Study of recirculation behaviors in Taylor cones based on numerical simulations

In this study, we focus on investigating the hydrodynamics mechanism of recirculation cells (RCs) inside the Taylor cone. Based on the flow fields quantitatively obtained in numerical simulations, the startup process of RCs is established within 1 ms of the voltage being switched on. The time evolution of the RCs indicates that their intensity, quantified as the local velocity-gradient tensor, is highly dependent on the surface charge density. This is accompanied by surface charge convection under tangential electrical stress, creating a high-pressure region at the tip and pushing the liquid to flow backward. The effects of main process parameters, including liquid flows, voltages, physical properties of liquids, and temperature of electrospray device on the RCs, are given numerically, and the local competition between the viscous shear stress and tangential electrical stress is analyzed qualitatively through nondimensional analysis. The research shows that at higher fluid electrical conductivities, the RCs quickly reach their maximum intensity as the surface charge saturates. Cone-jets with high flow rates reduce the surface charge, and high fluid viscosities lower the velocity gradient, both of which weaken the recirculation. It is also found that the recirculation can be eliminated by lowering the temperature because the fluid becomes less electrically conductive and more viscous.