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Abstract

Nanoparticle synthesis in spray flames is a promising approach to produce nanoparticles with specific properties. A liquid precursor solution, after injection, undergoes breakup and atomization resulting in the formation of the droplets. Further, heating and evaporation of these droplets occur. The droplets may experience thermal decomposition or puffing and possible micro-explosion, depending on the choice of the precursor solution and the initial conditions. Finally, gas-phase combustion occurs followed by formation of nanoparticles. It is significant to understand the different processes experienced by the droplet under such circumstances. The precursor solution consists of more than one liquid component. However, the thermophysical properties of the different components constituting the precursor solution droplets are often unknown, thus posing a major challenge to the modeling and simulation of the various processes undergone by the droplet. The most commonly used solvents for nanoparticle synthesis using spray flames are alcohols such as ethanol and butanol. Further, some of the precursors like iron(III) nitrate nonahydrate (INN), that are used to produce nanoparticles contain water molecules. Therefore, as the first step, a zero-dimensional multicomponent droplet evaporation model is employed in order to improve the understanding of the characteristics of heating and evaporation of single spherically symmetric ethanol/water and butanol/water droplets in convective air. The interior of the droplet is not physically resolved and the convective heating and evaporation of the multicomponent droplet is considered by extending the Abramzon and Sirignano model for single component droplets. The numerical simulation results of the ethanol/water droplet are compared with the experimental data. Further, the lifetime of single butanol/water droplets is analyzed for the initial conditions of the laminar matrix burner. The matrix burner was designed within the SPP1980 to understand the reaction kinetics of new precursor solutions associated with nanoparticle synthesis in spray flames and thus, develop reaction mechanisms. The gas phase simulations concerning the matrix burner assume that the droplets are completely evaporated at the inlet of the matrix burner. Therefore, in order to support this assumption, the droplet lifetimes are evaluated. Next, the numerical study concerning the heating, evaporation, and possible thermal decomposition of the precursor solution droplets of iron(III) nitrate nonahydrate and ethanol are performed. The precursor/solvent system of INN and ethanol is used to synthesize iron oxide nanoparticles using spray flames. There are two pathways through which the nanoparticles may be produced from the precursor solution droplet. In one pathway, the particle is formed inside the precursor solution droplet, whereas in the second pathway, thermal decomposition occurs that transfers the droplet into the gas phase, from where the nanoparticles may be generated. The numerical model is developed to capture both these pathways. Further, the precursor solution droplet consisting of titanium(IV) isopropoxide (TTIP) and p-xylene is considered, which is used to produce titanium dioxide nanoparticles using spray flames. The numerical results of the INN/ethanol and TTIP/p-xylene droplets computed using the zerodimensional model are parameterized using polynomial approximations for use in complex simulations concerning the production of nanoparticles using spray flames. The tabulated data is provided along with a code in the C programming language to read the data. Finally, in order to understand the puffing and possible micro-explosion of TTIP/p-xylene precursor solution droplets, a new one-dimensional model is developed. The novelty of the model is that it can distinguish between puffing and micro-explosion. The droplet heats up causing the higher volatile p-xylene to evaporate preferentially which results in the accumulation of the lower volatile precursor, TTIP, at the surface of the droplet and thus a liquid shell is formed that offers resistance to the process of evaporation. Also, the higher volatile component accumulates inside the droplet and puffing occurs once the boiling temperature of that liquid is reached. As the pressure in the droplet interior reaches above the ambient pressure, micro-explosion of the droplet occurs. Detailed investigation is conducted to identify the conditions for which micro-explosion of the droplet occurs.