A THERMODYNAMIC ANALYSIS OF NONEQUILIBRIUM ARGON PLASMA TORCHES

The nonequilibrium process of argon plasma torches is analyzed theoret ically. Thermodynamic diagrams of different degrees of ionization are developed to aid in understanding and analyzing the transition from ch emical equilibrium to frozen flow in dc plasma torch operations. A the rmodynamic model is developed to describe the nonequilibrium processes in a de argon plasma torch. In the model the ionization process is ap proximated as a constant-pressure heating process, with little deviati on from the equilibrium state upon completion of heating. If the plasm a flow is frozen shortly after heating, the entropy increase is small during the transition from equilibrium to frozen flow. In this case th e frozen flow will have nearly the same composition and entropy as the flow at the heating section exit. For singly ionized argon plasmas in the entropy range relevant to de torch operation, the frozen flow sol utions on the affinity-pressure diagram are found to be insensitive to entropy change. Therefore the present model predicts that argon plasm as generated at different power levels will have almost identical affi nity at the torch exit for the same operating pressure. This predictio n agrees with experimental observations except for very low torch powe r levels.