Numerical Study of the Vaporization and Combustion of Single p-Xylene Droplets in Hot Air

A single droplet heating, vaporization, and detailed combustion model is developed for pure $p$-xylene (p-C₈H₁₀) in hot air. p-C₈H₁₀ is a combustible solvent in precursor solutions, for instance, with titanium tetraisopropoxide (TTIP) for the production of $\mathrm{T}\mathrm{i}{\mathrm{O}}_2$ nanoparticles. In the present one-dimensional mathematical model, a spherically symmetric $p$-xylene droplet in hot air is considered, resolving both the droplet (liquid phase) and the ambience (gas phase). The calculation of the vaporization rate includes the Stefan velocity at the droplet surface. In the gas phase, a detailed chemical reaction scheme is used. Elementary reactions are combined with complex reactions that account for the thermal decomposition of the $p$-xylene. The reaction mechanism comprises 93 chemical reactions among 25 species. Variable thermo-physical properties are used for both the gas and the liquid phase. A parameter study is conducted by varying the hot ambient gas temperature and the initial droplet size. In a hot ambience, initial expansion of the $p$-xylene droplet occurs due to droplet heating. After initial heating and vaporization, autoignition and combustion in the gas phase take place. In contrast to similar studies of single droplet combustion in the literature, the present simulations are not only carried to the end of the droplet lifetimes but continued until the gas flame extinguishes due to lack of combustible fuel. The vaporization rate constant, the autoignition, and the flame standoff distance are analyzed.