Burning characteristics and micro-explosions of free-falling boron-based slurry-fuel droplets

Boron-based slurry fuels are promising high-energy-density propellants for advanced aerospace propulsion systems, yet their fundamental combustion behaviors remain insufficiently understood. In this paper, we have developed a novel flat-flame-supported experimental platform to provide a well-controlled hot environment with sufficient heating rates for free-falling, boron-based slurry fuel droplets. Three liquid fuels ethanol, exo-THDCPD (HD-01), and quadricyclane (QC) are investigated, with boron additions ranging from 0 to 10 wt%. The results indicate that boron addition exerts a fuel-dependent influence on the droplet burning rate: it increases linearly the rate for ethanol, while exhibiting a saturating or marginal effect for the opaquer high-energy-density fuels. A mathematical model partially captures the contribution of enhanced radiation absorption. Crucially, boron addition induces frequent micro-explosion events, which are absent in pure fuel combustion. These events facilitate early boron combustion, as evidenced by BO 2 spectral emissions. High-speed imaging reveals pronounced droplet deformation and surface rupture, culminating either in the ejection of secondary droplets within 3∼5 ms or, more rarely, in the complete disintegration of the parent droplet. Finally, a ‘deformation-recovery/ejection’ sequence is identified from direct observations, and an experimentally supported hypothesis attributing it to subsurface bubble dynamics, rather than to rigid shell formation, is proposed, thereby extending the classical understanding of slurry fuel micro-explosions.