Browse Publications Technical Papers 2018-01-0233

Observations of Soot Optical Property Characteristics Using High-Speed, Multiple Wavelength, Extinction Imaging in Heavy-Duty Diesel Sprays 2018-01-0233

Soot formation in high-pressure sprays of n-dodecane is quantified under conditions relevant to heavy-duty diesel engines. Sprays are injected from a single-hole diesel injector belonging to the family of Engine Combustion Network (ECN) Spray D injectors. Soot optical thickness and the total soot mass are quantified using a high-speed extinction imaging diagnostic with incident light wavelengths of 623 nm and 850 nm. The total soot mass is derived from the measured optical thickness using a non-dimensional extinction coefficient (ke) based on the Rayleigh-Debye-Gans approximation for fractal aggregates (RDG-FA). In a previous work, a near unity ke ratio was observed for extinction measurements using incident light at 406 nm and 520 nm-suggesting invariance in the complex refractive index (m = n + κi) of soot at these visible wavelengths; however, the present work demonstrates a significant difference in m for measurements with infrared incident light. During the quasi-steady period of the spray combustion event, the experimentally determined ke ratio (ke,623-nm/ke,850-nm) is 1.5 +/− 0.5. To compute a matching ke ratio via RDG-FA, a significantly smaller imaginary component (κ) must be implemented at 850 nm relative to that selected for 623 nm. A range of possible refractive index values are explored to meet the experimental requirement of ke = 1.5 and a value of m = 1.75-0.75i is recommended for soot extinction measurements in high-pressure spray flames with incident light near 623 nm. The identical optical diagnostic is applied to a non-premixed, steady, laminar ethylene flame for comparison. Larger ke ratios (ke > 1.8) are observed for immature soot in the near-burner region with a minimum of 1.36 at an axial distance 50 mm from burner surface. Downstream of 50 mm, the ratio increases due to the effects of oxidation on soot properties. Further analysis using RDG-FA, while accounting for differences in soot morphological properties among the two flame configurations, provides evidence that the refractive index of soot in a high-pressure n-dodecane spray flame differs from that in the atmospheric pressure ethylene diffusion flame.


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