Simulation study of superheating in evaporating droplets of (TTIP + p-xylene) in spray flame synthesis

Spray flame synthesis is an important technique for the production of metal oxide nanoparticles. Micro-explosions of the burning and evaporating droplet are an important breakup mechanism that is usually necessary in order to achieve a homogeneous product quality. There is an ongoing debate on the causes of these micro-explosions. With the present simulation study, we contribute to the clarification of this issue. Simulations of the evaporation of droplets of titanium (IV) isopropoxide (TTIP) dissolved in p-xylene, for which detailed information on thermophysical properties is available, are carried out for two scenarios representing single droplet combustion experiments and spray flame combustion, respectively. A one-dimensional model is developed, in which both the energy transport and the mass transport inside the droplet and the corresponding convective boundary conditions are considered. From the temperature and concentration profiles in the droplet, the superheating is calculated, considering the non-ideality of the liquid phase. The results show that superheating sets in inside the droplet. Hence, after reaching a certain threshold, a sudden evaporation will occur inside the droplet — and cause the micro-explosion. This effect is related to large values of the Lewis number, indicating that the heat conduction is much faster than the mass diffusion. As the Lewis number is not only large for the system studied here but also for other precursor solutions used in spray flame synthesis, the explanation of the observed micro-explosions is expected to hold in many cases.