The evolution of the Size Distribution Function (SDF) of incipient soot in a laminar non-premixed Planar Mixing Layer Flame (PMLF)

This work tracks soot inception in a Planar Mixing Layer Flame (PMLF) stabilized between adjacent slot jets of the fuel and oxidizer, sheathed from the surrounding air by nitrogen, which is compatible with spatially resolved dilution sampling thanks to its unconfined stabilization by flow impingement on a plate slotted to exhaust the hot flame products and anchor their position via buoyancy. The PMLF has a thick soot layer and the self-similar structure of a thermochemical boundary layer whose horizontal cross-sections are equivalent to 1D- Counterflow Flames (CFs) with strain rates lower than 10/s and negligibly affected by a downstream probe. The thermochemical advection transport histories leading to the soot present in a PMLF horizontal cross-section are also mimicked by those experienced in its equivalent 1D-CF. Multimodal SDFs are measured at 50 mm above the burner via dilution sampling followed by High-Resolution Differential Mobility Analysis, with results validated by in-situ laser diagnostics. Molecular clusters smaller than 2 nm growing into ∼2.5 nm proper particle nuclei form both while the flame temperature ( T ) decreases downstream of the ∼1740 K blue layer in the oxidizer stream, and at increasing T below 1000 K in the fuel stream. The formation and growth from ∼3 to ∼5 nm of coagulates of clusters and nuclei , commonly identified as incipient soot primary particles, occur during their advection in both reactant streams toward the particle mixing interface separating the particle streamlines in either of them. In both high and low T soot inception regions, the number concentrations of clusters and nuclei are comparable to those of 2 and 3–4 ring aromatics quantified by capillary sampling followed by GC/MS analyses, respectively.