Ultrasonic atomization for PocketQube applications

The growing demand for miniaturized satellites, particularly PocketQubes, has resulted in the need for a compact, low-power propulsion system which is capable of generating high thrust under severe spatial and energy limitations. This thesis presents the design, development, and optimization of a micropropulsion system based on Vibrating Mesh Atomizer (VMA) technology. Utilizing a high-frequency piezoelectric-driven ultrasonic atomizer, this study presents a novel propulsion approach that minimizes electrical input while avoiding complex mechanical parts or pressurized tanks.

To investigate and optimize the VMA-based system, a comprehensive experimental platform was developed. Multiple experiments were conducted by varying driving waveforms, aperture geometry (size and count), and excitation frequency to evaluate their influence on atomization dynamics and performance. Atomization rate was measured using a precision weighing balance over defined intervals, while size of the ejected liquid droplets was analysed using a particle size analyser. An electromagnetic calibrated torsional thrust stand was employed to measure the thrust output of the system. Spray characteristics were captured using a high-speed imaging technique, allowing in-depth observation of droplet behaviour. 

This study was conducted in three phases, first phase of the study explored the influence of driving waveforms—symmetrical, pulse, and asymmetrical—on atomization behaviour across different VMA configurations. Square waveforms consistently delivered the highest atomization rates (1.4 × 10-6 kg/s) due to their high energy, while pulse waveforms provided uniform droplets (span ~ 1.3) with low atomization rate (2.72×10-7 kg/s and 2.58×10-7 kg/s for the Gauss and Lorentz waveforms, respectively). Asymmetrical waveforms revealed the crucial role of negative cycle duration in controlling suction flow and droplet size. Altogether, this study demonstrates that waveform pattern can serve as a powerful tool for controlling atomization characteristics.

The second phase focused on mechanical design parameters, including aperture size (4 µm, 10 µm and 30 µm), aperture count, centre-to-centre aperture distance and excitation frequency. Smaller apertures produced uniform droplets larger than aperture size via pumping action (span < 1), while larger apertures resulted broad droplet size distributions likely influenced by cavitation and capillary wave formation (span >1). Increasing aperture count improved atomization output but resulted in reduced per-aperture efficiency. These findings underscore the complex trade-offs between geometric configuration and operational performance.

In the final phase, a fully integrated micropropulsion prototype was constructed and tested using deionized water and 50 wt% aqueous ethylene glycol (EG). The system achieved maximum thrust values of 121.16 µN (water) and 116.07 µN (EG), with a peak thrust-to-power-mass ratio of 2.16 µN/W·g. Water exhibited wider frequency responsiveness but lower stability, whereas EG delivered more consistent thrust due to its higher viscosity and lower volatility.

In conclusion, this research provides a comprehensive framework for designing VMA-based propulsion systems specifically for PocketQube applications. By integrating insights from waveform modulation, geometric configuration and fluid properties, the findings establish the feasibility and adaptability of ultrasonic atomization for space propulsion