PPPS-2013: Numerical simulations of ionic liquid electrospray thrusters

摘要

An innovative model of an ionic liquid electrospray thruster based on Molecular Dynamics was developed. Two coarsegrained potentials1 (a simple coarse-grained and an effective-force coarse-grained (EFCG)) models were used, rather than an all-atom model, to model an entire electrospray thruster. First, the impact of the coarse-grained model on the ionic liquid EMIM-BF4 was studied. A system representing an electrospray capillary was then equilibrated for both CG potentials, and an extrusion model was developed based on a repulsive wall potential to simulate a constant mass flow inside the capillary. Multiple constant electric fields were applied to the system and the mass flow rate was also varied. Three ionic species, monomers (EMIM+), dimers ([EMIM-BF4]EMIM+) and trimers ([EMIM-BF4]2EMIM+), as well as droplets (charged clusters larger than 8 ions) were observed and their distributions were recorded at a distance 0.1 µm away from the center of the capillary at a virtual extractor ring. Current values were derived from the measured distributions. The formation of a Taylor cone was observed when the mass flow was sufficiently high. The EFCG potential replicated an experimental setup best for an electric field of 0.5 V/nm, where solvated ions and droplets were observed. The computed values provided a good match for the values measured by Romero et al.2, especially at low mass flow rates. When the mass flow rate increased, larger droplets tended to break apart and lead to an increase in the monomer current. Higher values of the electric field resulted in almost exclusively monomers being extruded and the prediction that the electrospray would operate in a multi-jet mode, hence over-estimating the measured current values. When the polarity was reversed, very similar results were obtained, paving the way for the use of EMIM-BF4 as an efficien- propellant for arrays of emitters with switching polarities, that would require no neutralizing system. Thrust and specific impulses were estimated based on the exit velocities of charged clusters. The estimated thrust generally offered good agreement with measured values while the specific impulses were under-estimated. Finally, similar computations were done on a simple CG based potential model. The interatomic potential in the simple CG model required a higher external energy to extrude charged particles from the capillary, when compared to the EFCG model.