A modelling study is performed to investigate the characteristics of both plasma flowand heat transfer of a laminar non-transferred arc argon plasma torch operated at atmosphericand reduced pressure.It is found that the calculated flow fields and temperature distributions arequite similar for both cases at a chamber pressure of 1.0 atm and 0.1 atm.A fully developed flowregime could be achieved in the arc constrictor-tube between the cathode and the anode of theplasma torch at 1.0 atm for all the flow rates covered in this study.However the flow field couldnot reach the fully developed regime at 0.1 atm with a higher flow rate.The arc-root is alwaysattached to the torch anode surface near the upstream end of the anode,i.e.the abruptly expandedpart of the torch channel,which is in consistence with experimental observation.The surroundinggas would be entrained from the torch exit into the torch interior due to a comparatively largeinner diameter of the anode channel compared to that of the arc constrictor-tube.
Results observed experimentally are presented,about the DC arc plasma jets andtheir arc-root behaviour generated at reduced gas pressure without or with an applied magneticfield.Pure argon,argon-hydrogen or argon-nitrogen mixture was used as the plasma-forming gas.A specially designed copper mirror was used for a better observation of the arc-root behaviouron the anode surface of the DC non-transferred arc plasma torch.It was found that in thecases without an applied magnetic field,the laminar plasma jets were stable and approximatelyaxisymmetrical.The arc-root attachment on the anode surface was completely diffusive whenargon was used as the plasma-forming gas,while the arc-root attachment often became constrictivewhen hydrogen or nitrogen was added into the argon.As an external magnetic field was applied,the arc root tended to rotate along the anode surface of the non-transferred arc plasma torch.
