Hydrogen as a fuel additive in laminar premixed methane flames: Impact on the nucleation and growth of soot particles

This work reports new experimental data regarding the impact of H-2 on slightly sooting methane flames (Phi-1.91) characterized by the formation of soot particles undergoing growth processes. This study has been carried out in three atmospheric sooting premixed methane flames with added H-2 (2% of the total flowrate) or substituted (10% molar comparing to CH4 fuel). The mole fraction profiles of selected aliphatic and aromatic compounds (C-1 -C-16) have been measured by Gas Chromatography and Jet-Cooled Laser-Induced Fluorescence. Concerning the soot particles, we have implemented for the first time an analytical methodology based on in-situ measurements by Laser-induced Incandescence and Cavity Ring-Down Spectroscopy coupled with ex-situ measurements by Scanning Mobility Particle Sizing (SMPS). This original approach allows to determine the soot volume fraction profiles and corresponding evolution of the particle number density and particles size density function without requiring accurate knowledge of the dilution ratio during SMSP measurements. The reported data highlight that the introduction of H-2 strongly influences the formation of aromatic species and soot depending on the operating conditions (H-2 addition or substitution). The impact of H-2 was evaluated on the nucleation step and the soot growth processes. In that context, we notably show that the H-2 substitution inhibits while the H-2 addition favors the formation of nascent soot particles, and furthermore the H-2 addition generates substantial modifications in the dynamic of the soot growth processes. This work also supports the idea that the onset of the particle nucleation might be controlled by specific concentration thresholds of aromatic precursors.