Pioneered by the Nobel laureate John Fenn, electrospray has found application in various fields, including mass spectrometry, electrospinning and micro-nano encapsulation. Although invented in the mid-twentieth century, electrospray thrusters were unsuitable for satellites because they required very high voltage, greater than 10 kV. This was mainly attributed to the poor conductivity of glycerol, the commonly used propellant material during that time. Nevertheless, with the recent rapid development of micro-nano satellites as well as the growing application of ionic liquid characterized by high electrical conductivity and near-zero volatility, the potential use of electrospray thrusters has been revitalized. Unlike other space systems, electrospray thrusters are considered potential candidates for micro-nano satellites applications owing to their advantages of high specific impulse, low energy consumption and small size.
The structure of a typical electrospray thruster consists of a capillary emitter, an extractor electrode and the propellant. The impact of surface tension and electrostatic forces results in the formation of cone-jet mode and latter electrospray beam, which play a vital role in the functionality of the thruster. Unfortunately, the conventional electrospray thruster with a capillary emitter fails to produce a high specific impulse due to the tradeoff effects between the electrospray instability and mass flow. Besides, they operate at a single mode with limited thrust. To overcome this problem and achieve dual-modes propulsion, a hybrid emitter consisting of a capillary and externally-wetted needle has been proposed as a viable alternative. The hybrid emitters can operate at dual modes: capillary emission and externally-wetted emission modes. Despite the progress, a complete understanding of the fundamental electrospray properties and the underlying mechanism behind the two emission modes are still lacking.
Herein, Beihang University researchers: Mr. Jinrui Zhang, Professor Guobiao Cai and Professor Weizong Wang, in collaboration with Professor Aamir Shahzad from Government College University Faisalabad and Dr. Xuhui Liu from Beijing Institute of Control Engineering, investigated the electrospray behavior of ionic liquid in a hybrid emitter electrospray thruster using molecular dynamics (MD) simulation. In their approach, EMIM-Tf2N was utilized as the propellant because itis the popular propellant of electrospray thruster. The propulsion performances of the two emitters (capillary emitter and hybrid emitter) and their respective emission modes were compared at various flow rates. Their work is currently published in the International Journal of Heat and Mass Transfer.
The authors findings showed that compared with the capillary emitter, the hybrid emitter emitted ions more easily due to larger hydraulic impedance and stronger electric field. As such, the mean diameter range and charge to mass ratio of the emitted particles in the hybrid emitter was relatively smaller and larger respectively, in comparison with that observed in the capillary emitter. Interestingly, the hybrid emitter, which operated at the externally-wetted emission mode at the lower flow rates of 2.269 m/s and 4.537 m/s, changed to capillary emission mode at a higher flow rate of approximately 18.15 m/s, and the same scenario was observed when EMIM-Tf2N propellant was used in experiments of literature. Furthermore, the analysis of the particle charges revealed that the charge of nearly all the hybrid emitter particles exceeded the Rayleigh limit at externally-wetted emission mode. On the other hand, very few large particles were below the Rayleigh limit at capillary emission mode, and the number of particles increased with an increase in the flow rate. .
In summary, the research team is the first to study the behaviors of ionic liquid electrospray thruster with a hybrid emitter using molecular dynamics simulation. The hybrid emitter exhibited a better propulsion performance than the capillary emitter, with a 24% and 7% increase in the specific impulse and thrust, respectively. The proposed hybrid emitter operated efficiently at external emission mode at relatively lower flow rates. However, the operation mode changed to capillary emission mode at higher flow rates which was sufficient to trigger the liquid surface to completely cover the needle. In a statement to Advances in Engineering, Professor Weizong Wang noted that the study insights would contribute to the design of hybrid emitter electrospray thrusters with a wider thrust range for micro-nano satellites applications.
Zhang, J., Cai, G., Shahzad, A., Liu, X., & Wang, W. (2021). Ionic liquid electrospray behavior in a hybrid emitter electrospray thruster. International Journal of Heat and Mass Transfer, 175, 121369.