Water flow induced by a DC corona discharge above the water surface

Abstract The aim of this study is to characterize the flow of deionized water induced by positive and negative DC corona discharges ignited between the water surface and a needle located a few millimetres above it. Firstly, we provided an overview of the main published studies, and outlined the various physical mechanisms that could be responsible for the liquid flow. Secondly, we used a particle image velocimetry system to analyse the flow in relation to various parameters, such as the gap between the needle tip and the liquid surface, the discharge current, and the discharge polarity. Our results revealed the presence of an upward flow located directly beneath the discharge impact point on the liquid surface and an outward tangential flow originating from this point and propagating towards the vessel walls. This outward flow leads to the formation of two counter-rotating vortices on either side of the vessel. These correspond to a cross-sectional view of a single three-dimensional structure: a toroidal vortex ring. Regarding the mechanisms driving the flow, the ionic wind clearly plays a major role by exerting shear stress at the interface. However, it cannot alone explain the upward flow, and especially the strong increase in its velocity within the last millimetre below the water surface. In this article, we argue that this flow originates from the Coulomb force, acting on ions that are located immediately below the strong space charge induced by the corona discharge at the interface, in front of the needle tip. Moreover, we hypothesise that the charged species in this strong space charge region move towards the vessel sides beneath the liquid surface because the ions repeal each other, leading to an outward surface force. This also contributes to the tangential flow. These are only hypotheses that we are currently unable to definitively prove, and should therefore be treated with caution.