Effects of the Length of a Cylindrical Solid Shield on the Entrainment of Ambient Air into Turbulent and Laminar Impinging Argon Plasma Jets

Abstract

When materials processing is conducted in air surroundings by use of an impinging plasma jet, the ambient air will be entrained into the materials processing region, resulting in unfavorable oxidation of the feedstock metal particles injected into the plasma jet and of metallic substrate material. Using a cylindrical solid shield may avoid the air entrainment if the shield length is suitably selected and this approach has the merit that expensive vacuum chamber and its pumping system are not needed. Modeling study is thus conducted to reveal how the length of the cylindrical solid shield affects the ambient air entrainment when materials processing (spraying, remelting, hardening, etc.) is conducted by use of a turbulent or laminar argon plasma jet impinging normally upon a flat substrate in atmospheric air. It is shown that the mass flow rate of the ambient air entrained into the impinging plasma jet cannot be appreciably reduced unless the cylindrical shield is long enough. In order to completely avoid the air entrainment, the gap between the downstreamend section of the cylindrical solid shield and the substrate surface must be carefully selected, and the suitable size of the gap for the turbulent plasma jet is appreciably larger than that for the laminar one. The overheating of the solid shield or the substrate could become a problem for the turbulent case, and thus additional cooling measure may be needed when the entrainment of ambient air into the turbulent impinging plasma jet is to be completely avoided.

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.…”

“…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.…”

Modeling on the Momentum and Heat/Mass Transfer Characteristics of an Argon Plasma Jet Issuing into Air Surroundings and Interacting with a Counter-Injected Argon Jet

“…The main assumptions employed in this study include (i) the gas for the plasma jet generation and the counter-flow injection are argon, but the ambient gas is air; (ii) the flow is subsonic, steady and axisymmetrical; (iii) the plasma is in the local thermodynamic equilibrium (LTE) state and optically thin to radiation; (iv) the swirling velocity component is negligible; and (v) the diffusion of the surrounding air into the argon plasma jet can be handled by using the combined-diffusion-coefficient method [11,12] for the laminar case and using the turbulence-enhanced combined-diffusion-coefficient method [13][14][15][16] for the turbulent case.…”

“…If the gas mixture can be assumed to be in the LTE state, there is no loss of accuracy in this procedure [12]. In the present study, such a combined-diffusion-coefficient method proposed by Murphy [11,12] is employed to treat the diffusion of the ambient air into the laminar plasma reactor, while the turbulence-enhanced combined-diffusion-coefficient method proposed in [13] is used to deal with the diffusion of the ambient air into the turbulent plasma reactor, similarly to that employed in [13][14][15][16].…”