DC negative corona discharge characteristics in air flowing transversely and longitudinally through a needle-plate electrode gap

CThe electrical characteristics of a steady-state DC negative corona discharge in air flowing transversely or longitudinally in relation to the needle-to-plate (or mesh) electrode axis were experimentally investigated. The velocity of air flowing into the corona discharge chamber was limited to a range from 0 to 6 m/s, typical of electrostatic precipitators, gas ionisation sensors and EHD pumps. The investigations showed that in this airflow velocity range three discharge current modes of the DC negative corona could exist: the Trichel-pulse corona mode, the 'Trichel pulses superimposed on the steady current (or DC) component' mode and the steady glow (the steady current or DC) corona mode, regardless of the airflow direction. The average corona discharge current, which is a macroscopic corona discharge parameter, was almost not affected by the airflow. However, both microscopic corona discharge parameters: the electric charge and the repetition frequency of Trichel pulses were influenced by the airflow. The transverse airflow caused an increase in the electric charge and a decrease in the repetition rate of Trichel pulses, the both by about 25% at a velocity of 6 m/s. In the case of longitudinal airflow the average corona current slightly increased when the airflow was directed towards the mesh and decreased when the airflow was directed towards the needle tip. Similarly as in the case of transverse airflow, the electric charge and the repetition frequency of Trichel pulses were affected by the longitudinal airflow. The electric charge and the repetition frequency of Trichel pulses increased and decreased, respectively, with growing airflow in the direction towards the needle electrode. The airflow growing in the direction towards the mesh electrode produced the opposite effect - the electric charge and the repetition frequency of Trichel pulses decreased and increased, respectively. The opposite behaviours of the electric charge and the repetition frequency of Trichel pulses with growing airflow velocity could explain the weak dependence of the negative corona average current on transverse or longitudinal airflow. Therefore, the independence of the negative corona average current of the airflow cannot be considered as sufficient proof that the airflow (up to 6 m/s) does not affect the negative corona parameters.