Mechanisim And Experimental Study Of Ultrasonically Electrostatic Spraying

Electrostatic spraying can produce micron and nano scale charged droplets.These charged droplets have monodisperse and controllable size.Considering these characteristics,it has been widely used in the fields of electrostatic coating,thin film preparation,nanoparticles preparation,space propulsion(colloid thruster),etc.However,the efficiency of charged droplets emitted by electrostatic spraying is dependent on the number of the needle emitters.At the present stage,the limitation of micro-machining and micro-assembling leads to the difficulty and high cost of needle emitter integration.The low number density of integrated needle emitters results in problems like low work liquid flow rate,low spraying current and low efficiency of emitting charged droplets.This dissertation is aimed at improving the number density of emitters of electrostatic spraying and a new technique called ultrasonically electrostatic spraying(UES)is studied.UES is a novel technology which combines electrostatic spraying and ultrasonic vibration.Under the work of ultrasonic vibration,dense capillary standing waves can be generated on the emitting surface.And these capillary standing waves are used to replace the needle emitters of traditional electrostatic spraying.At the same time,a strong electrostatic field is applied to the capillary standing waves.Then,Taylor cones would be formed at the crest of the capillary standing waves and charged droplets are emitted from them.Adjusting the frequency of ultrasonic vibration to tens of thousands Hz,several millions of capillary standing waves can be obtained per square centimeter.The number density of emitters can be greatly improved by using UES.This dissertation analyzed the emission mechanism of UES theoretically in detail,including the formation process of capillary standing waves on the emitting surface with ultrasonic vibration,the formation process of Taylor cone at the crest of capillary standing waves under the strong electrostatic filed and the emission process of charged droplets.An emission model was built.Theoretical formulas about the radius,charge-mess ratio of the charged droplets and the spray current were deduced.Besides,theoretical formulas about specific impulse and thrust used in space propulsion were also proposed.Then,the factors which influenced the working performance of UES were theoretically discussed.Additionally,based on the theoretical emission model,numerical analysis was carried out to predict the working performance of UES using a kind of typical liquid.To explore the practical working performance of UES,an experimental setup with adjustable working parameters was built.The influences of voltage applied on the extractor,distance between the emission surface and the extractor,ultrasonic vibration frequency,ultrasonic vibration power,flow rate of the working liquid,size of the extractor and properties of the working liquid on the working performance of UES were experimentally investigated.Besides,the working performance of UES in vacuum was also studied.The experimental results showed that properly high voltage applied on the extractor could increase the spray current,while a too high voltage may lead to electric filed breakdown and experimental facilities damage.The distance between the emission surface and the extractor should not be too large or too small.Reasonable distance can improve the emission performance.Increasing the vibration frequency can increase the number density of the emitters and the charged droplets to obtain higher spray current.An optimal vibration power can be experimentally achieved to maximize the spray current.For the flow rate of working liquid,there existed an optimal flow rate to balance its consumption and supply.Low flow rate would bring emission efficiency degradation.Too high flow rate would result in waste of working liquid and instability of the emission.Proper size of the extractor was beneficial to emit charged droplets.The working liquid with high conductivity,low surface tension coefficient and low viscosity was more favorable to improve the emission performance.Compared with the emission performance in the air,that of the UES in a vacuum chamber was better.In a conclusion,the study of this dissertation showed that UES has a great potential in improving the emitting current and the efficiency of emitting charged droplets.It can be expected to solve the problem of low spray current in traditional electrostatic spraying.