  In the current work, we develop a single droplet model to describe particle
formation in multicomponent-liquid droplet combustion. Both the gas-to-particle
conversion and droplet-to-particle conversion routes of different solution
properties are investigated, together with the population balance model and the
droplet drying model. For multi-component droplets without precursors, the
droplet combustion is limited by the species diffusion in the liquid phase. The
model can well predict the droplet shrinkage history observed by previous PDA
measurements. For the precursor with a low boiling point than its thermal
decomposition temperature, the precursor in the droplet can then transform into
nanoparticles through the gas-to-particle conversion route. The population
balance model reveals that the generated nanoparticle size relies on both the
precursor mass fraction and the residence time, which is consistent with the
vapor-fed aerosol flame synthesis. For the precursor that tends to decompose or
precipitate in the liquid, it then undergoes the droplet-to-particle conversion
route. The droplet behavior can be classified by the ratio of droplet
evaporation time and precursor reaction or precipitation time. For small
droplets with short evaporation time, the nanoparticle formation obeys the
one-droplet-one-particle rule. For large droplets with long evaporation time,
the competition among precipitation, thermal decomposition, and evaporation
determines the final nanoparticle morphology. The single droplet model
established in this study can potentially guide the precursor design and be
coupled with the turbulent flame simulation of the whole flame spray pyrolysis
burner.
