UCLA

The electrospray (ES) thruster currently requires significant life and performance improvements. This dissertation aims to investigate the critical physics of low to high-conductivity cone-jet and droplet formation of ES thrusters to provide detailed descriptions of electrospray cone, jet, droplet, and ion formation for life and performance modeling. The leaky-dielectric model is incorporated in the Finite Volume Method (FVM) code, OpenFOAM, to investigate the electrospray emission behavior of low to high-conductivity liquids. This work extends FVM modeling to high conductivities by employing a new interface interpolation scheme devised in the Volume of Fluid (VOF) method to ensure charge conservation for accurate reproduction of charge accumulation and the resulting meniscus shape in the cone-to-jet region and jet breakup. Modeling 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 (TBP), respectively. The droplet diameter is shown to increase as the dimensionless flow rate increases or the electric Reynolds number decreases. Results are consistent with a parametric investigation of the meniscus shape, 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 provide valuable physical insights for varying liquid conductivity in the electrospray emission process. In particular, a low dimensionless flow rate or high electric Reynolds number leads to the emergence of convex-outward menisci associated with high charge density in the cone-to-jet region, resulting in high jetting velocity and high specific charge droplets.

The propellant temperature can significantly impact the cone-jet droplet emission and ionization process in the droplet breakup and transition region. The energy equations are employed for the governing equation of the finite volume model to accurately predict the energy of the charged droplets under different operating conditions. The new interface interpolation scheme devised in the Volume of Fluid (VOF) method is implemented to ensure energy conservation for accurate temperature prediction due to Joule dissipation, i.e., Ohmic heating and viscous dissipation. The increasing temperature is observed after the cone-to-jet region, in which internal and outer electric fields at the liquid surface are highest along the jet. Ohmic dissipation tends to reduce with increasing electrical permittivity where high charge relaxation impedes the electric field.

The axisymmetric two-dimensional leaky dielectric EHD model is extended to a three-dimensional EHD model to investigate steady tilted cone-jet emission. Modeling results agree well with experiments and scaling laws for jet diameters with increasing electrical Bond numbers. The tilted cone-jet results from the asymmetric tangential electrostatic force at the dilated surface with a higher radius of curvature of the tilted cone above the critical electrical Bond number. Increasing electrical permittivity leads to a lower critical electrical Bond number tilting the cone, though a higher charge relaxation results in a larger jet radius.