Comparison of Laminar and Turbulent Thermal Plasma Jet Characteristics—A Modeling Study

Abstract

Modeling results are presented to compare the characteristics of laminar and turbulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respectively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is comparatively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbulent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80-250 A and argon flow rates (1.8-7.0)×10 −4 kg/s.

Direct Measurement of the Gas Entrainment Into a Turbulent Thermal Plasma Jet

“…4-7 are roughly consistent with the modeling predictions of Ref. [19][20][21], although the entrained gases are somewhat different (nitrogen is used in this experiment while air is used in the modeling studies). As mentioned above, the plasma-forming gas (argon) of the plasma torch is supplied through three channels, i.e.…”

“…It was shown that the entrained air mass flow-rate almost linearly increases with increasing jet-inlet gas velocity (or mass flow rate) for a fixed jet-inlet temperature and with increasing axial distance. And the values of the entrained gas mass flow rate normalized to the jet-inlet mass flow rate (m 1 /m 0 ) were as large as about 16 at the axial distance of 100 mm [19]. The present experimental results presented in Figs.…”

“…7, where the variations of the entrained mass flow rate normalized to the jet-inlet mass flow rate (m 1 /m 0 ) are shown as the function of the jet-inlet mass flow rate (m 0 ) for the three different chamber lengths. It is seen that the value of the normalized mass flow rate m 1 /m 0 increases with The modeling study on the entrainment characteristics of turbulent argon thermal plasma jets issuing into ambient air was carried out in [19][20][21]. It was shown that the entrained air mass flow-rate almost linearly increases with increasing jet-inlet gas velocity (or mass flow rate) for a fixed jet-inlet temperature and with increasing axial distance.…”

“…A variety of diagnostic techniques have been employed to study the entrainment of ambient gas into the turbulent plasma jet, and many measured results have also been reported concerning the evolution of plasma parameters in plasma jets [5,6,9,10,[14][15][16]. The modeling results reported in [19][20][21] showed that since turbulent transport mechanism is dominant in the turbulent plasma jet, the mass flow rate of the ambient air entrained into the turbulent plasma jet is comparatively large (e.g. maybe about 20 times larger than the mass flow rate at the jet inlet for a turbulent thermal plasma jet with 100-mm length) and almost directly proportional to the mass flow rate at the jet inlet.…”

Three-dimensional modeling of heat transfer and fluid flow in laminar-plasma material re-melting processing

“…The main assumptions used in the modeling study of the plasma jet include steady, laminar and axi-symmetrical flow with negligible swirling velocity component; local thermodynamic equilibrium (LTE) and optically thin plasma; diffusion of the ambient air into the plasma jet can be handled by the combined-diffusion-coefficient method [9][10][11][12][13]. The governing equations including the mass, momentum, energy, and gas species conservation equations in the cylindrical coordinate system are similar to those used in [11][12][13] and can be written as follows:…”