Vittorio Colombo

Two-temperature thermodynamic and transport properties of argon–hydrogen and nitrogen–hydrogen plasmas

Two-temperature thermodynamic and transport properties of argon–hydrogen and nitrogen–hydrogen plasma mixtures are presented, chemical equilibrium being achieved. The calculations of transport properties are carried out using the Chapman–Enskog method up to the third order; when electron temperature differs from that of heavy particles, calculations are performed following both a recent two-temperature theory by Rat et al that retains the coupling between electrons and heavy particles and a simplified decoupling theory proposed by Devoto. No relevant discrepancies between results obtained using these two approaches have been found, allowing the simplified method of Devoto to be still used in the computation of non-equilibrium transport properties like thermal conductivity, electrical conductivity and viscosity, with the exception of some diffusion coefficients. Results for composition, mass density, specific heat, thermal conductivity, electrical conductivity and viscosity of atmospheric pressure plasmas in the electron temperature range 300–40 000 K are reported.

Three-dimensional modeling of inductively coupled plasma torches*

A 3D model for the simulation of inductively coupled plasma torches (ICPTs) working at atmospheric pressure is presented, using a customized version of the computational fluid dynamics (CFD) commercial code FLUENT®. The induction coil is taken into account in its actual 3D shape, showing the effects on the plasma discharge of removing the axisymmetric hypothesis of simplification, which characterizes 2D approaches. Steady flow and energy equations are solved for optically thin argon plasmas under the assumptions of local thermodynamic equilibrium (LTE) and laminar flow. The electromagnetic field equations are solved on an extended grid in the vector potential form. In order to evaluate the importance of various 3D effects on calculated plasma temperature and velocity fields, comparisons of our new results with the ones obtainable from 2D models and from an improved 2D model that includes 3D coil effects are presented. 3D results are shown for torches working under various geometric and operating conditions and with different coil shapes, including conventional helicoidal, as well as planar, elliptical, and double-stage configurations.

High-speed imaging in plasma arc cutting: a review and new developments

The aim of this paper is twofold: (i) to review all the achievements in our understanding of the phenomena related to plasma arc cutting (PAC) technology by means of high-speed camera (HSC) imaging and flow visualization techniques and (ii) to report on new studies that make use of recent and advanced instrumentation for HSC diagnostics, also highlighting some previously uncovered research subjects. In the last decade HSC imaging and flow visualization techniques have progressed considerably as a powerful qualitative diagnostic technique for investigating some of the fundamental phenomena typically occurring in PAC technology. More recently, HSC imaging has also been used to investigate pre-cut phases in PAC analysis, such as pilot arcing and piercing of mild steel and stainless steel plates with dual gas torches in various operating conditions, providing new insight into the process and highlighting some interesting plasma behaviour. HSC imaging of pilot arcing has been used to investigate the influence of the arc current, plasma pressure and swirl strength on the shape of the arc, on the type of the rotational motion of its attachment on the nozzle tip and to track trajectories and velocities of hafnium particles emitted from the electrode insert during that phase. HSC imaging can also highlight the behaviour of the arc during piercing phases and the possible presence of short non-destructive double arcing, otherwise impossible to recognize.

A two-dimensional nodal model with turbulent effects for the synthesis of Si nano-particles by inductively coupled thermal plasmas

Nano-particle synthesis by means of inductively coupled plasma torches is a material process of large technological interest. Numerous parameters are involved in the optimization of this process; hence the development of numerical models for the prediction of thermal and magneto-fluid dynamics fields, precursor powder trajectories and thermal history, as well as nano-particle formation and growth, is necessary for the up-scaling of these devices from laboratory batch production to an industrial continuous process. In this work, a two-dimensional (2D) discrete-type model (nodal model) for the analysis of nano-powder nucleation and growth is presented, taking into account convection, diffusion and turbulent effects on particle formation. Discrete-type models feature high precision and reveal a great deal of information useful for clarifying the nano-particle formation process. Using Si as the precursor material, 2D simulations of a nano-particle synthesis RF plasma apparatus with a reaction chamber are carried out. Good agreement is found when comparing results obtained with this model with those coming from a well-established nucleation-coupled moment method. Moreover, the extended amount of obtainable information that characterizes the nodal model is underlined.

Two-temperature thermodynamic and transport properties of carbon?oxygen plasmas

Thermodynamic and transport properties of different carbon–oxygen plasmas mixtures are presented for both thermal equilibrium and non-equilibrium conditions: to the knowledge of the authors, the latter data have not been reported in the literature. The calculations of transport properties are carried out using the Chapman–Enskog method up to the third order; properties for plasmas out of equilibrium have been obtained using both the two-temperature theory developed by Rat et al and a simplified theory by Devoto, that neglects the coupling between electrons and heavy particles; it is shown that for carbon–oxygen mixtures no differences between results obtained using these two theories can be appreciated for thermal and electrical conductivities; some discrepancies have been found for ordinary diffusion coefficients of the type electron-heavy particle. Moreover, local thermodynamic equilibrium results for transport properties obtained using Lennard-Jones potentials have been compared with results obtained using more recent potential data and with available results reported in the literature. Results for composition, mass density, specific heat, thermal conductivity, electrical conductivity and viscosity of atmospheric pressure plasmas in the electron temperature range 300–30 000 K are reported.

A three-dimensional investigation of the effects of excitation frequency and sheath gas mixing in an atmospheric-pressure inductively coupled plasma system

A three-dimensional numerical model for the simulation of the behaviour of a commercial inductively coupled plasma torch with non-axisymmetric reaction chamber has been developed, taking into account turbulence and gas mixing through RNG k– theory and the combined diffusion approach of Murphy, respectively.The effects of changing coil current frequency, the hydrogen mixing in an argon primary gas and the flow patterns and temperature distributions which take place in a reaction chamber with a lateral gas outlet system and two observation windows have been investigated, with the final aim of setting up a computational tool able to predict the main features of plasma assisted treating and processing of injected raw materials.Three-dimensional shapes of the temperature, velocity and mass fraction fields have been obtained and analysed for an Ar–H2 mixture at atmospheric pressure. Computations have been performed with two different coil current frequencies, i.e. 3 and 13.56 MHz, showing that for the lower value the 3D effects in the discharge are enhanced.Accurate mixing and demixing mechanisms have been investigated in both cases, including considerations on the relative importance of different thermal diffusion contributions due to mole fraction and temperature gradients.Temperature distributions in the reaction chamber for different cases have been correlated with different flow patterns and recirculation flows which take place as a consequence of the non-axisymmetry of the reaction chamber.

Two-dimensional time-dependent modelling of fume formation in a pulsed gas metal arc welding process

Gas-metal arc welding (GMAW) is a process in which a non-constricted plasma arc is ignited between a workpiece that works as the cathode and a metal wire that works as the anode; the latter is melted by the heat from the arc and a metal transfer occurs from the wire to the workpiece. Following recent studies on health effects for welding workers exposed to fume inhalation, a huge effort is currently being devoted to the investigation of fume formation during the welding process and to the development of processes with lower fume production rate. In the welding process, fumes are generated by nucleation and growth of nanoparticles from the metal vapour coming from evaporating weld pool, droplets and metal wire.Even if experimental try and fail approaches have been adopted to develop operating conditions that induce a lower formation of fumes, modelling is a valuable tool that provides insight on those physical processes occurring during the formation phase that cannot be easily monitored by diagnostics. In this work, the simulation of fume formation in pulsed GMAW process is reported taking into account the metal transport and metal vapour formation in a self-consistent approach using the Volume-of-Fluid (VoF) method and modelling fume nanoparticles production using the method of moments (MoM) for the solution of the aerosol general dynamic equation. While this method has been widely used for the modelling of nanoparticle synthesis in thermal plasma reactors, this is the first attempt to implement the MoM approach in modelling fume formation in pulsed GMAW.

Three-dimensional investigation of particle treatment in an RF thermal plasma with reaction chamber

Particle treatment in a commercial inductively coupled plasma torch with a non-axisymmetric reaction chamber has been studied using a three-dimensional numerical model, including particle turbulent dispersion. Computations have been performed for two different coil current frequencies (3 and 13.56?MHz), showing that for lower frequencies particle trajectories deviate from their initial axial direction as a consequence of non-axisymmetric thermo-fluid-dynamic fields: these effects arise when the first and last coil turns induce unbalanced Lorentz forces in the discharge that prevail over fluid-dynamic inertia. For higher frequencies, almost axial particle trajectories have been predicted.Moreover, computations for different mass flow rates of the precursor particles and for different axial positions of the injection probe are shown, these parameters having an important influence on the effectiveness of the powder treatment process.

Characterization of a Cold Atmospheric Pressure Plasma Jet Device Driven by Nanosecond Voltage Pulses

The structure, fluid-dynamic behavior, temperature, and radiation emission of a cold atmospheric pressure plasma jet driven by high-voltage pulses with rise time and duration of a few nanoseconds have been investigated. Intensified charge-coupled device (iCCD) imaging revealed that the discharge starts when voltage values of 5-10 kV are reached on the rising front of the applied voltage pulse; the discharge then propagates downstream the source outlet with a velocity around 107-108 cm/s. Light emission was observed to increase and decrease periodically and repetitively during discharge propagation. The structure of the plasma plume presents a single front or either several branched subfronts, depending on the operating conditions; merging results of investigations by means of Schlieren and iCCD imaging suggests that branching of the discharge front occurs in spatial regions where the flow is turbulent. By means of optical emission spectroscopy, discharge emission was observed in the ultraviolet-visible (UV-VIS) spectral range (N2, N2+ , OH, and NO emission bands); total UV irradiance was lower than 1 μW/cm2 even at short distances from the device outlet (<;15 mm). Plasma plume temperature does not exceed 45 °C for all the tested operating conditions and values close to ambient temperature were measured around 10 mm downstream the source outlet.

Modelling for the optimization of the reaction chamber in silicon nanoparticle synthesis by a radio-frequency induction thermal plasma

The optimization of the reaction chamber for the silicon nanoparticle synthesis process by a radio-frequency induction thermal plasma is addressed using a plasma thermo-fluid-dynamic model coupled with electromagnetic field equations and with a moment model for nanoparticle transport. Various reaction chamber geometries composed of two parts?a conical top and a cylindrical bottom?are evaluated in terms of the yield of the synthesis process, the presence of recirculation flow patterns that may affect the uniformity of the produced nanoparticles and the size distribution of nanoparticles at the chamber outlet. Turbulent diffusion is suggested as the physical phenomenon that leads to nanoparticle deposition onto the walls of the reaction chamber. The injection of a suitable gas along the walls of the reaction chamber at the axial position, where the nanoparticle nucleation takes place, is proven to be effective in increasing the synthesis process yield.