Simulation of electrospray emission processes for low to moderate conductivity liquids

Abstract

The leaky-dielectric model is incorporated in the Finite Volume Method (FVM) code, OpenFOAM, to investigate the electrospray emission behavior of low to moderate conductivity liquids. This work extends FVM modeling to moderate conductivities by employing a new interface interpolation scheme that is devised in the volume of fluid method to ensure charge conservation for accurate reproduction of charge accumulation and resulting meniscus shape in the cone-to-jet region and jet breakup. The model results agree well with experiments and scaling laws for droplet diameter and total current for low and moderate conductivity fluids, i.e., heptane and tributyl phosphate, respectively. The droplet diameter is shown to increase as the dimensionless flow rate increases or the electric Reynolds number decreases. The results are also consistent with a parametric investigation of the meniscus shape and the maximum charge density for key operating conditions (flow rate and extraction potential) and liquid properties (conductivity, surface tension, viscosity, and relative permittivity). These results show that the new interface interpolation scheme provides accurate results for a wide range of conductivities, fluid properties, and operating conditions. The results also provide valuable physical insight for varying liquid conductivity in the electrospray emission process. In particular, low dimensionless flow rate or high electric Reynolds number leads to the emergence of convex-outward menisci associated with a high charge density in the cone-to-jet region, resulting in high jetting velocity and high specific charge droplets.