Numerical Simulation Of Piezoelectric Droplet Ejection Process Under Coaxial Airflow

Droplet ejection technology evolved from inkjet printing technology,which mainly refers to printing ink droplets to form objects with specific shapes and patterns,while droplet ejection technology assists industrial production and manufacturing by printing micro fluids of specific materials.Droplet ejection technology is mainly divided into continuous droplet ejection technology and on-demand droplet ejection technology,among which piezoelectric-driven droplet ejection technology uses the unique inverse piezoelectric effect of piezoelectric materials to drive droplet ejection,which has three main advantages: fast response speed,high ejection efficiency and on-demand ejection.Because of these three advantages,piezoelectric-driven droplet ejection technology has been widely used in engineering applications and scientific research fields.However,the fluid shape changes and the flow field changes in the air domain involved in the piezoelectric droplet ejection process are extremely complex,so it is impossible to fully reveal its flow principle only through experimental research,while numerical simulation can accurately and intuitively study the droplet forming principle,fluid shape changes and the flow field changes in the air domain by simulating the actual process through simulation calculation.Therefore,in this paper,a two-dimensional numerical model of piezoelectric nozzle with coaxial airflow ejection slot is established to explore the influence of coaxial airflow on the ejection process of piezoelectric nozzle forming complete droplets According to the calculation results,the distribution characteristics of velocity field and pressure field in the air domain inside the nozzle and outside the nozzle are analyzed,and compared with the experimental results to verify the rationality of the simulation results Finally,neural network and genetic algorithm are used to optimize the key structural parameters of the sprinkler.The main research contents are as follows:(1)the ejection problem of piezoelectric sprinkler under the action of coaxial airflow is simplified as a two-dimensional numerical model corresponding to the actual sprinkler,and two-dimensional numerical simulation is carried out on the complete droplet ejection process and fluid flow law of piezoelectric droplet sprinkler and the flow field changes in the internal and external air fields of the sprinkler by using the multi-physical field simulation software COMSOL Multiphysics.In the simulation calculation,the influences of the support base,the upper cover and the sealing ring are not considered,but only the influences of the key components such as piezoelectric ceramic sheet,thin copper sheet,gas inlet,liquid inlet,liquid storage chamber,nozzle and air domain are considered.The material properties and boundary conditions are set in COMSOL software,and the grid is divided for calculation and solution.(2)Through the calculated simulation results,the variation characteristics of pressure field and velocity field in the air domain outside the spray chamber and nozzle are analyzed.The simulation results show that the coaxial airflow is dispersed to the left and right sides after meeting directly below the nozzle,so that axisymmetric paired vortices are formed in the air domain.With the continuous jet of airflow,the number of vortices increases,expands and moves downstream of the nozzle.With the introduction of coaxial airflow,the amplitude of piezoelectric plate increases,and the pressure in air domain decreases,while the pressure value in vortex region which is located just below the nozzle and formed by airflow diffusion on both sides decreases more significantly.(3)According to the numerical simulation results obtained in COMSOL software,the influence of coaxial airflow on droplets in the droplet ejection process of piezoelectric nozzle is analyzed,and the main conclusions are as follows: With the enhancement of the intensity of coaxial airflow introduced by coaxial airflow ejection slot,that is,the speed of air fluid increases,the moment when droplets break into droplets from the initial liquid column is delayed,and the shape of droplets gradually tends to be spherical from the ellipsoidal shape at the initial break,The initial extension length of droplet after flowing out of nozzle is increased,the diameter of droplet after breaking is increased,the velocity of front end and rear end of main droplet and satellite droplet are increased,the separation time between main droplet and satellite droplet is delayed,the droplet ejection velocity is increased,and the displacement and equivalent diameter of main droplet and satellite droplet are increased.(4)In this paper,a multi-objective optimization model of piezoelectric sprinkler under coaxial air flow is established by using multilayer feedforward neural network(BP neural network)trained by error back propagation algorithm and non-dominated sorting genetic algorithm with elite strategy(NSGA-?).Firstly,the numerical model of piezoelectric nozzle is introduced into COMSOL Multiphysics for numerical analysis.Then,the prediction model of piezoelectric nozzle under the action of coaxial air flow by BP neural network is taken as the objective function of NSGA-II for multi-objective optimization.Finally,the optimal parameters of piezoelectric nozzle with coaxial air flow injection slot are as follows: the diameter of piezoelectric ceramic plate is 22 mm,the diameter of nozzle is 0.15 mm,the velocity of coaxial air flow flowing from coaxial air flow injection slot is 0.45 m/s,and the nozzle cavity Under this optimal parameter combination,the droplet velocity is 0.85 m/s and the droplet divergence rate is 11%.This is very close to the result calculated by importing the numerical model under the optimal parameter combination into COMSOL software,and the error is less than 20%.At this time,the introduction of coaxial airflow does not interfere with droplet ejection,but assists droplet ejection in a better shape to accelerate straight down.To sum up,the numerical model of piezoelectric droplet ejection head under the action of coaxial airflow is established in this paper,and the pressure field and velocity field in the ejection chamber and air domain are numerically simulated According to the results of two-dimensional numerical simulation of piezoelectric nozzle under the action of coaxial airflow in COMSOL software,the influence of coaxial airflow introduced by coaxial airflow ejection slot on droplet ejection phenomenon of piezoelectric nozzle is analyzed BP neural network and NSGA-II genetic algorithm are used to optimize the key structural parameters of piezoelectric sprinkler under coaxial airflow,which lays a solid theoretical foundation for improving the structural parameters of piezoelectric sprinkler under coaxial airflow.