Numerical simulation of the flow characteristics inside a novel plasma spray torch

A novel direct current non-transferred arc plasma torch that can generate a long, stable and silent plasma jet of over 350 mm in length into ambient air is studied by numerical modelling. Numerical simulation of the plasma torch operating in the current of 160 A with a mixture of 70% nitrogen and 30% argon in volume at three different gas flow rates (8.5 SLPM, 10 SLPM and 14 SLPM) are performed in a 3D domain. The renormalization group method is employed to model the turbulent flow inside the plasma torch, particularly the torch's novel channel structure. The results show that a narrow circular gap at the boundary layer of the cathode led to a region of high flow constriction and large pressure and velocity gradients. The anode counts with a novel trumpet-like structure that separates the flow in the channel and induces turbulent fluctuations in the direction transverse to the flow, which can disrupt and decrease the cold boundary layer around the arc column. The arc attachment position that obtained in simulation is in agreement with the experiment observation. The maximum velocity at the torch nozzle in simulation is increased and the length of the plasma jet in experiment is decreased with the increasing of the total gas flow rates from 8.5 SLPM, 10 SLPM to 14 SLPM. However, the maximum temperature at the torch nozzle in three different gas flow rates are varied slightly. Our results suggest that generation of a long and stable plasma jet downstream of the nozzle exit should not only focus on a relatively low gas flow rate, but depend instead on the aerodynamic characteristics of the channel flow, particularly in the anode region and downstream of the anode.