Abstract
Evaporation of charged droplets is the fundamental aspect of many technologies, such as the electrospray cooling, spray drying, and ink-jet printing. In this study, the ionic wind flow properties, as well as the subsequent oscillating characteristics and evaporation performance of the hanging droplets influenced by the ionic wind were experimentally investigated. Deionized water was used as the working fluid and digital camera was used to capture the evaporation behavior of pendant droplets. Particle image velocimetry (PIV) was employed to characterize the ionic wind flow generated by a pin-to-plate non-uniform electric field, where the needle electrode was connected to a high-voltage power supply. Effects of electric field and gas environment on the evaporation rate, oscillation amplitude and oscillation frequency were systematically studied. The results showed that the ionic wind will not appear until the applied voltage is higher than the corona voltage (6 kV for air). The critical corona voltage varied with the ambient gas. Without ionic wind, the evaporation was found restrained by the electric body force. On the opposite, the evaporation process was enormously improved when the ionic wind was formed. In addition, the development state of ionic wind would change from unstable form to fusiform shape when the voltage raised. Both the increase in external electric field strength and the ion mobility in gas were found to increase the ionic wind speed which leaded to the enhancement of oscillation amplitude and evaporation rate. Effect of electric field intensity on droplet evaporation could stack with ion mobility with a maximum enhancement ratio of about 9.9 times compared with naturally evaporated droplets. This study demonstrated that the ionic wind significantly enhanced the evaporation of pendant droplets, which would contribute to a better understanding of the heat transfer enhancing mechanism under the non-uniform electric field.
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