Probing soot nanoparticle evolution in hydrogen-added counterflow diffusion flames

Hydrogen (H$_2$) addition to hydrocarbon flames alters soot yield, morphology, and nanostructure. This study investigates its influence on ethylene (C$_2$H$_4$) counterflow diffusion flames by quantifying soot volume fraction, refractive-index function for absorption, and size distributions, while simultaneously characterizing flame structure via OH*, CH*, and Swan band emission as well as temperature profiles. To disentangle the individual effects of H$_2$ addition, H$_2$-, He-, and He/Ar-modified flames were compared. The evolution from nascent, weakly light-absorbing nanoparticles of approximately 2–3nm to mature soot was tracked, and the observed differences were attributed to the disentangled effects. Higher flame temperatures were found to enhance soot formation, whereas H$_2$-specific chemistry and thermophysical changes associated with H$_2$ addition acted in a soot-suppressing manner. The amount of added H$_2$ determines whether the combined effects enhance soot formation at low addition levels or suppress it at higher addition levels, where smaller particles with a less graphitic nanostructure are formed.