Investigationon On Breakup Mechanism And Electrohydrodynamics Characteristics Of Charged Droplet

As a fundamental content of electrohydrodynamics (EHD), electrostatic atomization from a capillary channel for generating ultrafine, ideally monodisperse, easily controlled, and high-deposition droplets with low energy consumption has been widely applied to micro-and nanometer thin film and particle preparation, air purification, microcombustion, space micropropulsion, mass spectrometry, and biotechnology. Deformation, breakup, coalescence and departure of charged droplet are the important phenomena of charged multiphase flow which are produced by electrostatic atomization, and the electrohydrodynamic characteristics of coupling field are extremely complicated. Especially a detailed research on the transition and stability of micro-jet modes and the dynamic behavior of adjacent droplets are still lacking, some new findings are worthy of further discussions. With the development of modern flow measurement techniques, visualization on charged micro-jet with the help of high-speed microscopy and the analysis of coalescence and separation behaviors are one of the main method to reveal the electrostatic atomization mechanisms and provide theoretical foundation for developing efficient electrostatic atomization technology. This thesis shows a relatively comprehensive research on the evolution behavior and electrohydrodynamic characteristics of charged droplet, the main research work and innovation points are as follows.Based on single droplet induction charging device, visualization has been given on the formation of liquid bridge and the motion behavior of charged satellite droplet under the effect of applied electric field. The rupture morphology of liquid bridge has been accurately captured and the meniscus angle is firstly used to judge the rupture sequence. In addition, with the electric Bond number increases, the transition behavior differences of the dripping mode for various liquid properties have been discussed in details.A systematical study on the coalescence and noncoalescence behaviors of charged satellite droplet is reported. From the view of energy conservation, an electric Bond number is defined to analyze its action mechanism on the motion behavior of satellite droplet. Based on the main droplet charge measurement device, a calculating method of charge on satellite droplet is firstly proposed and provides a measurement basis for the future research work on the interaction of charged droplets.The mode transition and stability analysis charged micro jet is proposed in detail. The hydrodynamic relaxation time and charge relaxation time are first used to discuss the micro-jet transition mechanism with different conductivities and the evolution morphology characteristic of the micro-jet with the influence of surface tension and viscosity has been analyzed. Subsequently, droplet size and velocity changes with the influence of jet transition have been discussed to find the action mechanism of the breakup mode on the atomization characteristics of biodiesel. By combining high-speed microscopy with mini-PIV technology, the jet instability of ethyl ethanol is analyzed. The stability atomization range of charged micro-jet is obtained which gives a general law for controlling the stability of micro-jet.The interaction behavior of oppositely charged droplets is experimentally studied. A visualization system for measuring the contact behaviors of oppositely charged droplets has been developed to discuss the noncoalescence and breakup characteristics under the effect of electric field. The noncontact bouncing and breakup behavior have been first accurately captured by use of high-speed microscopy with high space-time resolution. From the perspective of ion migration, the contact bouncing behavior with the influence of conductivities is redescribed. Air ionization theory combined with Rayleigh limit theory is first introduced to discuss the action mechanism of noncontact bouncing and breakup. The contact behavior range is defined through a series of orthogonal experiment and the breakup strength is evaluated by defining the breakup volume and the elongation rate.Special attention has been paid to breakup behavior at negative side of oppositely charged droplet. By analyzing the influence of interface curvature, electrode polarity and solution pH value on the selection of breakup polarity, the authenticity of special breakup behavior at negative side is verified under the gas environment. The helium gas environment is first used for a second capture to the noncontact rebound and breakup behavior of oppositely charged droplet and the polarity selection feature of noncontact breakup behavior has been carefullly studied. The action mechanism of breakup behavior which always happens at the negative side has been reported under the gas environment. In addition, the influence of gas ionization on noncontact rebound and breakup characteristic under air/helium environment charged droplet has been obtained.