Two-Color Diffused Back-Illumination Imaging as a Diagnostic for Time-Resolved Soot Measurements in Reacting Sprays

Despite ongoing research efforts directed at reducing engine-out emissions, diesel engines are known to be one of the largest sources of atmospheric particulate matter (i.e., soot). Quantitative measurements are of primary importance to address soot production during the combustion process in the cylinder of diesel engines. This study presents the capabilities of an extinction-based diagnostic developed to quantitatively measure the soot volume fraction in n-dodecane sprays injected in a high-pressure, high-temperature vessel. Coupled with high-speed imaging, the technique yields time-resolved measurements of the soot field by relying on a diffused back-illumination scheme to improve extinction quantification in the midst of intense beam steering. The experiments performed in this work used two wavelengths, which, when combined with the Rayleigh-Debye-Gans theory, provide information about the optical and physical properties of soot. For validation of the extinction imaging technique and comparison with prior work, the path-averaged soot volume fraction has been simultaneously measured using laser extinction. We found the sensitivity of this diagnostic to soot volume fraction to be below 1 ppm despite beam-steering influences in the time-resolved data, and we were able to quantify higher soot concentrations in engine-relevant spray flames than previous studies by our group that used laser-induced incandescence (LII) in conjunction with path-averaged laser extinction. This work demonstrated a preference for shorter visible wavelengths, as soot-generated extinction is higher while broadband flame emission is lower at these wavelengths. Consequently, shorter visible wavelengths resulted in a greater signal-to-noise ratio. Analysis of two-color extinction indicated that the refractive index and morphological properties of soot change during the formation and oxidation processes.