On nanoparticles in iron dust flames of Bunsen-type: Evolution of size distribution and hetero-coagulation with micron-sized particles

This study investigates the evolution of nanoparticle size distributions in laminar iron dust flames of Bunsen-type as a function of equivalence ratio, oxygen concentration in the oxidizer, and the size distribution of seeded micron-sized particles. Experiments were conducted using a Bunsen-type burner. Spatially resolved nanoparticle size distributions were measured via on-line particle probing and differential mobility sizing. Thermophoretic sampling enabled morphology and phase analysis via scanning and transmission electron microscopy (SEM and TEM) and energy-dispersive X-ray (EDX) spectroscopy. Nanoparticles form in the boundary layer of burning micron-sized particles and are transported into the surrounding gas phase, where they undergo condensation-driven growth, leading to a broadening of their size distribution along the flame axis. The mobility size distribution, which includes both single nanoparticles and aggregates, shifts toward larger sizes, while the number concentration decreases due to coagulation. Hetero-coagulation with micron-sized particles leads to the formation of nanoparticle coatings on their surfaces, reducing the fraction of gas-borne nanoparticles. At the flame tip, higher equivalence ratios, elevated oxygen concentrations, and smaller seeded particles increase nanoparticle number concentrations. Additionally, smaller seeded iron particles result in a narrower nanoparticle size distribution. Further downstream, hetero-coagulation dominates over nanoparticle growth, leading to a continuous reduction in the gas-borne nanoparticle volume fraction. These findings indicate that mitigation strategies could be based on controlling hetero-coagulation, adjusting the oxygen concentration in the oxidizer, or carefully selecting the equivalence ratio and/or the size distribution of micron-sized iron particles. • On-line particle probing and mobility sizing reveal nanoparticle size evolution. • Nanoparticle size distribution broadens due to condensation-driven growth. • Hetero-coagulation with micron-sized particles reduces gas-borne nanoparticles. • Increased oxygen concentrations enhance nanoparticle formation. • Equivalence ratio affects nanoparticle formation and (hetero-)coagulation.