Conditions for Plasma Jet Formation in a Laminar Plasmatron

Abstract

The laminar plasma jet is an effective tool of plasma technologies for material processing thanks to great values of bulk temperature and high penetration of a plasma stream. However, laminar stream formation in an axial jet plasmatron is connected with a number of difficulties caused by physical restrictions. Conditions of formation of a laminar stream at the exit from a plasmatron nozzle with interelectrode inserts are found when using the most widespread plasma-forming gases. With use of a method of universal characteristics of arc and heat-transfer model of arc of jet plasmatrons, calculation of characteristics of the arc compressed by walls of the discharge channel is carried out; the necessary arc compression ratio in the stable laminar stream mode and its extreme values taking into account the permissible thermal load on walls of the discharge channel in the arc stabilization zone are defined. It is shown that reduction of the diameter of the plasmatron discharge channel reducing diameter of the laminar plasma stream exhausting from a plasmatron nozzle reduces the permissible values of the arc compression ratio. Influence of the cooling mode of walls of the plasmatron arc channel formed by interelectrode inserts on the arc compression ratio at separate water cooling of each interelectrode insert and at their joint cooling is shown. All calculations were carried out for the values of gas consumption and discharge channel length meeting a basic condition of laminar stream formation of Re < 150. The arc current values necessary for a laminar plasma stream formation, depending on the discharge channel section and a plasma-forming gas consumption taking into account thermal properties of the gas environment, are determined. The method proposed in this article makes it possible to calculate an arc column temperature profile in the stabilization zone of a laminar plasmatron and can be used when developing plasmatrons with a stable laminar stream of set plasma-forming gas.