Ignition and combustion characteristics of ultrasonically levitated single droplets of nano‑boron/oxygenated fuel slurries

Addition of boron nanoparticles to liquid oxygenated fuel lead to the formation of nano‑boron slurries with high energy density. This study focuses on ignition and combustion characteristics of single droplets of nano‑boron slurries during ultrasonic levitation and effects of different oxygenated fuels on ignition and combustion characteristics of single droplets. In this study, nano‑boron slurries of boron/ethanol, boron/1-hexanol, boron/dimethyl carbonate, boron/triacetate, boron/propylene glycol monomethyl ether and boron/tripropylene glycol monomethyl ether with 10 wt% solid loading were prepared. Moreover, an ultrasonic levitation system was designed and established, which was used for igniting single droplets in ultrasonic levitation state. The results indicate that ignition and combustion process of levitated single droplets of nano‑boron slurries can be divided into three stages: a stable endothermic stage, an oscillation and micro-explosion stage and an aerosol combustion stage. Smoke was observed before aerosol combustion stage of boron/triacetate and boron/tripropylene glycol monomethyl ether, whereas flame jets were observed during combustion of other fuels. Furthermore, emission spectra of BO2 with high intensity could be observed during aerosol combustion stage. A shorter main chain length of oxygenated fuel facilitates ignition of nano-B slurries. Boron/propylene glycol monomethyl ether exhibited shortest ignition delay, which was approximately 13.8% shorter than that of boron/tripropylene glycol monomethyl ether with the longest ignition delay. Furthermore, a longer main chain length of oxygenated fuel facilitates combustion of nano-B slurries. Nano‑boron slurries with longer main chains (boron/1-hexanol, boron/triacetate and boron/tripropylene glycol monomethyl ether) exhibited brighter and larger flames, regardless of whether they were alcohols, esters or ethers, and quenched zones with bright and clearly delineated edges were observed. Intensity integral and maximum intensity of emission of these three fuels were also higher than those of other fuels. Boron/triacetate demonstrated the best combustion performance, with maximum intensity and intensity integral at 547.8 nm being 3.88 times and 1.9 times those of boron/ethanol, respectively, which has the worst combustion performance. This study provides a theoretical basis and experimental reference for the application of oxygenated-fuel-based nano‑boron slurries and their base liquid fuel selection.