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
Reduction in pressure rise due to internal arcing using melting and vaporization of metal
Summary form only given. When an arc is ignited between the electrodes in a closed chamber, the internal pressure rises. In this paper, the reduction in the pressure rise due to internal arcing using the melting and vaporization of a de-energized metal plate was investigated experimentally. The results showed that allowing a Cu or Fe plate placed near the electrodes to be melted and vaporized by the arc, the reduction in the pressure rise increased as the amount of mass loss of the plate increased. The decrease level of the maximum pressure rise was more than 20% on average in our experiments. On the basis of these experimental results, the energy balance in the closed chamber was estimated in order to clarify the mechanism behind the reduction in the pressure rise. The dominant cause of the reduction in the pressure rise for the Cu plate was the increase in energy consumption by the melting and vaporization of the plate. For the Fe plate, it was the increase in the energy of consumption by the arc radiation.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.