The effect of sodium chloride on the nanoparticles observed in a laminar methane diffusion flame

The occurrence of nano-particles along the centerline of a laminar co-flow diffusion flame with a fuel jet diameter of 10 mm was investigated under two conditions: a methane diffusion flame with and without the addition of NaCl particles of the order of 100 nm in diameter in the fuel flow. Samples of nano-particles from different heights above the burner (HAB) were extracted through a small pinhole in a tubular probe and were immediately diluted several thousand times. Subsequent to sampling, particles were directed to a scanning mobility particle sizer (SMPS) to measure the particle size distribution. A rapid thermocouple insertion method was used to measure the temperature of the flow at different axial and radial positions. Extracted nano-particles from different HABs were collected on grids and their structure was studied using transmission electron microscopy (TEM). The spatial temperature field in the flow was not significantly changed with the addition of NaCl particles to the fuel flow. Particle size distributions for the methane-only flame showed different stages of nano-particle evolution, from inception and surface growth to coagulation and oxidation. TEM images showed that primary soot particles had core-shell structures. Particle size distributions for the methane-NaCl flame showed that considerable concentrations of sub-10 nm particles appeared at heights above the burner where soot inception is typically not expected in a methane flame. The TEM images showed that the structure of these nano-particles was similar to cubical NaCl particles. For higher HABs the particle size distribution followed a similar trend as the methane-only flame, except for regions of the flame where coagulation is the dominant process. The addition of NaCl reduced the mobility diameter of soot particles and increased their number concentration. These results indicate that the addition of NaCl to the flame reduces the coagulation between soot primary particles.