Theoretical And Experimental Study On Electrohydrodynamic Microjet Atomization

The process in which liquid ejected in different atomization modes from a nozzle with a small inner diameter under the action of a high voltage electric field and further broken into fine droplets by electrostatic force is electrohydrodynamic atomization（EHDA）.Electrohydrodynamic atomization can produce highly charged monodisperse droplets in a controlled manner,and the droplet size and spatial distribution can be regulated by adjusting the voltage.This technology is used in a wide range of industrial applications such as electric spinning,biopharmaceuticals,aerospace propulsion,film preparation of nanomaterials and 3D printing.In different applications,different droplet characteristics are required,which can be achieved by adjusting the operating parameters to obtain different electrohydrodynamic atomization modes.Among the typical atomization modes such as dripping,spindle,cone-jet and multi-jet,the cone-jet mode is considered to be one of the most promising atomization modes.However,the cone-jet mode usually occurs in a small range and cannot be used in applications with high liquid flow rates,such as desalination,pharmaceutical spraying,fuel injection,etc.The simple-jet mode is able to provide electrohydrodynamic atomization at higher liquid flow rates and meet the needs of specific fields.This paper uses theoretical and experimental methods to study the atomization process of the simple-jet.The evolution of various jet breakup patterns of anhydrous ethanol and deionized water is recorded in detail with a high-speed digital camera.The jet breakup patterns are systematically classified and analyzed,and the dispersion equations describing the unstable breakup of low viscous charged liquid jets are established by theoretical methods.The main research contents and conclusions of this article are as follows:（1）The occurrence of simple-jet mode and the evolution of breakup morphology of the simple-jet mode for different liquids（anhydrous ethanol,deionized water）are observed experimentally.The evolution of the charged liquid（anhydrous ethanol）from a cone-jet to a simple-jet is also observed with a special hemispherical nozzle.In the case of basically stable potential,with the liquid flow rate increases,the Taylor cone at the tip of the nozzle gradually elongates and disappears,and the cone-jet gradually transforms into a simple-jet.The formation of the stable simple-jet mode of anhydrous ethanol at different Weber numbers（We,related to the liquid flow rate）is investigated with a common metal nozzle.In the low Weber number of dripping state,when an electric field is gradually applied to the liquid by a high voltage electrostatic generator,an unstable simple-jet mode has developed.When the Weber number is above the critical value We_cr,a stable simple-jet mode will form after a certain strength of electric field is applied in both the dripping faucet and jetting states.As the electric potential increases,the jet undergoes typical break-up patterns including varicose instability,whipping instability,whipping assisted bifurcation and ramified.（2）The atomization characteristics of the simple-jet mode are investigated.The jet and its morphology of breakup into droplets are analyzed for different operating parameters.The spectra of anhydrous ethanol and deionized water undergoing steady simple jets are determined.The critical values of the transition of the break-up morphology are discussed.The droplet size distribution,the jet stable length and the atomization angle of the simple-jet mode are investigated.The influence of the operating parameters（flow rate,voltage）on the different atomization characteristics are analyzed.It is shown that the simple-jet mode is able to produce charged monodisperse droplets in a controlled manner.Under certain external parameters,the anhydrous ethanol simple-jet can produce a large number of charged monodisperse droplets with a particle size of about 40μm.The stable length of the simple-jet decreases with the increase of the electric potential and increases with the increase of the flow rate.The jet atomization area can be divided into two parts:the central area and the peripheral area.The central and peripheral atomization angles increase with the increase of potential and decrease with the increase of flow rate before the appearance of ramified instability.（3）A dispersion equation describing the instability of a low viscous charged liquid jet is developed using the Laplace equation and the differential equation of motion for viscous fluids.Electrodynamic and hydrodynamic boundary conditions are considered.The equation systematically describes the effects of flow velocity,electric potential and viscosity on charged jets.The analysis of the equation explores the effect of different physical parameters on the stability of the charged jet and compares the analysis with the experimental content with MATLAB.The increase in viscosity promotes the stabilization of the charged jet.The increase in surface tension increases the instability of the jet and makes it more difficult to form a stable simple-jet.Variations in external parameters such as electrode spacing and initial diameter of the jet also have an effect on the surface wave growth rate.The variation of the dominant effects of axisymmetric and non-axisymmetric surface waves in the breakup process of the charged jet under different experimental conditions is analyzed.Theoretical and experimental values of charged jet wavelengths have been compared.The results show that the dispersion equation derived in this paper predicts the wavelength of axisymmetric surface waves in good agreement with the experimental observations.