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Atmospheric pressure needle-to-plane negative corona discharges have been studied considering the effect of photoionization in electrode gap spacing from 2.9 to 3.9 mm. Trichel pulses, spatial distributions of electron density and electric field, pulse duration time, and pulse frequency under different gap spacing were investigated in this work. It is found that peak value and duration time of the first Trichel pulse decrease with the increase of gap spacing. The movement of charged particles is faster in narrow gap spacing, which leads to higher pulse frequency. Moreover, the effect of photoionization on negative corona discharge under different gap spacing is also studied by numerical simulation. It is found that electric field, as well as electron density, is weakened due to the effect of photoionization. Besides, the magnitude of Trichel pulses, duration time of the first pulse, and pulse frequency are relatively small when taking the effect of photoionization into consideration. These results are of great importance to fully understand the mechanism of negative corona discharge under different conditions. [ABSTRACT FROM AUTHOR]

For practical application, studying the mechanism governing the macroscopic dielectric properties of polymer composites at low concentration is of great significance. A three-dimensional (3D) representative model with randomly dispersed grains in polymer matrix has been established and calculation results of effective permittivity of BT/PVDF composites with finite element method (FEM) show good consistency with experimental measurements, which demonstrate that the distribution of electric field in ceramic particles/polymer composites at low concentration can be approximated by the electrostatic field. Furthermore, the relationship between the internal electric field and dielectric properties of composites has been established. The effect of grain size in submicron and its dielectric constant to the effective permittivity of composites are fully investigated. The former shows of negligible significance compared with the evident effect of volume fraction, while the latter presents saturation as increasing the permittivity ratio between ceramic particle and polymer. [ABSTRACT FROM AUTHOR]

The saturated reactor is used in the DC converter valve to protect thyristors. A fluid-solid coupling heat dissipation model was established for the saturated reactor in this study, which included water cooling, heat conduction, heat convection and heat radiation. By the means of finite element method, the temperature and flow field distribution were calculated, which indicated that the low thermal conductivity of epoxy resin hinders the heat dissipation of the saturated reactor. To research effects of thermal conductivity on heat dissipation, the heat dissipation performance of saturated reactor with different epoxy resins was numerical simulated by using this model. The results showed that the heat dissipation performance of the saturated reactor can be effectively improved by the rise of the thermal conductivity of epoxy resin, but the marginal benefit of increasing the thermal conductivity of epoxy resin is gradually reduced. Meanwhile, high-thermal conductivity epoxy resin could have the longer service life under the same operation conditions. [ABSTRACT FROM AUTHOR]