Impact of atomization and spray flow conditions on droplet μ-explosions and temporal self-similarity in the FSP process

Flame spray pyrolysis (FSP) is a technique for the synthesis of metal oxide nanoparticles by combusting precursor solutions in a spray flame. The combustion of certain precursor solutions is known to lead to severe droplet disruptions (μ-explosions) in the spray flame that are linked to the synthesis of homogeneous and phase-pure nanoparticles. In this work, a broad spectrum of suitable subsonic operating conditions for the synthesis of iron oxide nanoparticles by FSP is investigated to understand the influence of the jet Reynolds number and turbulence on the onset of μ-explosions and droplet dynamics in spray flames. In order to enable a coherent comparison between differently operated spray flames using an iron(III) nitrate nonahydrate solution, the gas-to-liquid mass ratio and, hence, the oxygen/fuel ratio have been kept constant in order to identify the influence of flow conditions on the droplet dynamics. From the analysis of the droplet sizes in the spray and in the spray flame, it is found that in all combusting sprays, the droplet sizes convert from unimodal (after atomization) to bimodal droplet size distribution (DSD) due to the presence of μ-explosions. The occurrence and evolution of the bimodal DSD reveal that high jet Reynolds numbers result in narrower DSD and in a sharper separation of both DSD probability peaks (modal values). A straightforward 1-step kinematic model is presented to describe the conversion of unimodal to bimodal DSD considering the evaporation of droplets as well as the disruption of droplets to mimic the effect of μ-explosions. The temporal evolution of droplets in FSP is investigated by spatially resolved velocity data that reveal the formation of a temporal self-similarity. The resulting iron oxide nanoparticle size decreases with increasing jet Reynolds number. The turbulent mixing and residence times in the flame, primarily set by the jet Reynolds number, are identified as key design parameters for FSP