Numerical analysis of shroud gas effects on air entrainment into thermal plasma jet in ambient atmosphere of normal pressure

A numerical analysis of the influence of air entrainment into the plasma jet on the thermal plasma characteristics is performed to provide a design basis for nontransferred plasma torches operated in an ambient air of atmospheric pressure along with shroud gas injection. The assumption of steady-state, axisymmetric, local thermodynamic equilibrium, and optically thin plasma is adopted in a two-dimensional modeling of thermal plasma flow with an annular shroud gas shell. A control volume method and a modified semi-implicit pressure linked equations revised algorithm (known as SIMPLER) are used for solving the governing equations, i.e., the conservation equations of mass, momentum, and energy along with the equations describing the so-called K-epsilon model for flow turbulent kinetic energy (K) and its dissipation rate (epsilon), and the mass fraction equations for gas mixing. The two-dimensional distributions of temperature and flow velocity of the thermal plasma jet as well as the air mole fraction mixed with the plasma are found in an exterior jet expanding region outside the torch, and they are compared for the two cases with and without shroud gas injection. As a result of calculations, the flow rate of the injected shroud gas and the location of its injector turn out to be major parameters for controlling ambient air entrainment. The calculations also reveal that the annular injection of shroud gas surrounding the plasma jet reduces air entrainment into the plasma jet remarkably while it does not significantly affect the plasma temperature and velocity. The present numerical modeling suggests the optimum design and operating values of an argon shroud gas injector for minimizing air entrainment into the thermal plasma flame ejected from the nontransferred plasma torch operated at normal pressure in the ambient atmosphere. (C) 1999 American Institute of Physics. [S0021-8979(99)05509-7].