An experimental and theoretical study on ablation controlled arcs in cylindrical tubes is presented. Measuring techniques for stagnation pressure, electric field strength, mass ablation rate, and arc cross section are described with which a comprehensive set of experimental data is obtained for blackened PTFE as a reference material. These data are interpreted with an isothermal two-zone model that consistently accounts for the balance of mass and axial momentum and yields simple scaling laws for the arc characteristics. Consistent agreement with the experimental data is found for an arc temperature TA = 19 000 ± 2000 K, a vapor layer temperature Tv = 3400 ± 200 K, and a transparently radiated fraction of the arc power of v = 0.32 ± 0.03. The vapor temperature can be explained with a photoablation mechanism. The ablation arc model allows quantifying of the phenomena related to nozzle clogging in gas-blast circuit breakers, namely flow blocking and reverse flow heating. How these phenomena determine the pressure rise in self-blast circuit breakers is shown.

Ablation Controlled Arcs

Arc plasma torches are the primary components of various industrial thermal plasma processes involving plasma spraying, metal cutting and welding, thermal plasma CVD, metal melting and remelting, waste treatment, and gas production. They are relatively simple devices whose operation implies intricate thermal, chemical, electrical, and fluid dynamics phenomena. Modeling may be used as a means to better understand the physical processes involved in their operation. This article presents an overview of the main aspects involved in the modeling of DC arc plasma torches: the mathematical models including thermodynamic and chemical nonequilibrium models, turbulent and radiative transport, thermodynamic and transport property calculation, boundary conditions, and arc reattachment models. It focuses on the conventional plasma torches used for plasma spraying that include a hot cathode and a nozzle anode.

https://link.springer.com/content/pdf/10.1007%2Fs11666-009-9342-1.pdf

Arc Plasma Torch Modeling

Arc plasma torches are the primary components of various industrial thermal plasma processes involving plasma spraying, metal cutting and welding, thermal plasma CVD, metal melting and remelting, waste treatment and gas production. They are relatively simple devices whose operation implies intricate thermal, chemical, electrical, and fluid dynamics phenomena. Modeling may be used as a means to better understand the physical processes involved in their operation. This paper presents an overview of the main aspects involved in the modeling of DC arc plasma torches: the mathematical models including thermodynamic and chemical non-equilibrium models, turbulent and radiative transport, thermodynamic and transport property calculation, boundary conditions and arc reattachment models. It focuses on the conventional plasma torches used for plasma spraying that include a hot-cathode and a nozzle anode.

The dynamics of the electric arc inside a direct current non-transferred arc plasma torch are simulated using a three-dimensional, transient, equilibrium model. The fluid and electromagnetic equations are solved numerically in a fully coupled approach by a multiscale finite element method. Simulations of a torch operating with argon and argon–hydrogen under different operating conditions are presented. The model is able to predict the operation of the torch in steady and takeover modes without any further assumption on the reattachment process except for the use of an artificially high electrical conductivity near the electrodes, needed because of the equilibrium assumption. The results obtained indicate that the reattachment process in these operating modes may be driven by the movement of the arc rather than by a breakdown-like process. It is also found that, for a torch operating in these modes and using straight gas injection, the arc will tend to re-attach to the opposite side of its original attachment. This phenomenon seems to be produced by a net angular momentum on the arc due to the imbalance between magnetic and fluid drag forces.

/pdf/multiscale-finite-element-modeling-of-arc-dynamics-in-a-dc-i2kd03w18f.pdf

Multiscale Finite Element Modeling of Arc Dynamics in a DC Plasma Torch

Analytic formulae are derived from the controlling physical equations to give the principal characteristics of free burning arcs. Predictions of arc radius, voltage, central temperature, electric field and centre plasma velocity are in reasonable agreement with published experimental results for arcs in air from 1 A to 20,000 A at a pressure of 1 atmosphere and an electrode spacing of 1 cm. The theory enables useful predictions to be made for applications of arcs in welding, switchgear and arc furnaces.

http://iopscience.iop.org/article/10.1088/0022-3727/12/11/016/pdf

Simple theory of free-burning arcs

High current density (> 10 A mm-2) arcs in channels of solid material are determined by (i) arc induced evaporation of wall material, (ii) axial convection, and (iii) radiative energy transfer. Based on these processes, an integral arc model is developed from first principles that relates the arc characteristics to the arc current, the channel geometry, and the properties of the channel material. The model is shown to agree satisfactorily with measurements. For engineering applications approximate scaling laws are derived for the arc field strength, the pressure in the channel and the wall evaporation rate.

Evaporation Dominated High Current Arcs in Narrow Channels

An axially blown arc is studied under physical conditions typical of HV circuit breakers. The experiment is especially designed to produce a stable and reproducible arc with good accessability for diagnostics. After a stationary phase the current is ramped down with adjustable slope. The vicinity of current zero is especially investigated. For the first time, the most effective recovery zone is localized and its physical parameters are evaluated. The limiting curve for thermal reignition is determined. Based on the observed phenomena a theoretical description is derived which explains quite well the experimentally known extinction characteristics of this type of arc.

Investigation on the physical phenomena around current zero in HV gas blast breakers

In a previous paper (Hermann et al 1974) a detailed experimental and theoretical investigation of an arc in a supersonic nozzle flow was initiated by discussing the high-pressure arc section near the stagnation point. This paper is a continuation of this work for the section further downstream, especially within the nozzle. Extensive measurements of temperature, flow velocity, pressure and electric field strength are described. Theoretical results are obtained by a two-zone model. A detailed comparison between theory and experiment allows for the first time a complete quantitative discussion of all effects influencing the arc and the flow. Compared with the arc in the high-pressure section the influence of radiative energy transfer decreases. Instead, flow turbulence becomes the dominating exchange mechanism.

http://iopscience.iop.org/article/10.1088/0022-3727/7/12/317/pdf

Experimental and theoretical study of a stationary high-current arc in a supersonic nozzle flow

A detailed comparison between experiment and theory is made on the basis of the results of Parts I and II. As a first step, a theoretical solution is derived which is based on experimental results for the turbulence onset and for the radial temperature profile at current zero. This solution is compared with the various measured data, e. g., the radius of the hot-gas channel as a function of time and the dielectric recovery characteristics. The influence of space-charge effects on the recovery are shown to depend on the time after current zero and the shape of the applied voltage. Recommendations for further research are made.

Dielectric recovery of an axially blown SF6-Arc after current zero: part III-comparison of experiment and theory

A gas-blast switch having a hollow support insulator, and and at least one switching chamber housing supported thereon and within which there is arranged a set of fixed contacts, surrounded by a blast chamber, and a set of movable contacts. A blast nozzle coacts with the movable contacts and is connected with a pump device having a pump cylinder surrounding a pump chamber and movable with the movable contacts. The pump cylinder is displaceably arranged upon a stationary piston. A check valve opens towards the pump chamber. During the cut-on stroke pressurized gas is sucked by the valve into the pump chamber. The latter is connected by the check valve with a supply compartment communicating via inlets with the blast chamber and the internal space of the support insulator. A further valve is responsive to the pressure increase in the blast chamber, which valve, when responsive, interrupts the connection between the internal space of the support insulator and the inlets. The supply compartment is preferably formed by a cylindrical hollow body whose one end face serves as the pump piston, whereas at the end region of the outer surface of the hollow body furthest from such piston there are formed the throughpassages or inlets as axially lengthwise extending slots. The valve may be a plate valve whose valve plate coacts with a valve seat affording access to the internal space of the support insulator and further, in its open position, subdivides the inlets into two sections, one opening into the supply compartment, the other by means of the valve seat into the internal space of the support insulator.

Lutz Dr Niemeyer

Papers

Evaporation Dominated High Current Arcs in Narrow Channels

Investigation on the physical phenomena around current zero in HV gas blast breakers

Experimental and theoretical study of a stationary high-current arc in a supersonic nozzle flow

Dielectric recovery of an axially blown SF6-Arc after current zero: part III-comparison of experiment and theory

Gas blast switch