Abstract
Liquid atomization determines the initial conditions for flame formation and particle synthesis. Without a stable flame, high droplet velocities and thus short droplet residence time in the flame may lead to droplets being injected into an extinguished flame, which influences synthesis and final particle output. An experimental investigation of spray formation and flame stability is performed through high-speed visualization. Targeted variation of nozzle geometry is applied to improve spray-flame interaction and compared to a standardized burner. Timescales of spray density and flame fluctuations are quantified and compared, where the latter were significantly larger and hence not correlated. Instead, dispersion gas forms a barrier between spray phase and pilot flame; hence, ignition depends on large liquid lumps with high radial momentum to break through the dispersion gas for spray ignition. Angling of dispersion gas flow increases radial shear and turbulence and leads to refined atomization and improved flame stability. To investigate the nozzle influence on particle formation, particle characteristics are examined by online and offline analytics with focus on particle structures and product purity. The modified nozzle produced smaller primary particle sizes, thus indicating a sensitivity of sintering dominance on the nozzle geometry. Impurities impact the examination of particle structures and general particle functionality. Carbon contamination was apparent in synthesized particles and also indicated sensitivity to nozzle geometry. Discrepancies to literature data are discussed regarding differences in flame activity and droplet characteristics. The report highlights, how product characteristics can differ crucially due to changes in nozzle geometry despite comparable operating conditions.Graphic abstract
Highlights
Spray-flame synthesis of nanoparticles is known for its flexibility in producing a wide range of nanoparticle powders with high purity (Strobel and Pratsinis 2007)
The high-speed videos show a highly in-stationary flame behavior with characteristic repeating sequences of ignition, combustion, extinction and re-ignition. Such flame fluctuations will have a direct influence on the local reaction conditions that the synthesized particles experience on their trajectory through the flame
The question arises if the reaction conditions already change during the residence time of single particles or if almost constant flame conditions can be assumed for single particles
Summary
Spray-flame synthesis of nanoparticles is known for its flexibility in producing a wide range of nanoparticle powders with high purity (Strobel and Pratsinis 2007). To exploit the full potential of spray-flame synthesis towards sophisticated product output with unique functionalities, a fundamental understanding of the underlying process chain is required (Stark and Pratsinis 2002). The overall process complexity in the flame is very high: particle characteristics are determined in the flame within a few milliseconds residence time and influenced by various sub-processes including atomization and spray formation, droplet evaporation, flame chemistry and the particle formation (Kammler et al 2001). If the rate of fuel mass consumption exceeds fuel evaporation rate, the flame is prone to fuel starvation which can lead to flame extinction
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