Soot nucleation in low-sooting high-pressure flames: Experiment and modeling

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Combustion is still by far the major source of power generation and therefore retains long-term practical importance, especially for the long-distance transportation sector. The related combustion processes are known to generate carbonaceous particles associated with negative climate impact and health hazards, especially for the small particle size classes. Therefore, the better understanding and reliable predictive modeling of soot formation pathways – particularly in pressure regions relevant for engine combustion – is of high interest. Especially the processes governing the initial steps of soot nucleation, and the balance between formation and oxidation in flames that operate close to the fuel/air ratio of soot formation are poorly understood, and there is still a controversy about these initiation mechanisms. Existing kinetics models for soot nucleation and growth based on hypotheses about PAH–PAH, radical–radical, and radical–PAH interactions are not experimentally verified. Therefore, the objective of this proposal is to improve the understanding of soot formation in flames at elevated pressure (2–20 bar) by supporting model development with an extended experimental database of spatially-resolved measurements of temperature, species concentrations, mean soot particle size, and volume fraction. This is accomplished using in situ optical diagnostics and thermophoretic soot sampling from up to now not investigated premixed methane/ethen/air flames at high pressures (1 – 20 bar) in regimes with low-sooting tendency achieveable by proper selection of equivalence ratios (i.e., low C/O ratio). To modify chemical routes towards soot formation soot promoting/suppressing additives (ethyne and hydrogen, respectively) will be mixed into the fresh gas flow. Measurements will be performed in two high-pressure burners with optical access supporting flat premixed flames. Time-resolved laser-induced incandescence (LII), optical extinction, tuneable diode laser absorption spectroscopy in the near- and mid-infrared, and soot pyrometry will be used to determine soot volume fraction, mean particle size, gas temperature and carbon monoxide and water vapor concentrations, respectively. For independent validation of the particle size distribution, spatially resolved soot sampling is carried out with subsequent electron microscopy analysis. To increase the range of studied total pressures and species measurements, we plan for a strong international collaboration with other expert groups: Researchers from Lille University (France) conduct measurements in sub-atmospheric flames and help establishing the high-pressure soot-balanced flames and associated models. Partners from Texas A&M University (USA) will provide temperature, CO, atomic hydrogen, and oxygen concentration profiles from an identical burner. The growing database will form the basis for chemistry modeling with partners from the University of Milan (Italy).