Ignition and combustion properties of ultrasonically levitated single nano-Aluminum-based slurry droplets with various liquid oxygenated fuels and solid contents

Nano-aluminum (nAl)/oxygenated slurry is a promising fuel for aerospace and internal combustion engines. This study investigated the ignition and combustion of single nAl-based slurry droplets containing various oxygenated fuels (dimethyl carbonate (DMC) and triglyceride triacetate (TA)) and varying solid contents (1 wt. %, 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.%). Droplets were levitated using an ultrasonic levitator, and their deformation and flame evolution were recorded using a digital high-speed camera. A fiber-optic spectrometer was employed to measure the optical signals from the droplets, and a dual-color infrared thermometer was used to measure the surface temperatures of both the droplets and nAl particles (nAls). The experimental results indicate that the ignition and combustion processes can be categorized into three consecutive stages: a stable endothermic stage, an oscillation and micro-explosion stage, and an aerosol combustion stage. The expansion, deformation, and breakup of the bubbles within the droplet resulted in micro-explosions. The burning solid nAls appeared bright orange, and the aerosol flame of the droplet exhibited distinct signs of heterogeneous combustion. Among the two oxygenated fuels, the sample with TA (Al/TA) exhibited a longer ignition delay and higher temperature integral during the ignition process. During combustion, it also demonstrated higher flame brightness and overall spectral integrated intensity at 900 nm, with the spectral integrated intensity approximately 77.6% higher than that of Al/DMC, indicating superior combustion performance. As the solid content increased from 1 wt. % to 20 wt.%, the ignition delay and maximum temperature during the ignition process of Al/TA initially increased and subsequently decreased, reaching a maximum at 15 wt.%. The 15 wt.% Al/TA sample also exhibited bright flame and highest integrated intensity at 900 nm, indicating the best combustion performance. This study provides valuable insights for the application of nAl/oxygenated slurries.