Simulation Of Nozzle Internal Flow And Spray Formation For Vacuum Spray Method

Vacuum spray method is a new method for preparing polymer thin films. It has attracteda great deal of interest because of its advantages such as dense films fabrication, uniform andcontrollable film thickness and preparation of gradient thin films, etc. We first review theexisting vacuum spray methods to study the film-forming process. It is found that althoughthese methods have built experimental apparatus and obtain polymer films with goodperformance, the film-forming process has no detailed analysis. Numerical simulation is agood choice to investigate the vacuum spray method as the long period and big cost ofexperimental study. The spray process can be divided into the nozzle internal flow, external flow, anddeposition of droplet. Our simulations mainly research the nozzle internal and external flow,and nozzle internal flow provide the initial conditions for external flow. First, we mainlysimulate the nozzle internal under different outlet pressures, injection pressures, nozzlediameters, nozzle lengths, and simulation fluids. The cavitation has a great impact on thenozzle internal flow. We study the influence of cavitation on the nozzle internal flow bycomparing the vapor and turbulence kinetic energy distribution on the different positions ofnozzle under different simulation parameters. In addition, the nozzle discharge coefficientunder different simulation parameters is analyzed. The simulations of external flow arecorresponding to the simulation results of internal flow. We research the effect of simulationparameters on the spray characteristic by analyzing the spray beam width, spray penetration,Sauter Mean Diameter of droplet, and the overall distribution of droplet diameter. It is found that the nozzle internal flow has no obvious changes when outlet pressure isbelow0.1MPa, the overall distribution of droplet diameter shows little variation when backpressure is less than1KPa. A higher injection pressure gives a better spray characteristic. Thevapor and turbulence kinetic energy increase with the injection pressure increases, however,when injection pressure is above30MPa, the vapor is no significant change. Inner wall andradial vapor fraction at the nozzle exit decrease as the nozzle diameter decreases, but the axial vapor fraction increases as the nozzle diameter decreases. Nozzle with small diameter shows a better spray characteristic. The external flows of nozzle with different lengths have no difference. Nozzles with short length give bigger vapor on inner wall and outlet radial, but have smaller vapor on axis. The turbulence kinetic energy decreases as the nozzle length increases. Both the internal and external flow of the three organic solvents is better than water. The simulation researches provide some basic information for the experimental study of vacuum spray method, and make the direction for further research.