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 (ke,406 nm/ke,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 ke ratio (ke,623 nm/ke,850 nm) is 1.42 ± 0.27. To compute a matching ke 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 ke ratio requirements, making the determination of an accurate soot refractive index from these data implausible. Representative ke 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 ke 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 ke, and consequently on the soot volume fraction or total soot mass derived from extinction measurements, is on the order of ±20%.