Experimental Investigation of Large Scale Aerodynamics on Soot Emissions in Swirling Flows
Soot are microscopic airborne particles that impact human health and global climate, and are released into the atmosphere as a result of incomplete combustion. Reducing soot formation from burners improves both their efficiency and emissions standards. Modern burners utilize the Rich-Quench-Lean (RQL) technology that employs turbulent swirling flames with downstream dilution air. These turbulent flames exhibit spatial and temporal fluctuations not present in laminar flames. This adds a level of complexity to turbulent soot studies that is yet to be fully understood. In this study, a comprehensive analysis of soot formation in swirl-stabilized non-premixed turbulent flames at atmospheric pressure is performed using both Ethylene and an alternative Gevo alcohol-to-jet kerosene fuel referred to as Jet C-1. The goal is to extend the understanding presently available in the literature on dilution air by investigating the impact of systematically varying the amount and location of downstream radially injected dilution air on soot formation and oxidation for the first time. Firstly, Cold flow PIV was used to study the flow field and data revealed an increase in velocities and turbulence within the central recirculation zone (CRZ) as a result of dilution air. The impact of the dilution jets shifts downstream leaving the CRZ and shear layer unaltered as they are moved farther downstream, making them less effective, even if the dilution momentum through them is increased. Next, OH-PLIF, PAH-PLIF and soot LII experiments were used to study the Ethylene flames. Past work has looked at dilution jets positioned at a fixed downstream location. In this thesis, the location and amount of dilution air were systematically varied. Results revealed a strong correlation between OH, PAH and soot distributions, which are affected by the location of the dilution jets relative to the CRZ, where soot primarily forms. Dilution jets positioned up to two bluff body diameters from the burner inlet have a greater influence on residence times, mixing, and OH dispersion within the CRZ, and thus have a higher impact on soot reduction. Sampling experiments were performed for the first time on an RQL type burner to link in situ and ex situ diagnostics and measure soot concentrations at the exhaust of the burner. Particle size distributions (PaSDs) revealed that all the Ethylene flames produced a higher concentration of small particles under 4 nm. PaSDs also revealed that the flames that are not visibly sooty still produced soot volume fractions in the ppb range at the exhaust of the burner. A combination of sampling, extinction and LII measurements showed that while dilution is effective at reducing soot formation within the burner, LII data on its own misrepresents the degree of efficacy of dilution air as smaller particles fall outside the detection range of this technique. Finally, to mimic spray patterns in industrial applications, novel investigations into the influence of dilution air on a liquid swirling spray flame were performed. OH* visualization, Mie scattering and LII of soot experiments on Jet C-1 swirling spray flames showed that soot forms on either side of the spray cone. Understanding these spray-flame interactions is of the utmost importance and are dynamics not captured in gaseous swirling flames. In these flames, dilution air did not always reduce soot concentrations, which highlights the importance of choosing the correct position and amount of dilution air relative to the primary combustion zone. In all the tested Jet C-1 flames soot concentrations were in the low ppt range and were lower than in the Ethylene flames. This alternative kerosene fuel may prove to be a viable low soot alternative to the conventional jet fuels available today.
Turbulent swirling flames, LII, Particle Size Distributions, Soot from an alternative jet fuel, Soot Volume Fraction, Soot oxidation, RQL combustor
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