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 je t on the thermal plasma characteristics is performed to provide a design ba sis for nontransferred plasma torches operated in an ambient air of atmosph eric pressure along with shroud gas injection. The assumption of steady-sta te, axisymmetric, local thermodynamic equilibrium, and optically thin plasm a is adopted in a two-dimensional modeling of thermal plasma flow with an a nnular shroud gas shell. A control volume method and a modified semi-implic it pressure linked equations revised algorithm (known as SIMPLER) are used for solving the governing equations, i.e., the conservation equations of ma ss, momentum, and energy along with the equations describing the so-called K-epsilon model for flow turbulent kinetic energy (K) and its dissipation r ate (epsilon), and the mass fraction equations for gas mixing. The two-dime nsional distributions of temperature and flow velocity of the thermal plasm a jet as well as the air mole fraction mixed with the plasma are found in a n exterior jet expanding region outside the torch, and they are compared fo r the two cases with and without shroud gas injection. As a result of calcu lations, the flow rate of the injected shroud gas and the location of its i njector turn out to be major parameters for controlling ambient air entrain ment. The calculations also reveal that the annular injection of shroud gas surrounding the plasma jet reduces air entrainment into the plasma jet rem arkably while it does not significantly affect the plasma temperature and v elocity. The present numerical modeling suggests the optimum design and ope rating values of an argon shroud gas injector for minimizing air entrainmen t into the thermal plasma flame ejected from the nontransferred plasma torc h operated at normal pressure in the ambient atmosphere. (C) 1999 American Institute of Physics. [S0021-8979(99)05509-7].