A study of the temperature effect on the spray characteristics in the cone-jet mode of electrohydrodynamic atomization (EHDA) with viscous liquids

Abstract

In this work, the near-field spray characteristics of electrohydrodynamic atomization (EHDA) for viscous liquids (ethanol, G20, G40, G50, G66, and glycerol) under various temperatures T (T is from 293 to 343 K), electric Bond numbers BoE (BoE is from 0 to 3.5) and dimensionless flow rate Q* (Q* is from 11 to 400) have been investigated by employing a high-speed imaging technique. The transition of the spray modes, variations of the spray angle θ1, semi-angle of Taylor cone θ2, and spraying droplet size (the Sauter mean diameter D32 and the probability density function) in the cone-jet mode have been studied experimentally. The results indicate that the stable cone-jet mode disappeared regardless of increasing BoE for working fluids with relatively higher viscosity (G50, G66, and glycerol). Nevertheless, the temperature elevation promotes the appearance of the stable cone-jet mode, e.g., G66 fluid could form the stable cone-jet as liquid temperature increases to 343 K. Moreover, the temperature plays a significant role in improving the spray angle and the semi-angle of the Taylor cone, as well as droplet size distributions. Specifically, in the case of G40 fluid, the spray angle increased from about 20.8°–23.9° at room temperature (293 K) to around 34.1°–37° at 343 K. Meanwhile, the droplet size distributions were shifting from 9.73–35.49 μm at 293 K to 4.39–23.84 μm at 343 K. The increase in temperature causes a dramatic viscosity reduction in highly viscous fluids, and the viscous dissipation during the atomization reduced substantially. As a result, more kinetic energy was retained to overcome the surface energy and thus improve the quality of the spray. In addition, the dimensionless droplet size D* in the stable cone-jet mode shows a linear scaling relationship with Q*. By introducing an Arrhenius-type equation to account for the temperature effect, a model to predict dimensionless droplet size D* under various dimensionless flow rates Q* and liquid temperatures T has been proposed. The model is in good agreement with the experimental data under the stable cone-jet mode in the EHDA for viscous fluids at a range of temperatures.