Detection and Characterisation of Combustion Formed Nanoparticles Using Time-Resolved Laser-Induced Emission

Abstract

This thesis provides an improved characterisation of the evolution of soot nanoparticles in laminar and turbulent flames with a particular focus on the transition from nucleation to solid particles where intermediate structures occur. Laser light at 266 nm is primarily used for Laser-Induced Fluorescence (LIF) measurements as a means to detect and characterise incipient soot nanostructures that retain molecular qualities. These species have a size in the order of a few nanometres and do not absorb light at longer visible and infrared (IR) wavelengths. In addition, the 266 nm laser is used for Elastic Light Scattering and Laser-Induced Incandescence that prevail where larger solid particles are present. A 1064 nm laser, on the other hand, is used to exclusively excite solid soot particles that do absorb in the IR and preferentially incandesce. The combination of these Laser-induced-Emissions (LIE) is used to track the presence and character of both soot and soot precursor nanostructures. A key feature of these measurements is the ability to track the temporal decay of LIEs. Measurements taken in different combustion conditions (premixed, diffusion and turbulent) highlight different behaviours and properties of combustion formed particles. In all cases, the decay time of the majority of LIF signals is found to be much longer than that expected for molecules present with these flames at the same temperature, yet much shorter and spectrally different than that of soot particles, as is expected from semi-rigid structures. In some cases an obvious transition from smaller aromatic species to larger more rigid nanostructures is inferred from redshifted spectra downstream and longer fluorescent decay times. Collectively, these findings confirm the hybrid nature of nanostructures (semi-solid structure retaining molecular qualities) that dominate the early evolution of soot and highlight the multiple pathways in which soot precursor nanostructures initiate and evolve