Pattern of bubble evolution in liquids under repetitive pulsed power

Pulsed electric fields (PEFs) are employed in a newly developed food sterilization technology via repetitive electric pulses. The evolution of bubbles and the consequent electrical breakdown of the liquid treatment chamber are the main technical hurdles for industrial applications of this technology. The evolution pattern of the bubbles and the effect of pulse parameters on the bubbles under repetitive pulses are studied. Repetitive bipolar rectangular pulses (6-10 kV, 20-80 μs in duration, with a 50-200 Hz repetition rate) are applied to a gap (1 cm) containing sodium chloride solution (with a conductivity of 30 μS/cm). A high-precision bubble monitoring system is developed to recognize and count microbubbles with diameters greater than 50 μm. The pattern of the bubble evolution can be divided into 4 stages: the initial stage, developing stage, converging stage and discharging stage. The saturated time, number, area and average diameter of the bubbles are introduced as feature parameters to better characterize the bubble evolution, and the relevant time-varying curves for different pulse parameters are obtained based on experimental images. The experimental results show that a stronger electric field, a longer pulse duration and a higher repetition rate speed up the evolution of the bubbles. The growth of the bubbles depends more on the effect of continuous long pulses, and the electric field might be a deciding factor concerning the initiation of microbubbles, while the increase in bubble volume is mainly a result of the thermal effect of current.