Nano-organic carbon measurements in a laminar co-flow diffusion flame

The formation of carbonaceous particulates in a co-flow laminar diffusion flame has been studied using UV-visible spectroscopy and laser scattering/extinction techniques for measurements of volume fraction and particle sizes. Measurements were performed in a non-smoking ethylene-air flame at atmospheric pressure. The results were compared to those obtained in similar flame studied by other groups using absorption and LII measurements. UV-visible spectroscopy allowed the identification of two classes of particles: soot particles, which absorb light in the whole spectral range and nano organic carbon particles (NOC) which are transparent to the visible radiation. The mean size of nano organic carbon particles was estimated to be about 2-3 nm. This was in agreement with previous results obtained in rich premixed flames with equivalence ratios across the soot formation threshold. The experimental results indicate that nano-organic carbon particles are formed in the fuel side of the flame front closer to the flame centre line than soot particles and with a concentration level comparable to that of soot suggesting a role mainly responsible for soot formation. The experiments reported here constitute a spatially-resolved, "in-situ" evidence of the presence of nano-organic carbon particles in a diffusion flame and are essentially based on the knowledge of the refractive index of NOC particles in the UV region. They also evidence the lack of information obtained using some optical techniques such as LII for total particle volume fraction.