Fundamental investigation on the thermal transfer coefficient due to arc faults
In order to determine the pressure rise due to arc faults in electrical installations, the portion of energy heating the surrounding gas of fault arcs has to be known. The ratio of the portion of energy to the electric energy, the thermal transfer coefficient, well known in literature as kp factor, is adopted here. This paper presents a theoretical approach for the determination of the pressure rise in electrical installations. It is based on the fundamental hydro- and thermodynamic conservation equations and the equation of gas state taking into account melting and evaporation of metals as well as chemical reactions with the surrounding gas. With respect to the dependence of the arc energy on gas density, the radiative effect of fault arcs on the energy balance is introduced. In consideration of the radiation, the more reasonable arc energy is applied for the estimation of the gas temperature and of the thermal transfer coefficient in the energy balance. In order to identify conveniently which factors essentially influence the development of pressure, the thermal transfer coefficient is studied and evaluated as an alternative variable of the gas pressure. The results for a test container show that factors such as the kind of insulating gases and of electrode materials, the size of test vessels, and the gas density considerably influence the thermal transfer coefficient and thus the pressure rise
Experimental and theoretical investigation of high-pressure arcs. I. The cylindrical arc column (two-dimensional modeling)
The properties of the free-burning arc column are studied for ambient pressures of 0.1 MPa (i.e., atmospheric) to 10 MPa for applications in underwater welding and cutting as well as arc discharge lamps. The influence of transverse magnetic fields is studied in Part II. A DC current of 50-100 A is applied to an argon discharge with a conical tungsten cathode and a plane water-cooled anode which are separated by several millimeters. The electrical properties are measured, and the temperature distribution is determined by spectroscopic means utilizing a two-dimensional charge-coupled device (CCD) sensor. A self-consistent numerical solution of the conservation equations yields the temperature, velocity, pressure, and current distributions. The predicted arc temperatures agree well with the measured temperature distributions. An analysis of the conservation equations shows that the arc column becomes radiation dominated with increasing pressures resulting in small temperature gradients within the column and large gradients at the boundaries. It is found that a net emission coefficient might be used to account for the radiative heat transfer in the investigated parameter range. The arc constricts due to increased convective cooling especially at the cathode, while temperatures and velocities are decreasing. The power expended in the column scales approximately with the square root of the ambient pressure in line with the radiation dominance, whereas the voltage drop across the electrode sheaths exhibits no pressure dependence for a given current.
Spectroscopic measurements in nitrogen arcs with respect to local thermodynamic equilibrium
A wall-stabilized nitrogen arc at atmospheric pressure was studied spectroscopically in the current range from 20 to 70 A with the aim of detecting departures from LTE. Measurements of the relative intensity of several N(I) lines showed that the populations of the upper excited states are in equilibrium for currents greater than approx. 25 A. Comparisons of the excitation temperatures obtained from relative intensities with the electron temperature and with the temperature curves calculated for different departures from LTE show that deviations from equilibrium are small at the arc center for electron densities above approx. 4 × 1016 cm-3. Deviations between the radial profiles of the electron and excitation temperatures were observed for currents between 35 and 55 A.