Theoretical & Experimental Study On The Phenomena/Mechanism Of Discharge In Air-water Two-phase Mixture

Some discharge phenomena in multiphase mixtures are often observed in nature or our everyday life, such as the lightning in rain, the discharge in electrostatic precipitation or electrostatic spraying of liquid, the water and wastewater treatment process by impulse discharge, the activation of desulfurization catalyzer for flue gas by discharge plasma, and the discharge phenomena on outdoor high-voltage transmission lines under the bad weather conditions (rain, snow or sandstorm), and so on. The gas discharge theories were developed well, but the investigation to multiphase mixture discharge has been started just now. Based on the primary processes of gas discharge, this paper analyzes several main influencing factors for the inception and development of discharge in air-water two-phase mixture, and then a preliminary discharge mechanism is presented. The primary discharge processes of both are considered as similar, but the discharge parameters may be different because of the water drops'immergence, which macroscopically results in some different discharge phenomena. On the one hand, the deformation of local electric fields induced by water drops causes more violent collision ionization in the area with enhanced electric fields, and makes electron drift/diffusion velocities be faster, so it is possible that discharges begin at the enhanced areas. On the other hand, the attachment (electrical charging) processes by water drops reduces the free electrons, which weakens the discharge. Therefore, both of the deformation of local electric fields and the attachment process are the most important factors. A simple and usable way to calculate the local electric fields in a two-phase mixture in a uniform external field is presented. If the mixture has a low permittivity mismatch and a low volume fraction of particles (i.e. the electrostatic interactions among particles are weak), then the dipole approximation, neglecting the interactions, is acceptable. Or else it is appropriate to adopt the dipole-enhanced approximation considering the interactions. The validity range of the dipole approximation forms the division between the two models. The perturbation parameter a in the general mixing rule is renewed by the dipole-enhanced approximation, and then a modified mixing formula is derived. The dipole-enhanced approximation is proved out by that the predicted effective permittivities with this modified formula agree well with not only the exact numerical solutions but the experimental results. The theoretical formulas for calculating the electrical charging for high volume fraction mixtures are derived based on the dipole-enhanced approximation. These formulas show that the quantity and velocity of charging rise with not only the electric field, the particle radius and the permittivity mismatch, but also the volume fraction of particles. The discharge phenomena in the rod-plane gaps in air-water two-phase mixture are investigated applied by DC high voltage experiments. The experimental results show that, the corona discharge current in air-water mixture is always greater than that in air under the same conditions, which mainly results from the deformation of local electric fields. The corona discharge current will decrease because the water drops and water molecules can attach free electrons in space. Generally, the positive space discharges whose drift velocities are relatively slow compared with the electrons drift velocities have a great influence on the tip discharge. To some extent, the water drops with high velocities may dissipate the positive space discharges, which cause greater positive polarity discharge current and smaller negative polarity discharge current. The discharge phenomena in the rod-plane gaps applied by standard lightning impulse voltage in air-water two-phase mixture are also investigated experimentally. The study content involves several important aspects, such as the 50% breakdown voltage (U50), the V-T characteristics, the polarity effect, the leader shape and velocity. It is reasonable to ignore the effect of positive space discharges since they can not accumulate in so short impulse width, which differs from the situation of DC high voltage discharge. If the over-voltage is not too high, the time to breakdown in air-water mixture is longer than that in air under positive impulse voltage, while the situation under negative impulse voltage is just reverse. For the same gap, the time to breakdown under negative impulse voltage is longer and more dispersive than those under positive impulse voltage, but the V-T characteristics curves of negative impulse voltage are not steeper than those of positive impulse voltage. The discharge's developing processes are recorded by means of the chop-wave-still-camera method. It is found that the discharge streamers'distribution in air-water mixture is wider than that in air, and the branches of streamers are more than those in air too. In the end, according to the experimental results, a useable leader velocity model is presented by bringing the mixture coefficient k3 and the over-voltage coefficient k4 into the Baldo model.