Temporal and spatial evolution of EHD particle flow onset in air in a needle to plate negative DC corona discharge
In this paper we present images showing the temporal and spatial evolution of the electrohydrodynamic (EHD) flow of dust particles (cigarette smoke) suspended in still air in a needle to plate negative DC corona discharge arrangement just after the corona onset, i.e. in the first stage of development of the EHD particle flow. The experimental apparatus for our study of the EHD flow onset consisted of a needle to plate electrode arrangement, high voltage power supply and time resolved EHD imaging system based on 2D time resolved particle image velocimetry equipment. The time resolved flow images clearly show the formation of a ball like flow structure at the needle tip just after the corona discharge onset, and its evolution into a mushroom like object moving to the collecting electrode. After a certain time, when the mushroom like object is still present in the interelectrode gap a second mushroom like object forms near the needle electrode and starts to move towards the collecting electrode. Before the first mushroom like object reaches the collecting electrode several similar mushroom like objects can be formed and presented simultaneously in the interelectrode gap. They look like a series of mushroom like minijets shot from the needle electrode vicinity towards the collecting electrode. The simultaneous presence of mushroom like minijets in the interelectrode gap in the corona discharge in particle seeded air resembles the negative ion charged 'clouds' (induced by the Trichel pulses) traversing simultaneously the interelectrode gap of the corona discharge in air, predicted a long time ago by Loeb, and Lama and Gallo and recently by Dordizadeh et al. Analysing the time behaviours of the mushroom like minijets and current waveform in the corona discharge in particle seeded air, we found that the Trichel pulse trains, formed just after the corona onset initiates the mushroom like minijets. The first stage of development of the EHD particle flow, the area of which is practically limited to the interelectrode duct, ends when the first mushroom like minijet reaches the collecting electrode.