How boiling happens in nanofuel droplets

We report detailed analyses of evaporation and atomisation characteristics of nanofuel droplets in a contactless environment (acoustic levitation) under external radiative heating. Two base fuels, ethanol and n-dodecane with a significant difference in their respective vapour-pressures, are considered. Nanoparticles (NPs) of cerium oxides (CeO2) are utilised as nano-additives at a dilute particle loading rate (PLR) of 0%-0.5% by weight. Pure ethanol droplets vaporise at a faster rate than pure dodecane droplets and do not exhibit any secondary atomisation. However, pure dodecane droplets exhibit two modes of secondary breakup; Kelvin-Helmholtz instability induced stripping and catastrophic breakup beyond a certain threshold value of the initial droplet size. Nanofuel droplets of ethanol neither exhibit any significant change in the vaporisation rate nor exhibit secondary atomization. Contrarily, dodecane-based nanofuels show enhanced vaporisation due to heat absorption by nanoparticles and consequently different modes of secondary breakup. Interestingly, dodecane-based nanofuel droplets exhibit internal boiling induced atomization. A time scale analysis considering orthokinetic NP aggregation, evaporation lifetime, and bubble growth rate is presented to elucidate the mechanism of such internal boiling. The theoretical non-dimensional time scale (τ*) so coined is extended to estimate the minimum value of the droplet size necessary for exhibiting boiling. The analysis shows excellent agreement with the experimental observations. Furthermore, we propose a unique three-dimensional regime map to correlate the breakup modes with droplet sizes, PLR, and heating rates.