| The micron and nanometer droplets produced by the cone-jet in electrohydrodynamic(EHD)atomization have unique advantages such as good monodispersity and high charge.They are widely used in biomolecular mass spectrometry,electric fluid printing(2D and 3D),space propulsion,nanostructured material preparation,micro-scale spraying combustion and heat transfer,and even the production of food and medicine in daily lives.In different application fields,the physical parameters of the atomized base liquids(surface tension,conductivity,viscosity and dielectric constant),flow rate,applied voltage and other experimental parameters,as well as the electrode form,are all important to the formation and stability of the Taylor cone as well as jet break up.The influence of the atomization mode,atomization characteristics and controllability are the key basic issues of EHD cone-jet application.In this paper,an electro-fluid atomization device under the needle plate electrode was designed and built.A high-speed photography system equipped with a 12x micro-zoom lens was used to visualize the atomization phenomenon of the electrohydrodynamic(EHD)cone-jet of the metal capillary with various diameters.This research discusses the shape of Taylor cone and the dynamics of the charged microjet in the stable cone jet mode(jet flow initial jet velocity)and spatial characteristics(jet length and position),etc.,and further studied the fragmentation morphology transformation and size distribution characteristics of the charged microjet.The main research content and conclusions of this article are as follows:(1)Through experimental research,the operating parameter envelope of several atomization medias(ethanol,diethylene glycol(DEG),propylene glycol,etc.)for electric fluid stabilized cone-jet atomization has been determined.The research results show that the chocked cone jet is obtained at a small supply flow rate(usually less than the minimum flow q0)and a small applied voltage(usually less than the minimum?m);the pulsating cone jet is mainly formed at a larger flow rate(usually slightly greater than the minimum flow q0)and a smaller voltage(usually less than the minimum?m).For the given flow rate,the Taylor cone in the stable cone-jet mode has a‘convex cone-right cone-concave cone'transition with the increase of applied voltage.The analysis of the interface stress of the Taylor cone shows that the tangential electric field force is the dominant force for the formation of the cone interface and the jet acceleration.In addition,the cone shape is also affected by the supply flow rate.At a lower flow rate,the liquid cone is fuller;at a higher flow rate,the liquid cone is sharper.(2)Several typical cone jet modes(pulsating cone,stable cone,droplet or coulomb splitting at the tip of the spindle,etc.)under different physical parameters have been obtained.The research analyzes the pulsating characteristics of the pulsating jet,the initial jet velocity,the length of the jet breakup,and the dynamic characteristics when the jet end is unstable and the varicose breakup occurs.Studies have shown that the pulsation characteristics of cone jets are related to flow rate and applied voltage.The diameter and length of the charged microjet are positively correlated with the increasing flow rate.In addition,the occurrence of unsteady charged micro-jets modes(bifurcated jets,unsteady multi-jet,oscillating jets,etc.)are related to the liquid medium and electric field.(3)In the EHD cone jet mode,the charged microjets form droplets due to two typical breaking modes:varicose(axisymmetric)instabilities and kink(inaxisymmetric)instability breakup modes.Research shows that the breakup mode of droplets mainly depends on the flow rate.With the increase of the flow rate,the charged jet breakup mode gradually transits from the varicose instabilities to the kink instability.the‘size distribution mechanism'in the cone-jet mode is also showed(the size segregation effect is the most obvious in the kink instability).In the stable cone jet mode,jet breaks up into droplets at the jet end,the droplet size decreases with decreasing flow rate(or the increasing applied potential).The dimensionless diameter(d/d0)as a function of the dimensionless flow rate(q V/q0)is in good agreement with the universal scaling laws. |
PDF Full Text Request