Influence of precursor concentration on spray and particle formation in flame spray pyrolysis

Liquid atomization is expected to be a key process in Flame Spray Pyrolysis, as it determines the primary droplet size and velocity distribution, which represent initial conditions for flame shape and temperature field. A well-defined manipulation of atomization may be achieved by variation of nozzle geometry, injection parameter or fluid properties. However, change of fluid properties, e.g. varying solvents or increasing the precursor concentration, also influences the final particle properties. In this work, the influence of precursor concentration on fluid properties, spray and particle formation in Flame Spray Pyrolysis is experimentally investigated. High-speed shadowgraphy and Phase-Doppler measurements are conducted in a semi-closed combustion chamber with a well-defined burner geometry (SpraySyn burner) that is based on a coaxial atomization principle. By adding precursor to the flammable solvent, resulting spray characteristics, flame shape and appearance are changed in color, shape and mean droplet size distribution. Compared to resulting mean droplet sizes of pure solvent mixtures (D32, 22 μm), mean droplet sizes in the center of the precursor flame at 30 mm above the burner are generally smaller (D32, 18 μm). Persisting high share of small droplets in the droplet size distribution are presented, that presumably origin from an accumulation of metal organic precursor at the droplet surface during combustion, that acts as a diffusive barrier for high volatility solvent and thus reduces droplet evaporation. Sufficiently high above the nozzle exit, increasing the precursor concentration does not affect the radial distribution of mean droplet sizes but leads to increased mean particle sizes. More precursor presumably leads to higher concentrations of prime particles in the flame, thus probability of particle collusion and agglomeration. High-speed imaging at nozzle exit revealed random fluctuations of initial jet diameter, paired with jet axis shifting around 1000 Hz. These superposed fluctuations determine the initial conditions for atomization, induce pulsations in the flame and thus initiate the entire chain of the particle synthesis process.