Study On Atomization Performance Of Coaxial External-mixing Air-assisted Atomizer

In air-assisted atomizing nozzle,according to the different mixing parts of gas-liquid in the nozzle,it can be divided into external mixing type,internal mixing type and compound type.Coaxial external-mixing air-assisted atomizer is widely used in the fields of pesticide spraying,medical pharmacy,metal spraying and so on due to its high atomizing quality and high viscosity atomizing fluid.Most of the previous researches on nozzles focus on the internal mixing nozzles or the mechanical atomizing nozzles without the assistance of gas,while relatively few researches on the coaxial external-mixing air-assisted atomizer.Moreover,the research object is mostly Newtonian fluid and the research variable is relatively single.Based on the experimental measurement and numerical simulation,by changing the working condition and nozzle structure,the relevant parameters,like spray angle,droplet size,pressure and speed in the flow field,are analyzed in this paper,in order to explore the influence of external environment and internal structure on the atomization performance of coaxial external-mixing air-assisted atomizer,and to improve the atomization performance of coaxial external-mixing air-assisted atomizer on the twophase flow field and target surface in practical projects.The research contents are mainly divided into the following two aspects:Firstly,based on the atomization mechanism of coaxial external-mixing airassisted atomizer,the experimental platform of spray angle high frame speed image shooting and droplet size measurement was built.According to the experimental results,the research conclusion can be summarized as the following four points:(1).when the nozzle is sprayed under the condition of no shaping air hole pressure,the spray angle is directly proportional to the liquid viscosity,and inversely proportional to the fluid flow and annular hole/auxiliary atomization hole pressure;(2).when the nozzle has shaping air hole pressure,the greater the viscosity is,the smaller the spray angle is,and the more difficult it is to atomize the liquid;the fluid flow and the pressure of the shaping air hole are directly proportional to the spray angle,while the pressure of the annular hole/auxiliary atomization hole is inversely proportional to the spray angle;(3).when the atomization condition of the nozzle is different,the change trend of droplet size at different positions in the flow field will be obviously different;(4).the influence trend of the average particle size of the same axial plane under different working conditions is also different.Secondly,based on the computational fluid mechanics technology,the coaxial external-mixing air-assisted atomizer was modeled and simulated,and the reliability of the simulation results was verified by the experimental results.Based on the importance of shaping air holes to the coaxial external-mixing air-assisted atomizer,the influence of shaping air holes on atomizing performance of nozzles was analyzed.The results are summarized as follows:(1).with the increase of the distance between the junction point and the liquid hole(Δ),the maximum pressure at the airflow intersection of the shaping air holes decreases,the high-speed tapered area in the flow field is obviously extended,and the velocity at the same axial position of the nozzle increases;(2).the larger the value ofΔ,the smaller the peak value of droplet particle size distribution on the target surface,and the more dispersed the particle size distribution,which is not conducive to atomization;(3).the greater the value ofΔ,the more obvious the aggregation effect of droplet particles on the target surface,and the more likely it is to cause local overspray.