Standardized Optical Constants for Soot Quantification in High-Pressure Sprays 2018-01-0233

Soot formation in high-pressure n-dodecane sprays is investigated 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 is quantified using a high-speed extinction imaging diagnostic with incident light wavelengths of 623 nm and 850 nm. Previously, soot measurements in a high-pressure spray using 406-nm and 520-nm incident light demonstrated a minimal wavelength dependence in the complex refractive index of soot (m), as demonstrated by a near unity ratio of the non-dimensional extinction coefficients (k_{e,406 nm}/k_{e,520 nm}). The present work, however, 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 k_e ratio (k_{e,623 nm}/k_{e,850 nm}) is 1.42 ± 0.27. To compute a matching k_e ratio via the Rayleigh-Debye-Gans approximation for fractal aggregates (RDG-FA), a substantially smaller imaginary component of m is required at 850 nm relative to that selected for 623 nm. We demonstrate that a wide range of complex refractive index pairs meet the experimental k_e ratio requirements, making the determination of an accurate soot refractive index from these data implausible. Representative k_e values for soot measurements in high-pressure spray flames are derived from extinction measurements performed on a well-characterized, steady, laminar, non-premixed, coannular ethylene flame. Although the measurements at several incident wavelengths do not yield single complex pairs for m, the range of potential refractive index pairs yields wavelength-dependent E(m) values consistent with the literature. Consistency in soot morphological properties and the observed k_e ratios between the spray flame and the oxidation region of the laminar non-premixed flame support the hypothesis that high turbulence and rapid mixing of the spray flame create an environment in which rapid flamelet-type soot oxidation processes occur throughout the radial and axial extent of the spray, beyond the inception region. Non-dimensional extinction coefficients over a range of visible wavelengths and into the near-infrared are proposed as a standard for quantitative soot measurements performed within the ECN. The associated uncertainty in k_e, and consequently on the soot volume fraction or total soot mass derived from extinction measurements, is on the order of ±20%.