Numerical analysis on burning and particle formation of a single ethanol-based droplet in flame spray pyrolysis

Ethanol is a common inexpensive solvent for industrial flame spray pyrolysis. This work presents a numerical study of a single ethanol-based droplet burning with different precursor concentrations under both dry and wet air conditions. The evaporation and decomposition of precursors, the deposition of water vapor and its diffusion and transport in the liquid phase are considered. The micro-explosion behavior and droplet burning constant were well predicted as compared to existing experimental data in literature. The results show that surface condensation, dissolution, and re-evaporation of water were controlled by the disparity of mass transfer and evaporation priorities of different components in the droplet at different temperatures. Water in the ethanol droplet increases first during the initial burning stage and then decreases with increasing temperature as water preferentially vaporized than the formed less-volatile viscous layer on the droplet surface during the droplet combustion. The ratio of particle synthesis through evaporation and thermal decomposition decreases with increasing precursor concentration. It is further identified that the presence of water from either combustion products or humid air suppresses evaporation of the precursors in ethanol droplets because water dissolved in droplets evaporates preferably than the precursor, thereby potentially inhibiting the gas-to-particle route and providing another potential option for the control of flame synthesis process.