Ionic liquid electrospray behavior in a hybrid emitter electrospray thruster

Significance 

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.

Ionic liquid electrospray behavior in a hybrid emitter electrospray thruster - Advances in Engineering

About the author

Mr Jinrui Zhang received his bachelor degree in Flight Vehicle Propulsion Engineering from Beihang University in China in 2019. Since that, he studies as a Master Student in the School of Astronautics and a member of the Advanced Space Propulsion and Energy Laboratory at Beihang University. His research interest focuses on molecular dynamics simulation and the experimental investigation of ionic liquid electrospray thrusters. Mr Zhang has published 5 papers in peer-reviewed journals and received the National scholarship for Postgraduates in 2021.

About the author

Prof. Dr. Weizong Wang received double Ph.D. degrees in electrical engineering from Xi’an Jiaotong University in China and University of Liverpool in United Kingdom in 2013. After that, he worked as a research fellow at Qian Xuesen Laboratory of Space Technology, China Aerospace Science and Technology Corporation (CASC). In 2015, he entered the PLASMANT research group at the University of Antwerp in Belgium supported by the European Marie Skłodowska-Curie (MSCA) Individual Fellowship. In 2018, he became a professor of Aerospace Science and Technology and founded the Advanced Space Propulsion and Energy Laboratory (ASPEL) at the School of Astronautics, Beihang University.

His current research activities include the fundamental physics, chemistry and applications of low temperature plasmas, by numerical modelling and experiments, for various applications, especially space propulsion and energy applications (electric propulsion and gases conversion into value added chemicals and renewable fuels). Prof. Wang has authored over 70 publications in peer-reviewed journals and received the National Excellent Patent Award of China in 2013 and Excellent Doctoral Dissertation Award of Shaanxi Province in China in 2014. He received the outstanding paper award from 2018 Asia-Pacific Conference on Plasma and Terahertz Science (APCOPTS) and China Top Cited Author Award from IOP Publishing in 2019. Prof. Wang is also in the advisory/editorial board of several journals, including Journal of Physics D: Applied Physics, Space: Science & Technology and guest editor for 2 special issues in academic journals.

Reference

Zhang, J., Cai, G., Shahzad, A., Liu, X., & Wang, W. (2021). Ionic liquid electrospray behavior in a hybrid emitter electrospray thrusterInternational Journal of Heat and Mass Transfer, 175, 121369.

Go To International Journal of Heat and Mass Transfer

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