John J. Lowke

Streamer propagation in air

A theory is presented for the development of the first streamer when a positive voltage is abruptly applied to a point in air at atmospheric pressure. The continuity equations for electrons, positive ions and negative ions, including the effects of ionization, attachment, recombination, electron diffusion, and photoionization, are solved simultaneously with Poissons equation. With an applied voltage of 20 kV across a 50 mm gap, the streamer does not reach the cathode. An intense electric field front propagates away from the point into the gap to a distance of 35 mm in 200 ns. During the next the streamer only moves a further 2 mm into the gap, and the electric field at the head of the streamer collapses. Finally, only positive space charge remains which moves away from the point, allowing the field near the point to recover after ; free electrons can thus give rise to a secondary discharge near the anode. The electric field distribution is shown to be quite different from that found previously for in that the electric field in the column of the streamer is generally only a fraction of the critical field for which ionization equals attachment. Streamers for a given applied voltage have a far greater range in air than in . The results presented for air also apply to flue gas mixtures, since the important material properties of both gases are very similar.

Theoretical analysis of removal of oxides of sulphur and nitrogen in pulsed operation of electrostatic precipitators

An investigation has been made of the various plasma chemistry reactions that occur in the corona discharge of an electrostatic precipitator operating in a typical flue gas. Calculations have been made of the rate coefficients for electron dissociation of the principal gaseous components, namely, nitrogen, oxygen and water vapor as functions of electric field. In addition, calculations have been made of the rates of ionisation and attachment and also the rates of excitation of the principal excited states. The calculations indicate that sulphur dioxide is removed principally by reactions with OH radicals to produce sulphuric acid, while nitrogen oxides are removed principally by reduction via the N radical to molecular nitrogen. However, for these reactions to occur, values of E/N of 70 Td or more are necessary, which is higher than the E/N of 30 Td at which electrical breakdown normally occurs; E is electric field strength and N is the gas number density. Approximate calculations indicate that, for an E/N of 100 Td, voltage pulses of width less than 1 /spl mu/s need to be applied to avoid breakdown. It is also shown that small quantities of nitrous oxide are produced and that the presence of water vapor has a significant effect on the plasma chemistry and increases the breakdown voltage. >

Predictions of weld pool profiles using plasma physics

This paper gives a review of recent papers which have led to the capability of the prediction of weld depths for gas tungsten arc welding, for any given arc current, electrode shape or separation and welding gas. The methodology is given for deriving plasma composition as a function of temperature and pressure from basic atomic and molecular properties. Transport coefficients of density, specific heat, enthalpy, electrical conductivity, thermal conductivity, viscosity and radiation emission coefficients can then be derived as a function of temperature. The conservation equations of fluid dynamics are then used to derive weld profiles for stainless steel for welding gases such as argon, helium, carbon dioxide and a 10% mixture of hydrogen in argon. The markedly different weld depths which are obtained are related to basic material functions such as specific heat, electrical and thermal conductivity. The temperature dependence of the surface tension coefficient has a marked effect on weld depth and profiles because it can influence the direction of circulatory flow in the weld pool. Electric arcs in helium and carbon dioxide are more constricted than arcs in argon and as a consequence the magnetic pinch pressure of the arc, transmitted to the weld pool, can force strong downward flows in the weld pool and thus lead to a deep weld. It is found that because of the interactions of the arc and the weld pool through effects such as viscous drag forces of the plasma on the weld pool, it is necessary to treat the arc, the electrode and the weld pool in a unified system.

A simplified unified theory of arcs and their electrodes

A recently developed unified theory of arcs and their electrodes, with cathodes which are thermionic emitters, has been simplified so that there is a reduction in computation times by approximately a factor of 100. Electrode and arc regions are treated together and points at the surface of the electrodes are treated in a special way to account for electrode effects; no assumptions are made concerning the current density at the cathode surface. The theory is used to make predictions of arc and electrode temperatures and arc voltages for arcs in argon as a function of current in the range 50 - 400 A. The maximum temperatures of the arc and the cathode and also the current - voltage characteristics are in reasonable agreement with experimental results for cathodes with a included angle. For a cathode with a included angle, we predict a maximum in the temperature several millimetres from the tip of the electrode, in approximate agreement with experiment. Temperatures of the cathode tip are predicted to be much higher for tungsten cathodes than for thoriated tungsten cathodes and are in reasonable agreement with experimental results. Tungsten electrodes are hotter than thoriated tungsten electrodes, partly due to increased ion heating, but largely due to greater heat conduction from the arc to the electrode due to the arc plasma covering a greater area of the electrode surface for tungsten.

Theory of free-burning arc columns including the influence of the cathode

Previous predictions of the properties of free-burning arcs have usually required assumptions to be made of conditions at the cathode, in particular, the current density. Numerical calculations presented show that the current density at the cathode, for a 200 A arc in argon with a tungsten cathode having a 60 degrees conical tip, is determined to a good approximation by the arc column and cathode shape. The cold non-equilibrium sheath next to the cathode is very thin ( approximately 0.02 mm) and can be neglected. Predictions are made of arc column temperatures, velocities, pressure and potential by solving in two dimensions the conservation equations of mass, momentum and energy. Predictions are in fair agreement with experimental results with maximum arc temperatures of approximately 22000 K. Current densities depend strongly on the electrode radius at the cathode tip. For a radius of 0.1 mm the current density is approximately 20000 A cm-2.

Predictions of metal droplet formation in arc welding

A two-dimensional time-dependent model has been developed for the prediction of droplet formation in gas metal arc welding. The model is a unified treatment of the arc, the welding wire, taken as the anode, and the workpiece, taken as a plane cathode. Predictions are made of the formation and shape of the welding droplets as a function of time, accounting for effects of surface tension, gravity, inertia and magnetic pinch forces. The wire feed rate and gas flow rates are also incorporated into the model. Calculations are made of current densities, electric potentials, temperatures, pressures and velocities in two dimensions both in the arc and also within the molten drop and solid electrodes. For an arc in argon with a mild steel wire of 1.6 mm diameter and a current of 325 A or more, we predict the formation of small drops of diameter 1.2 mm or less and large drop frequencies consistent with the spray transfer mode observed in welding. At currents of less than 275 A, we predict large drop sizes of about 3.8 mm in diameter or more, consistent with the globular transfer mode in welding. At a current of 300 A, in a transition zone between the two modes, we predict the presence of both small and large drops.

Modelling of thermal plasmas for arc welding: the role of the shielding gas properties and of metal vapour

The methods used to model thermal plasmas, including treatments of diffusion in arcs in gas mixtures, are reviewed. The influence of thermophysical properties on the parameters of tungsten–inert-gas (TIG) welding arcs, particularly those that affect the weld pool, is investigated using a two-dimensional model in which the arc, anode and cathode are included self-consistently. The effect of changing each of six thermophysical properties on the characteristics of an argon TIG arc is assessed. The influence of the product of specific heat and mass density is found to be particularly important in determining the arc constriction. By examining the influence of the different properties on the heat flux density, current density and shear stress at the anode, it is concluded that the weld pool depth can be increased by using shielding gases with high specific heat, thermal conductivity and viscosity. The effect of metal vapour on the arc and weld pool properties is assessed. The most important effect of the metal vapour is found to be the increased electrical conductivity at low temperatures, which leads to lower heat flux density and current density at the weld pool, implying a shallower weld pool.

Prediction of properties of free burning arcs including effects of ambipolar diffusion

A method is described to predict the two-dimensional distributions of temperature, velocity and potential of free burning arcs and their electrodes for cathodes of tungsten and thoriated tungsten. The effects of non-equilibrium due to the ambipolar diffusion of charged particles are included for the calculation of the plasma electrical conductivity. The electron diffusion current is explicitly included in the solution of the current continuity equation. The plasma for the arc and the electrode sheath regions is treated as a continuum, so that the thickness of the non-equilibrium regions near the electrodes is determined within the model, depending upon the arc current and the arc and electrode configuration. This new treatment allows the calculation of the negative anode fall that may occur across the anode sheath when the electron diffusion current near the anode surface becomes larger than the total arc current. For a thoriated tungsten cathode we take the work function for cooling by electron emission to be that of tungsten, as, for small percentages of thoria in tungsten, cooling effects from electrons passing through the interfaces for tungsten-thoria and then thoria-plasma will add up to be that of a tungsten-plasma interface. Calculations have been made for arcs in argon at currents between 2.5 A and 200 A. For currents above 120 A, we calculate the anode fall voltage to be negative, being -2 V at 200 A. For currents less than 50 A, non-equilibrium effects in the plasma extend across the whole arc and electron number densities can be several orders of magnitude below the values for local thermodynamic equilibrium. Calculated arc voltages, arc temperatures and electrode temperatures are in agreement with experimental measurements to within 20%.

Theory of electrical breakdown in air-the role of metastable oxygen molecules

A theory is given of electrical breakdown in air for non-uniform electric fields. It is proposed that space-charge effects tend to make the electric field in the streamer column uniform at a value of E/N for which effective ionization and attachment rates are equal (E is the electric field and N the gas number density). It is also proposed that a1 Delta g metastable states of molecular oxygen, produced in the pre-breakdown corona and streamer processes, have a dominant role in determining E/N in the streamer channel, because of their ability to detach electrons from O2- ions and thus reduce the effective attachment coefficient. Assuming an effective value of alpha /N=10-22 cm2 ( alpha being the ionization coefficient) at E/N=20 Td, predictions of breakdown fields in dry air and humid air are in fair agreement with experimental results for rod-rod breakdown with electrode spacings of from 0.5 m to 2.5 m. The effect of the metastables is to reduce the breakdown field applicable to plane parallel electrodes by a factor of about 6. It is predicted that 1% water vapour in air increases breakdown fields by about 11%. A breakdown criterion is derived defining the critical E/N for breakdown in terms of electron transport coefficients and metastable rate constants.

A computational investigation of the effectiveness of different shielding gas mixtures for arc welding

Tungsten?inert-gas welding arcs are modelled using a two-dimensional axisymmetric computational code. Both electrodes (the tungsten cathode and the metal anode workpiece) and the arc plasma are included self-consistently in the computational domain. The influence of adding helium, hydrogen and nitrogen to the argon shielding gas is investigated. It is found that addition of any of the gases increases the heat flow to and the current density at the anode. The shear stress and the arc pressure at the anode surface are increased by adding hydrogen or nitrogen or up to about 50?mol% helium, but decrease when more helium is added. It is predicted that the effect of adding any of the gases is to increase the depth of the weld pool, in agreement with the experimental evidence. The results are explained by referring to the thermodynamic and transport properties of the gas mixtures.