNor-malization Group (RNG) k-εmodel, the RNG k-εmodel taking into account the low Reynolds number effect and the realizable k-εmodel. The results of the RSM and the RNG k-εmodel taking into account the low Reynolds number effect are in reasonable agreement with the experiment. They both predict very close voltage, shock wave location and temperature variation along the axis to experiment. On the other hand, the other three models overestimate the turbulence effects and predict much lower velocity and temperature, especially the standard k-εmodel, which predicts the temperature is about 10000 K lower than the experiment in certain plasma jet regions.After comparing the turbulence model, we use the RNG k-εmodel taking into account the low Reynolds number effect to investigate the plasma cutting arc. The effects of plasma-gas swirl flow, the process parameters, torch geometry and the shielding gas on the plasma cutting arc are investigated. When the inlet gas swirl number increases, the maximum pressure location moves to the plenum chamber edge due to centrifugal force, the minimum pressure location moves much closer to the axis. The difference between the maximum and minimum pressure increases with swirl number. This will constricts the arc and leads to large current density and Ohm heating near the axis, and causes much higher temperature near the cathode. It is indicated that the swirl gas accelerates cathode erosion for two reasons: increasing plenum chamber pressure and changing the flow patterns in the vicinity of cathode. It is also found that, the gas with large swirl number leads swirl velocity component to be larger at cutting location according to angular momentum conservation. It has been suggested that this causes the different bevel angles for the left and right edges of the cutting kerf. In the parameters study, it is shown that gas flow rate, arc current, nozzle bore length and radius have essential effects on plasma arc characteristics. Long nozzle torch can provide high velocity plasma jet with high heat flux. Both arc voltage and chamber pressure increase with the nozzle length. Small radius nozzle torch increases the plenum chamber pressure and providing off-axis maximum velocity after shock wave. High arc current increases plasma velocity and temperature, enhances heat flux and augments chamber pressure and thus, the shock wave. Strong mass flow has pinch effect on plasma arc inside the torch, enhances the arc voltage and power, therefore increases plasma velocity, temperature and heat flux. After comparing two different torch geometries, it is found that the shielding flow has no significant effects on plasma velocity and temperature except the shock wave region. The shielding flow decreases the shock wave, and increases the arc voltage due to cooling effect. In the impinging geometry, shielding flow will crash the plasma jet after the nozzle exit and slightly increases the pressure in the torch. It is shown that the component of shielding gas has no significant effect on plasma cutting arc. The mole fraction of oxygen decreases very slowly along the axis and is still more than 90% at 10 mm downstream the nozzle exit.High-power hydrogen plasma torch (5 MW-10 MW) is of great importance in coal pyrolysis for producing acetylene and other industrial applications. Considering the large computational domain, the RSM is used to investigate high-power plasma torch. The calculated arc voltage and the electric potential distribution at electrodes are very close to the experimental data. It is found the swirl number has no significant effect on the plasma characteristics except the first two electrodes region. Both the arc voltage and energy-efficiency increase with the mass flow rate. It is shown that large mass flow will constrict the plasma arc, and therefore enhance the maximal temperature and velocity at the torch outlet.