Probe sampling to map and characterize nanoparticles along the axis of a laminar methane jet diffusion flame

Nanoparticles generated by a laminar methane jet diffusion flame were characterized by direct sampling through the pinhole of a probe and sized with a nano-scanning mobility particle sizer. Particular attention was paid to verify the validity of the probe-sampling technique. Results showed that the particle size distributions (PSD) were strongly affected by sample dilution immediately after extraction. Within the flame, the soot PSD did not become independent of dilution ratio until the sample was diluted several thousand times (depending on initial concentration) within a few milliseconds of extraction. A method for on-line measurement of dilution ratio was also introduced by measuring CO2 concentration continuously during the tests.High spatially resolved experimental results at different heights (typically 1 mm apart) along the central axis of the flame showed the evolution of PSD for particles larger than 2 nm covering early nucleation and growth to oxidation regions. Moving upward from the fuel-side, prior to the visible flame, the PSD in the particle nucleation region was bimodal (with one of the modes being smaller than 2 nm) and gradually turned into a unimodal PSD with a median of 10 nm due to particle growth. Nearer to the visible flame, the PSD became bimodal again with the larger-diameter mode growing in magnitude and being made up of ever-larger particles, while the smaller-diameter mode gradually shrunk in size until it vanished. Eventually, as the flow was leaving the visible flame, all the particles gradually oxidized until at the tip of the flame they reached a median diameter of 9 nm and in very low concentrations.