Experimental and numerical studies of electrostatic liquid spraying

Electrostatic liquid spraying has been successfully used in many industrial applications such as painting and pesticide spraying, and it continues to find new applications in many fields. The basic process of electrostatic liquid spraying includes droplet formation and charging, transportation and deposition. This thesis investigates the electrostatic spraying process both experimentally and numerically. Of the various droplet charging methods, conduction and induction charging are clarified and a comparison is made between these two methods regarding the charging mechanism, electric field distribution, electrode wetting, energy conversion and space charge effect for a two-electrode and a three-electrode geometry. It is shown that for a two-electrode arrangement with the same geometry and under similar conditions, conduction and induction charging have similar nozzle and target currents. However, for the three-electrode arrangement which is more commonly used in practice, the electric field distribution and space charge cause differences in leakage current and target current. Experiments showed that for the three-electrode geometry, conduction charging produces larger target current and smaller electrode leakage current when compared to induction charging.;In the charged droplet transportation process, the effect of space charge on the performance of an air-assisted embedded electrostatic induction-charging spray nozzle is investigated. The experimental work shows that the space charge does not affect the total current induced upon the liquid within the nozzle; rather, it affects the distribution between the space charge cloud current and the electrode current. The main reason for this is that a number of highly charged droplets escape the air entrainment by the repulsion of the space charge field and are repelled radially or are deposited on the nozzle's external surface, adding to the electrode leakage current. The stronger the space charge field, the more droplets are repelled. The result of this "feedback" phenomenon is that under constant atomizing air pressure and within a certain voltage range, the space charge effect makes the target current constant at a certain distance away from the nozzle, irrespective of the electrode voltage. For the same target current, a lower electrode voltage can be applied.;A further study was carried out on the droplet size distribution and charge-to-mass ratio of the above mentioned nozzle under different atomizing conditions by using a grounded shield to collect the droplets repelled to the nozzle's surface by the space charge force. The results showed that these droplets have a much higher charge-to-mass ratio and a relatively smaller mean drop size than those reaching the target. The charge-to-mass ratio of the droplets reaching the target is much lower than that delivered at the nozzle outlet because of space charge force caused drift which is greatly affected by the nozzle-to-target distance.;An analytical model of the air-assisted electrostatic spraying process in terms of electrogasdynamic (EGD) energy converter is put forward in which a one-dimensional model is used to analyze the electric field and potential distribution, the electric energy contained in the space charge cloud and the electromechanical power conversion. The charge deposition efficiency is expressed as the ratio of target current to nozzle current. It is shown that the generated electric field is bi-directional and depending upon the position in the spray cloud, it either acts against the mechanical force or provides an additional force to drive the droplets to the target. As the nozzle-to-target distance increases, the space cloud expands, resulting in more drift and lower deposition efficiency which was confirmed by experimental results.;Finally, a numerical simulation was carried out in which a CFD software---FLUENT---was used to simulate the electrostatic liquid spraying process with an air-assisted electrostatic induction-charging spray nozzle. The air flow and the droplet discrete phase were simulated by FLUENT solver. User defined functions (UDF) were employed to solve the space charge field formed by the charged droplets. The electrostatic field exerts an electrostatic body force on the charged droplets which in turn modifies the motion of the charged droplets. Coupling between the air flow, the droplet discrete phase and the electrostatic field yielded the trajectories of the charged droplets. The simulation work confirmed that under constant atomizing air pressure and liquid flow rate, the droplet trajectories diverge as the charge-to-mass ratio and nozzle-to-target distance increases. Smaller droplets are more likely to drift to nearby grounds.;Keywords. electrostatic spraying, liquid, conduction charging, induction charging, space charge, charge-to-mass ratio, drop size, electrogasdynamics, numerical simulation.