Numerical thermofluid simulation on tandem type of induction thermal plasmas with and without current modulation in a lower coil

Abstract

This paper describes numerical calculations on tandem-type induction thermal plasmas with and without current modulation of the lower coil. The tandem type of induction thermal plasma (tandem-ITP) has been developed with two independent rf coils in one plasma torch for stable operation and spatial control of temperature field in ITP. Furthermore, the coil current modulation for tandem-ITP, i.e. tandem type of pulse-modulated induction thermal plasma (tandem-PMITP), is expected to offer a temporo-spatially varying temperature field in the plasma torch. First, as described in this paper, a numerical thermofluid model was developed for tandem-ITP/PMITP with an upper coil and a lower coil to investigate unique temperature and gas flow fields. Secondly, the influence of gap length between the upper coil and the lower coil was studied in the tandem-ITP under steady state conditions. Results show that a shorter gap length between two coils of the tandem-ITP is favourable for materials processing because it suppresses unfavourable radial gas flow to the wall in the plasma torch. Thirdly, transient calculation was conducted for the tandem-PMITP with current modulation in the lower coil to investigate effects of an off-time modulation parameter on the temperature field. Results showed that coil current modulation provides unique temporo-spatially varying temperature and gas flow fields. Finally, calculation results for a tandem-PMITP were compared with those for a single-coil PMITP. Comparison revealed that tandem-PMITP has higher robustness in modulated induction thermal plasma by maintaining a higher-temperature region than that of the single-coil MITP. The tandem-PMITP can provide more controlled temperature and gas flow fields.