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
Study of a circuit-breaker arc with self-generated flow. I. Energy transfer in the high-current phase
This study deals with the first phase of operation, corresponding to the energy transfer between arc and gas and to the pressure rise. The experimental study is devoted to measurements of current, arc voltage, and pressure variations in N/sub 2/ and SF/sub 6/. For currents of the order of 10 kA the mean measured electric field was about 32 V/cm in SF/sub 6/ and 36 V/cm in N/sub 2/. Through a bibliographical study and a modeling approach of the interruption arc, an analysis of the role of the different mechanisms of energy transfer between the arc and SF/sub 6/ was conducted. With a 10 kA pulse, about 80% of the transfer is found to be due to convection and the rest to radiation from the arc. This transfer results in an overpressure of nearly 1 bar in SF/sub 6/ and 2 bar in N/sub 2/. It appears that the operation of this type of circuit breaker is limited to high currents: for currents below 7 kA the overpressure is lower than 0.5 bar, which does not provide efficient blowing at current zero. >
Laminar heat or mass transfer in rectangular channels and in cylindrical tubes for fully developed flow: comparison of solutions obtained for various boundary conditions
Abstract A comparison is made of results obtained with various boundary conditions at the wall of the channels. By comparing blood Sherwood numbers for finite wall resistance with those for constant concentration boundary condition only a small variation is found for various points along the blood channel.