Nanoparticle formation from precursor-loaded droplets: flow dynamics, population dynamics of particles and droplets, experimental validation.

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In flame spray synthesis, the liquid-to-gas transfer and subsequent particle growth is a still largely unexplored research area. Here, both suitable experimental as well as numerical methods which describe this phase transformation in all details are lacking. As a result, key process steps in the chain from spray to particle remain speculative. This project has formulated the goal to perform a detailed investigation of the transition from the liquid (droplet) phase to the solid (particle) phase in a flame spray reactor. Therefore, a combination of experimental and numerical tools will be used, which perfectly complement each other. These aim at investigating the transition from the liquid/droplet phase in the particle phase and thereby gaining a better understanding of the particle formation mechanisms. This allows to identify the key parameters for an efficient flame spray synthesis, which can then be used for optimization and scaling-up of the process.The first project period focused on the modeling of the particle formation mechanisms in the lower flame region. The fluid dynamics simulations yielded data on the temperature and concentration gradients in the particle formation zone. The application of these data led to a re-interpretation of the particle formation models. The particle sizing methods proved suitable for sampling from the lower flame region, where primary particles form. The close collaboration between the applicants allows a thematic coverage of the essential processes in particle formation in spray flames: the Kruis group focuses on the complex particular processes relevant for the particle dynamics, whereas the Thévenin group investigates the spray, the evaporation and the reactive gas phase in detail; both numerical projects benefit from the systematic experiments of the Wiggers group in the sense of comparison and validation, also leading to new questions for the modeling approach.In the second period the influence of the composition of the pilot flame and the dispersion gas will be investigated; furthermore, alternative solvents will be investigated. The numerical models will be extended with multicomponent evaporation, reactions in the liquid phase and detailed consideration of the gas-phase close to the droplets. The stability of the spray combustion will be investigated with DNS as well as with the dual-population Monte-Carlo method, in order to support our understanding of the cause of the suspected micro explosions.