Investigation On Characteristics Of Cone-jet In Electrohydrodynamic Atomization And Jet Breakup Instabilities

The cone-jet regime in electrohydrodynamic（EHD）atomization is widely applied into many fields such as bimolecular mass spectrometry,3D printing,nano-materials preparation,space propulsion and micro-scale fuel atomization due to its micro-scale or even nano-scale monodisperse droplets with high charge-mass ratio.A thin electrified jet was emitted from a Taylor cone apex and further disintegrates into many fine drops in the form of different jet breakup instabilities.The two typical instability breakup modes are varicose instabilities and whipping instabilities.The former produces droplets with uniform particle size at the end of the straight jet;the latter forms a curved kink jet at the end of the straight jet and break into fine droplets.It is helpful to promote the engineering application of the electrostatic cone jet atomization by adjusting the operating parameters to obtain different breakup modes of the micro-jet and study the atomization characteristics.In this paper,the complete recording of the atomization process of the EHD cone-jet under the needle-plate electrode was completed by a high-speed digital camera.The effects of nozzle structure,atomizing medium physical parameters（liquid surface tension,conductivity,density,permittivity and viscosity,etc.）and operating parameters（applied potential,liquid flow rate,etc.）on the steady cone-jet mode are investigated.The atomization characteristics such as the evolution of the liquid cone and jet characteristics under the capillary with the sheathed quartz tube are discussed in detail.The evolution law of the instabilities breakup shape of the cone-jet under the hemispherical nozzle with the applied potential,liquid flow rate and conductivity is analyzed.Based on the interface stress analysis,the criterion number for determining the transition of jet instability breakup mode is obtained by introducing scaling laws such as jet diameter and current.The main research contents and conclusions are as follows:（1）The applied potential and liquid flow rate of the steady cone-jet mode of anhydrous ethanol under the common capillary,the sheathed quartz tube and the hemispherical nozzle were compared.The results show that the sheathed quartz tube and the hemispherical nozzle can effectively expand the applied potential and liquid flow rate range of the steady cone-jet atomization,and can increase the minimum liquid flow rate of the steady cone-jet atomization.（2）A steady wheat shaped cone-jet atomization pattern can obtain by using the sheathed quartz tube to atomize the anhydrous ethanol.The Taylor cone angle decreases with the increase in the“electro-Weber”number,while increases as electric Bond number increasing;the jet breakup length decreases with an increase in the“electro-Weber”number and conductivity,while increases as electric Bond number increasing;the diffusion angle increases as“electro-Weber”number increasing,while decreases as electric Bond number and conductivity increasing.In addition,for a large liquid flow rate,it can be found the jet breakup shape changes from varicose to whipping instabilities.For high conductivity,an intermittent electrified jet appears and shows an umbrella plume,and the breakup length sharply shortens.（3）The electrohydrodynamic atomization were carried out on a variety of liquids such as ethanol,ethylene glycol（EG）,propylene glycol（PG）,and diethylene glycol（DEG）with the hemispherical nozzle,and different cone-jet instabilities breakup shape are recorded in the certain parameters ranges（feeding liquid flow rate,electric potential and conductivity）.The charged micro-jets show typical varicose and whipping instabilities successively and realizing the transition of instabilities breakup mode with an increase in liquid flow rate and conductivity for a fixed electric potential for most of liquids.In addition,the jet breakup length scaling laws show a single increasing trend with an increase in the liquid flow rate,however,the jet breakup length obtained experimentally shows a trend of first increasing and then shortening with the mode transition of the electrified jet.The same trend occurs when the conductivity increase gradually and a transition of the breakup mode.（4）Based on a large number of electrohydrodynamic atomization experiments,the charged jet breakup shape is divided into varicose instabilities mode,transition regime and whipping instabilities mode.Within the experimental range of this paper,the jet breakup mode hardly changes with the increase of applied potential.The jet breakup mode gradually changed from varicose to whipping instabilities mode with the increase in liquid flow rate and conductivity.By analyzing the electrified jet surface stress and comparing the orders of magnitude,the non-dimensional parameter G=Γ²δ_μ^0.52 including electric stress,surface tension and viscosity effects is obtained to characterize the criterion for breakup mode transition,whereΓis the electric force to surface tension ratio andδ_μis the inertia to viscous forces ratio.The results show that the varicose instabilities,transition regime,and whipping instabilities mode are classified with first critical threshold values G=40 and second critical threshold values G=102,respectively.