Laser-based diagnostics for investigating soot formation in combustion processes

Abstract: In this work, laser-based diagnostic techniques were developed and applied to investigate soot formation in small-scale laboratory burners as well as in practical combustion devices. Quantitative 2D laser-induced incandescence (LII) measurements often use a cylindrical lens to shape the beam and illuminate the planar region of interest. The varying laser fluence and sheet width in the imaged flame region created due to this focusing was found to introduce significant bias in the 2D LII signal distributions, especially for short focal length lenses combined with long imaged regions. Experimental data were recorded with variations in laser fluence and detector gate, which were reproduced through numerical simulations using a heat-and-mass-transfer-based LII model. It was shown that adequate choices of experimental parameters could minimize bias in the 2D LII signal distribution. Combined measurements using LII and extinction as well as laser-induced fluorescence of polycylic aromatic hydrocarbons (PAH-LIF) were applied to characterize soot formation in methane-air diffusion flames at elevated pressures. A special focus was on the evaluation of optical properties of soot at various spatial locations in the flames through the analysis of LII fluence curves. The absorption function E(m) was found to increase strongly with increase in pressure at the position of maximum soot volume fraction (fv), and also along the centreline of the flame until the position of maximum fv. It was demonstrated that an assumption of constant E(m) in 2D LII measurements in sooting diffusion flames leads to large uncertainties in evaluated soot volume fractions.Previous studies have shown that addition of potassium chloride (KCl) reduces soot concentration and primary particle size. As PAHs are precursors of soot, this work focused on the influence of potassium and sodium salt additives on PAH formation, and measurements were primarily made using PAH-LIF and elastic light scattering (ELS). Using different combinations of excitation and fluorescence wavelengths, the detected fluorescence could be classified to originate from two groups: small PAHs (~ 2-3 rings) and large PAHs (≳ 4 rings). Addition of potassium and sodium salts were found to have no observable influence on the formation of smaller PAHs. However, the concentration of larger PAHs decreased in the sooting flame region with the addition of these salts, where the largest effect was seen for potassium salts. Additionally, different salts of potassium (or sodium) showed similar fluorescence and scattering response, thus showing that it is the metal ion which is primarily responsible for the influence on PAH concentrations observed in this study. Laser diagnostic techniques have also been applied in an optical diesel engine to study the effects of injector aging and oxygenated fuel additive tripropylene glycol monomethyl ether (TPGME) on spray and soot formation as well as soot oxidation. Studies in the literature have shown that aging of injectors adversely affects diesel engine combustion. TPGME has also been found to lower soot emissions. The spray and soot formation were investigated using high-speed imaging measurements of Mie scattering and natural luminosity, whereas late cycle soot oxidation was studied using semi-quantitative 2D LII and extinction measurements. Aged injector showed higher fuel injection rates resulting in higher soot formation and faster soot oxidation, predominantly at low load conditions. The fuel with added TPGME produced lesser soot in the cycle and showed faster soot oxidation rates.