Imaging measurements of soot particle size and soot volume fraction with laser-induced incandescence at Diesel engine conditions
This work focuses on measurements of soot particle size and volume fraction at Diesel engine conditions. A combination of laser-induced incandescence (LII) imaging, line-of-sight laser extinction, soot pyrometry, and transmission electron microscopy (TEM) measurements of thermophoretically-sampled soot was used. Particle sizing strategies were developed with LII model for the analysis of particlesize poly-dispersity with time-resolved LII signal that is suitable for point-wise measurements at atmospheric pressure, and for spatially-resolved characterization with two-time-step LII imaging. Measurements were performed with these strategies in a flame at atmospheric pressure and in Diesel engine combustion to investigate their applicability. Additional measurements were performed for temperature and soot volume fraction. A novel method, called two-exponential reverse fitting (TERF), is introduced to extract information about the size distribution. The method is based on mono-exponential fits to the LII signal decay at a delayed time. It approximates the particle-size distribution as a combination of one large and one small mono-disperse equivalent mean particle size and does not require a distribution assumption. It also provides a ratio of the contribution of both size classes. The systematic error caused by describing LII signals by mono-exponential decays was calculated as less than 2% for LII signals simulated for mono-disperse aggregated soot with heat-up temperatures for which evaporation is negligible. The method was applied to LII data acquired in a laminar non-premixed ethylene/air flame at various heights above the burner. The particle size of the large particle-size class evaluated with the method showed good consistency with TEM results, however the size of the small particle-size class and its relative contribution could not be compared due to insufficient information in the TEM results for small particles. Simultaneous line-of-sight laser extinction measurements and LII imaging were performed to derive the soot volume fraction in a high-temperature high-pressure constant-volume pre-combustion vessel under the Engine Combustion Network’s (ECN) "Spray A" conditions with parametric variations of gas temperature and composition. Extinction measurements were used to calibrate LII images for quantitative soot distribution measurements. OH-chemiluminescence imaging was used to determine the lift-off length, and used to interpret the soot measurements. Maximum soot volume fractions around 2–3 ppm were obtained at the nominal ambient temperature defined for Spray A (i.e. 900 K) that rise to 12 ppm at elevated temperature (1030 K). Variations of ambient temperature and oxygen concentration were carried out showing effects on soot formation and oxidation that are consistent with the literature. The method for particle-size imaging is based on evaluating gated LII signals acquired with two cameras consecutively after the laser pulse and using LII modeling to deduce particle size from the ratio of local signals. A strategy was developed with a model-based analysis: the dependence of LII particlesize imaging on the assumed boundary conditions was identified such as bathgas temperature, pressure, particle heat-up temperature, thermal accommodation coefficients, and soot morphology. Various laser-fluence regimes and gas pressures were considered. Effects of laser attenuation were evaluated. A combination of one detection gate starting with the particle-heating and the other starting with 11 ns delay with twice as long gate width was found to provide the highest sensitivity for particle sizing at 60 bar. The optimum gate delays for different pressures were calculated. The effects of timing jitter for laser pulse and poly-dispersity were investigated. Systematic errors in pyrometry imaging at 60 bar was evaluated. Parallel to the model-based analysis, experiments were conducted in near Spray A conditions with parametric variations of injection pressure, gas temperature and composition. The results were compared to particle-size measurements from TEM of soot sampled at multiple axial distances from the injector. The discrepancies between the two measurement techniques are discussed to analyze uncertainties and related error sources of the two diagnostics methodologies. It is found that in such an environment where particles are small and pressure is high, LII signal decay times are such that LII suffers from a strong bias towards large particles.