Paper PS-FrM1
Positive Streamers Propagating Inside Bubbles in Liquids

Friday, November 13, 2009, 8:20 am, Room B2

Pulsed discharges in liquids are often in the form of streamers. In most cases, streamers do not directly propagate through the liquid phase. Rather breakdown occurs inside bubbles and near gas–liquid interfaces. Often bubbles are purposely injected to facilitate breakdown. Such discharges have been extensively studied for their use in water treatment, surgery, decontamination and sterilization. Recent experiments have shown that streamers often preferentially propagate along the surface of a bubble immersed in a liquid instead of propagating along the axis of the bubble. In this talk, we discuss results from a computational investigation of the propagation of streamers inside bubbles immersed in liquids. We show that dielectric constant of a liquid determines patterns of streamer propagation. The model, nonPDPSIM, is a 2-dimensional simulator executed on an unstructured mesh in which Poisson’s equation for the electric potential, and transport equations for charged and neutral species, and electron temperature, are solved. Radiation transport and photoionization are included by implementing a Green’s function propagator. A bubble of humid air, N₂/O₂/H₂O = 55/15/30 at atmospheric pressure is placed at the tip of a positive corona discharge immersed in a liquid of specified conductivity, σ, and permittivity, ε/ε₀. The bubble radii are 0.45 - 0.9 mm. We found that for low values of σ and ε/ε₀ the streamer propagates along the axis of the bubble. For large values of σ and ε/ε₀ the streamer propagates along the surface of the bubble. For essentially non-conducting liquids, the transition between axial and surface-hugging streamers occurs at 4 < ε/ε₀ < 8, depending on the size of the bubble and voltage. Increasing conductivity lowers the value of ε/ε₀ at which the streamer becomes surface-hugging. These trends largely result from the refraction of the electric field by the curved interface of the bubble in the presence of a diverging electric field. The final pattern of the streamer path is additionally a function of bubble size and its position relative to the tip of the electrode, applied voltage and polarity.