The Study On The Entrainment Characteristics Of High Temperature Partial Ionization Gas Jets

Thermal plasma jets have been extensively employed in materials processing, such as plasma spraying, thermal plasma chemical vapor deposition synthesis, plasma welding and cutting, plasma re-melting or cladding hardening of metallic surfaces, etc. When a gas jet is issuing into an ambient gas originally at rest, the ambient gas will be entrained into the jet due to the momentum transfer between the jet fluid and the ambient gas, leading to the increase of the axial mass flux of the jet and the spatial evolution of jet temperature, velocity and species concentration profiles. The gas entrainment controls the mixing rate of the jet fluid with the ambient fluid and affects the development of the flame and the residence time of reacting gas in a reaction system and the overall chemical reaction. When materials processing is conducted in air environment, the ambient air will be entrained into the materials processing region and leads to the oxidation of the metal materials exposed to the high temperature jet. The excessive oxidation of metallic materials is often unfavorable for materials processing.The SIMPLE algorithm is used to solve the governing equations, and the K -εtwo-equation turbulence model can be employed to describe the turbulence in this study. Modeling study is preformed to reveal the entrainment characteristics of turbulent free and impinging jets at ambient temperature by choosing different jet-inlet gases, i.e. air, nitrogen, hydrogen and methane. It is shown that the computed results for the free jets agree well with the experimental correlation. In the modeling study of turbulent impinging jets, there is only a little difference between the air jets and nitrogen jets in their flow fields due to the small difference between the air and nitrogen in their molecular weights. The existence of a substrate in the impinging jet enhances significantly the mass flow rate of the ambient gases entrained into the jet due to the additional contribution to the gas entrainment of the wall jet formed along the substrate surface.The paper also conducts the modeling study to reveal how the length of the cylindrical solid shield affects the ambient air entrainment, and at a constant shield length, how the maximum temperature and the maximum velocity at the jet-inlet respectively affects the ambient air entrainment. It is found that the solid shield can appreciably change the flow pattern in the region outside the plasma main-jet, but the plasma temperature, axial velocity and argon mass fraction distributions in the main jet region are less influenced by adding the solid shield. The present study demonstrates that the mass flow rate of the ambient air entrained into the laminar or turbulent plasma jet cannot be appreciably reduced by adding a solid shield. Based on the modeling study, the paper also conducts the experimental study on the entrainment characteristics of turbulent plasma jets using a specially designed porous chamber technique. By measuring the entrained mass fluxes, it is found that the entrained mass fluxes incrases with the increase of the mass fluxes at jet-inlet, and the entrained mass fluxes also increases with the increase of the length of solid shield.