Detection of nanostructures and soot in laminar premixed flames

This paper presents both in-situ and ex-situ measurements of nanostructures (also loosely referred to as nanoparticles) in laminar flames with the purpose of providing an understanding of their evolution to soot. Two laminar flame burners are studied covering a range of C/O ratios and hence different sooting propensities. Ex-situ measurements, utilizing soot sampling and analysis using differential mobility analyzers, are performed to yield information on the particle size distribution (PSD). A broad range of in-situ measurement techniques are employed including laser-induced fluorescence (LIF), laser-induced incandescence (LII), and elastic laser scattering. In-situ measurements completed at the University of Sydney, utilise fast response photomultiplier tubes to monitor time-resolved emission signals simultaneously in four different spectral regions, whilst measurements performed at the University of Naples Federico II are spectrally resolved. The temporal lifetimes of the LIF signals are found to be much longer than that expected for molecules at the same temperature, yet much shorter and spectrally different than that of soot particles. The laser based measurements, combined with the PSD results, suggest that LIF is able to track nanostructures as condensed phase matter with sizes in the order of few nanometers and with internal structures exhibiting the spectroscopic behavior of small PAHs. Conversely, LII is more suited for the detection of solid state particles which are larger in size and have a more aromatic character. It is found that close to the burner exit plane in the early regions of the flames LIF is measured both in the visible and ultraviolet (UV) bands, but not LII, implying the existence of nanostructures rather than soot. Further downstream, these nanostructures continue to exist but now in the presence of soot as is evident by the persistence of the LIF-UV and LIF-visible in conjunction with LII and laser scattering. Collectively, these findings confirm the hybrid nature of nanostructures that dominate the early evolution of soot.