Extinguishing phenomenon and critical discharge boundaries of argon and molecular-gas-seeded argon pulse-modulated induction thermal plasmas

摘要

A stable discharge region of pulse modulated induction thermal plasma at atmospheric pressure has been determined in terms of the duty factor (DF) and the shimmer current level (SCL) both experimentally and numerically. The experiment was performed with a MOSFET-employed inverter-feed pulse modulated power supply unit having a maximum power of up to 50 kW. The on-time of the pulsing signal was fixed at 10 ms while the off-time was varied to determine the critical/minimum DF up to which plasma discharge sustained, for a fixed SCL. In this way, for different SCLs, critical/minimum DFs were determined for different gas combinations. The gas flow was composed of 100 lpm argon and 2.5 lpm H-2, N-2 or CO2 as secondary gas through the sheath channel only. Steady state coil currents for Ar, Ar-H-2, Ar-N-2 and Ar-CO2 were 244 A, 263 A, 296 A and 296 A, respectively. In the numerical part, a two-dimensional local thermodynamic equilibrium (LTE) code was used to predict the discharge boundary for the same operating conditions as those of the experiment. Both experimental and simulated results reveal that injection of dissociative molecular gases greatly shrinks the stable discharge region, which is attributed to the lowering of temperature and particle density. It was found that the stable discharge region predicted numerically was smaller than that of the experimental one. This implies that a non-LTE plasma has a discharge zone wider than that of an LTE plasma. [References: 17]