Probing gas-to-particle transition in a moderately sooting atmospheric pressure ethylene/air laminar premixed flame. Part I: gas phase and soot ensemble characterization

A comprehensive investigation was conducted on a sooting ethylene laminar premixed flame: the present contribution focuses on the characterization of the gas composition by gas microsampling and GC/MS analysis, and of soot properties by optical measurements; Part II presents detailed measurements of soot nucleation by thermophoretic sampling and differential mobility analysis. Stable species up to 3-ring aromatics were quantified and compared with a computational model using two distinct chemistry models and accounting for buoyancy. Disagreement with respect to critical soot precursors was found, whose concentrations were typically overestimated by one order of magnitude in the models. Reliable thermocouple measurements of temperature, when matched by multi-color pyrometry results, provided information on soot volume fractions and optical properties complementing laser light extinction measurements. Results evidenced a sharp increase of the soot volume fraction in the inception zone of the flame followed by a plateauing behavior, similar to the trend of aromatic species, which is consistent with the well-known link between aromatic growth kinetics and soot inception. In addition, fit of pyrometric soot temperature to gas phase thermocouple measurements provided information on the light absorption dispersion exponent, revealing the aging of soot from near transparency in the visible spectral range for young soot particles to a more graphitic-like material further downstream. The presence of the stagnation plate used to stabilize the flame induced a sharp increase in the soot volume fraction and its dispersion exponent in the thermal boundary layer of the plate. This evidence suggests the occurrence of condensation of semi-volatile material under the cooling of flame products occurring near the plate, whose origin is yet to be firmly established but may be mediated by flame ions.