S. Ghorui

Non-equilibrium modelling of an oxygen-plasma cutting torch

A two-temperature, axi-symmetric, chemical non-equilibrium model has been developed for an oxygen-plasma cutting torch in two dimensions to obtain distributions of different plasma quantities inside the torch. Apart from mass, momentum and potential conservation equations, separate energy balance equations are considered for electrons and heavy particles. The κ–e model has been used to account for turbulence. Non-equilibrium properties required for fluid dynamic simulations are obtained from a non-equilibrium property code that includes chemical non-equilibrium. The results show distributions of temperature, velocity, pressure, potential, current density and different species densities inside the plasma torch for an arc current of 200 A. Plasma pressure inside the torch varies from several atmospheres to near-atmospheric pressure. It has been observed that the electron and the heavy particle temperatures differ less near the axis of the torch and appreciably near the wall. Interesting features, observed for other investigated quantities, found consistent with the recent experimental observations are discussed.

In Situ Optical Emission Spectroscopic Investigations During Arc Plasma Synthesis of Iron Oxide Nanoparticles by Thermal Plasma

Investigations using in situ precursor spectroscopy during the growth of nanoparticles of iron oxide by thermal plasma induced gas phase condensation method have been shown to be useful for correlating the size of nanoparticles with existing plasma parameters. The relative abundance of ionized Fe species inside the plasma plume is seen to directly establish the relation between particle size, arc current, arc length, and ambient pressure of the reacting oxygen gas. The argon plasma from a transferred arc reactor is made to impinge on the anode that is allowed to vaporize and react with oxygen. The spectral line profiles of both Ar and Fe along the plasma column during the synthesis of nanoparticles have been proved to be useful in understanding the growth mechanism. Band intensities of FeO molecular states indicated the inverse relation with particle sizes that have been correlated to the two competitive processes in which energy is released, namely: 1) one involving the radiative transition and 2) the other that of the growth by coagulation. Atomic Boltzmann plots are used for estimating the temperatures of the zones, whereas particle sizes have been inferred using transmission electron microscopic measurements

Collision integrals for charged-charged interaction in two-temperature non-equilibrium plasma

Choice of an appropriate form of shielding distance in the estimation of collision integrals under screened coulomb potential for two-temperature non-equilibrium plasma is addressed. Simple expressions for collision integrals for charged-charged interactions are derived. It is shown that while some of the formalisms used earlier completely ignore the presence of ions, the others incorporating it may result in negative collision integrals for the interactions involving particles at higher charged states. The parametric regimes of concern and impact of different formalisms on the computed transport properties are investigated with specific reference to nitrogen plasma. A revised definition of the shielding distance is proposed, which incorporates both electrons and ions, avoids the problem of negative collision integrals in all practical regimes of interest and results in calculated property values in close agreement with experimentally observed results.

Theory of dynamic behavior in atmospheric pressure arc plasma devices: part-II: validation of theory with experimental data

The results of an application of the developed nonlinear theory for atmospheric pressure arc plasma instability to various experiments are presented in this paper. Most of the important experiments on atmospheric pressure arc plasma instability reported in literature are addressed. In all cases, a good match has been observed between experiment and theory. General nature of the theory to explain observed instability features is brought out in this paper.

In Situ Studies of Emission Characteristics of the DC Thermal Arc Plasma Column During Synthesis of Nano-AlN Particles

The growth process of nanoparticles and nanowires of AlN by thermal-plasma-assisted gas phase condensation reaction has been investigated by optical emission spectroscopy. The concentrations of the reacting precursors in the plasma have been correlated to the crystalline phases of nanoparticles of AlN found from X-ray diffraction analysis. The size and morphology of the nanoparticles have been studied by transmission electron microscope investigations of as-synthesized powder at a set of reactor parameters, which included arc current, reactor pressure, and standoffs of the arc column. An attempt has been made to correlate the growth of AlN to that of the precursor density present in the plasma reaction zone

Role of arc plasma instability on nanosynthesis

Recent studies have shown that use of direct current arc plasma jet is a promising technique for bulk generation of nanostructures. This paper presents a dynamical study on such systems during the synthesis and attempts to correlate the role of inherent arc fluctuations on the properties of nanostructures produced. Different fluctuations detected during the process of nanosynthesis have been characterized in terms of behavior in real time, phase space, frequency space, fractal dimension, Lyapunov exponent of evolution and diagnosed to be chaotic. For the first time, it has been shown that size of nanoparticles are strongly related to the Lyapunov exponent of inherent voltage fluctuations realized during the synthesis. Interesting features of such dependences under different process conditions have been brought out. Since chaotic systems are controllable, the study brings out the possibility of development of new size control strategies for nanosynthesis.

Current transfer in dc non-transferred arc plasma torches

Fundamentals of current transfer to the anodes in dc non-transferred arc plasma torches are investigated. Specially designed anodes made of three mutually isolated sections and external dc axial magnetic fields of various strengths are utilized to explore the conditions for different diffused and constricted attachments of the arc with the anode. A number of new facts are revealed in the exercise. Under constricted attachment, formation of arc root takes place. Spontaneous and magnetically induced movements of the arc root, their dependence on the arc current and the strength of the external magnetic field, most probable arc root velocity, variation of the root velocity with strength of the applied magnetic field, the effect of swirl on the rotational speed of the arc root are some of the important features investigated. Two new techniques are introduced: one for measurement of the arc root diameter and the other for determination of the negative electric field in the boundary layer over the anode. While the first one exploits the rigid column behaviour of the arcs, the second one utilizes the shooting back of the residual electrons over an arc spot. Sample calculations are provided.

Study of stagnation enthalpy profiles in a plasma plume: comparison between computation and experiments

Calorimetric measurements have been carried out in air to measure the heat flux delivered by an impinging plasma jet on a substrate placed perpendicular to the jet axis. The total heat flux delivered to the substrate has been resolved spatially using both Gaussian and exponential profiles with the profile constants determined from inversion of experimental data. It has been observed that the exponential profile gives a slightly better match with the experimental results compared to a Gaussian profile, which is more commonly used to represent concentrated energy flux beams such as plasma, electron or laser beams. Simultaneously, the conservation equations of mass, momentum and energy for the same plasma plume have been numerically solved to compute the stagnation heat flux profiles with conduction, convection, diffusion and radiation as the basic transport processes. An isothermal homogeneous, cylindrical, plasma column approximation has been applied in a modified form to take into account the energy lost due to radiation. The results from the simulation are compared with the experimental results. The degree of validity of the simplifying assumptions made in the analysis and their contribution to the discrepancy between the theory and the experiment are discussed.

Unique erosion features of hafnium cathode in atmospheric pressure arcs of air, nitrogen and oxygen

Experimental investigation of cathode erosion in atmospheric pressure hafnium-electrode plasma torches is reported under different plasma environments along with the results of numerical simulation. Air, nitrogen and oxygen are the plasma gases considered. Distinct differences in the erosion features in different plasmas are brought out. Cathode images exhibiting a degree of erosion and measured erosion rates are presented in detail as a function of time of arc operation and arc current. Physical erosion rates are determined using high precision balance. The changes in the surface microstructures are investigated through scanning electron microscopy (SEM). Evolution of cathode chemistry is determined using energy dispersive x-ray spectroscopy (EDX). Numerical simulation with proper consideration of the plasma effects is performed for all the plasma gases. The important role of electromagnetic body forces in shaping the flow field and the distribution of pressure in the region is explored. It is shown that the mutual interaction between fluid dynamic and electromagnetic body forces may self-consistently evolve a situation of an extremely low cathode erosion rate.

A Two-Temperature Chemical Non-Equilibrium Model of an Oxygen Cutting Torch Including the Region above the Work-Piece

Summary form only given. There is increasing evidences of deviations from thermal and chemical equilibrium in highly constricted arcs. Although, equilibrium models of such arcs inside cutting torches have been studied, non-equilibrium models including the region above the work-piece are rare. In this paper, we present a two-temperature chemical non-equilibrium model of an oxygen cutting-torch to find the distribution of velocity, temperature, current, potential and pressure over a work-piece. We consider the plasma to consist of two sub-gases: one made of electrons and the other made of all other heavy particles including ions, atoms and molecules. While all the electrons have a temperature Te, all other heavy particles follow a different temperature Th for the same location. Together with mass continuity and momentum equations, separate energy conservation equations are solved for electrons and heavy particles. Apart from electromagnetic body force terms in the momentum equations and joule heating terms in the energy equation, additional energy exchange terms are introduced in the energy equations to account for transfer of energy from one sub-gas to the other. A dedicated property routine that includes chemical as well as temperature non-equilibrium is used to supply properties required for fluid dynamic simulations. Results are presented for different arc currents and geometries. Boundary conditions, appropriate for the model, are discussed.