Showing papers by "Xi'an Jiaotong University published in 1982"

On the Mechanism of Treeing Inhibition by Additives in Polyethylene

Abstract: On the basis of the experimental observation that the conductivity of acetophenone-doped polyethylene is field dependent over a range of applied electric stresses, a simple analysis shows that the increase in resistance to electrical treeing by additives is associated with this field-dependent conductivity. The additives can be considered as weak electrolytes. The rate of their dissociation into positive and negative charge carriers which causes an increase in conductivity is field dependent. A model for the behavior of the additives which leads to the lowering of the electrical stress at sites of high and divergent electrical stresses is given. Presently available experimental results are in good agreement with our proposed theory. Other factors affecting this mechanism are also discussed.

On the mechanism of tree initiation in polymers

Abstract: The tree initiation voltage, the initial tree penetration length and breakdown voltage in polyethylene, cross-linked polyethylene and acetophenone (1%) doped polyethylene have been measured under dc and ac voltage conditions using both the needle-plate and the needle-needle electrode configurations. The results show that all these high-field dielectric properties are strongly dependent on the polarity of the applied voltage at the needle, the defects created by cross-linking or impurity-doping in polyethylene, the geometric arrangement of the electrodes and the symmetry of the initial electric fields applied to the samples. The strong dependence of the dielectric properties on these parameters indicates that tree initiation is associated with electron and hole injection from electrodes and impact ionization by electrons in high-field regions with free volumes or mechanical cracking. The latter may be created during the manufacturing process of the materials (pre-existing defects) or by the high. field effects (post-introducing defects). In this paper we shall present our model for tree initiation, some new results and a brief discussion.

An Investigation on the Accuracy of the Double Disc Grinding

Abstract: In this paper, the process of non-orthogonality reduction on double disc grinder is discussed. The dominant parameters have been determined in terms of their effects on the grinding accuracy, and series of tests performed in order to verify theoretical prediction. Some recommendations have been formulated, which yield an improvement of the grinding accuracy. By adopting some effective measures, the amplitude of the non-orthogonalities has been reduced three times or more.

The experimental study of an air bearing medium——pressure expansion turbine

Abstract: This paper describes the experimental results of an air bearing expansion turbine which is installed in 150Nm~3/h oxygen plant.The experimental study has been carried out with rigidly-mounted and rubber O-ring stabilized bearings.The influence of some parameters,such as bearing radial cleara- nce 2Cr,bearing supply pressure Ps,shaft material and weight,and the properties of rubber O-ring etc,on the bearing instability onset speed(Self- excited whirl threshold)has been studied.In addition,the authors have also measured the vibration of bearing,the variation of bearing gas consumption, and the turbine thermodynamic performance.The experiments have shown that the performance of this air bearing turbine is excellent.

Impulse ratios of uniform and non-uniform field gaps in compressed gases

Abstract: Impulse ratios of nearly uniform field gaps were measured in compressed N2, SF6 and in ambient air surrounded by an insulating cylinder. For a 5-mm sphere gap irradiated by a mercury-are lamp an erroneous impulse ratio as high as 2.5 was recorded in both air and N2, while a considerably lower impulse ratio (≈ 1.5) was obtained in SF6, all measurements being made at 100 kPa. When 1 mCi caesium was used as an irradiation source the impulse ratio for the sphere gap in N2 and SF6 became unity at 100 kPa and slightly above unity at 400 kPa. For a 30mm point-sphere gap impulse ratios were measured in He, N2, CO, CO2 and SF6. Impulse ratio as high as approx. 2.9 was recorded in both He and N2 under negative polarity and as low as 0.62 was obtained in SF6 within the corona-stabilized pressure range. Also, in weakly attaching gases, CO and CO2, impulse ratios lower than unity were found in some pressure ranges under positive polarity.

Anti-explosion and heat transfer of plate-fin type condenser-evaporator in air-separation plant

Abstract: The correct design and operation of plate-fin condenser evaporator should meet the requirements of anti-explosion and heat transfer This paper deals with the two-phase flow calculating models and procedures for LOX boiling heat transfer and hydrodynamics and the exit liquid film Reynolds number is proposed as a reasonable criterion for anti-explosion design and operation on the basis of the analysis of LOX circulating condition which avoids deposition of hy-drocabon impurities on boiling surfaces By calculating with different design parameters and operating conditions, a series of characteristic curves are presented and some effective measures for preventing explosion and increasing heat transfer performance are proposed