An experimental investigation on the electrospray characteristics in a meso-scale system at different modes

Abstract

An experimental study on electrospray characteristics is carried out using a steel capillary as the nozzle and the grounded steel mesh as the collector of droplets in a meso-scale system. The liquid ethanol is atomized by the electrostatic field. Four spraying modes (pulsed-jet, cone-jet, skewed cone-jet, multi-jet) are visualized along with the increase of applied voltage on the nozzle. The droplet size, velocity and RMS (root mean square) velocity are measured using PDA (Phase Doppler Anemometer). It is found that both specific charge and axial electric field around the nozzle increase when the four different spraying modes change in sequence. The most of atomization of liquid ethanol often occurs around the nozzle outlet, which is inferred from the calculated results of axial electric field distributions. The droplet size decreases when the four different spraying modes change in sequence, because the increasing specific charge and electric field force lead to further break up of liquid droplets. At the same cross section of z = 10 mm, the axial velocity at the pulsed-jet mode is the largest and the axial velocity is nearly the same for cone-jet mode, skewed cone-jet mode and multi-jet mode. The distribution of RMS velocity at four spraying modes indicates that electrospray is fluctuant at the pulsed-jet mode and it is stable at the cone-jet mode, skewed cone-jet mode and multi-jet mode. The droplet is accelerated from the nozzle due to the stronger electric field intensity around the nozzle, but the droplet velocity has little change due to the lower electric field intensity after z = 10 mm. Theoretical analysis is performed based on the Rayleigh limit model, and it is found that the Rayleigh limit coefficient is between 0.4 and 0.6 at the cone-jet mode, between 0.2 and 0.4 at the pulsed-jet mode and the skewed cone-jet mode, and below 0.2 at the multi-jet mode.