Effects of oxygen enrichment on diesel spray flame soot formation in O2/Ar atmosphere

Abstract

In this study, diesel spray combustion at oxygen-enriched conditions (oxygen volume fraction of 21–70 %) with argon dilution is experimentally investigated in a constant-volume combustion chamber. Optical diagnostics are employed to study flame development, stabilization, and soot formation at oxygen-enriched conditions. To further verify the experimental observations, two-stage Lagrangian simulations are used to analyze the effects of oxygen on the formation and oxidation of soot precursors, polycyclic aromatic hydrocarbons. Results show that replacing nitrogen in air by argon leads to a 50 % reduction of the flame lift-off length, an increased soot flame temperature by 300 K, and higher soot concentrations. Flame morphology and structure still follow the classic conventional diesel combustion model in the oxygen range of 21–40 %, while changes are observed when oxygen levels are higher than 50 %. The width and length of the soot flame are shortened, and chemiluminescence from intermediate species like CO dominates the flame natural luminosity at the spray head, where the flame temperature reaches near 3000 K. Soot reduction mechanisms at high-degree oxygen-enrichment conditions are investigated. The intrinsic mixing-limited combustion of diesel sprays leads to unavoidable fuel-rich areas locally, but the shortened flame lift-off length and sufficient oxygen supply confines soot-forming conditions to a smaller, upstream region. The residence time of fuel parcels in this confined soot-forming area is shortened due to the larger local spray velocity. Thereafter, fuel parcels enter a high-temperature fuel-lean region, where the formed soot is oxidized rapidly.