Thermal features of low current discharges and energy transfer to insulation surfaces

The thermal features of low current AC discharges, of between 2 and 5 mA, between water droplets are presented in this paper. The `Best-fit´ method of optical emission spectroscopy (OES) analysis is applied to measure the discharge temperature. The discharge temperature increased from 1200 K to 1500 K when the current level rose from 2.5 mA to 5 mA, and its temperature was less sensitive to arc length than arc current. Measurements of the distribution of temperature along a 4 mm discharge showed that the regions including the arc roots do not have a higher temperature than the discharge column, despite appearing brighter. The introduction of a silicone rubber insulation surface adjacent to the discharge increased the discharge temperature, typically by ~160 K, and its energy per half cycle by 150%. The resulting temperature distribution on the insulation surface was captured by infrared thermal imaging. Finite element analysis (FEA) software successfully simulated heat transfer between discharge and surface, and showed good agreement with experimental results: highest temperatures being seen under the centre of the arc, where damage is also first seen in practice. FEA can thus be used to model, for example, the impact of thermal conductivity on insulation surface temperature.