Experimental characterization of diffusive-thermal instabilities in CO₂-diluted H₂―CH₄―CO unstrained diffusion flames

The combustion of multi-fuel mixtures is experimentally studied for the first time in unstrained diffusion flames, where the parasitic hydrodynamic effects present in common research burners are negligible. A broad range of H₂-CO-CH₄ fuels highly diluted in CO₂ is investigated to provide an understanding of the intrinsic diffusive-thermal instabilities (DTIs) that onset in low calorific biomass-derived syngas. For each fuel blend, the burning intensity or the Damköhler number ($D_a$) is gradually reduced, going through the marginal stability state where DTIs first appear, down to the lean extinction limit. Flame stability limits are provided. From the large difference between the Lewis numbers of the multiple fuel species ($Le_i$), the cells that onset due to H₂ are seen to interact and compete with the pulsations from CO and CH₄, leading to superimposed cellular-pulsating instabilities. These are thoroughly characterized by measuring the pulsations amplitude, frequency, cell size, number of cells, and fraction of the flame sheet actively burning. An effective fuel Lewis number ($Le_{F,eff}$) calculated from the fuel mixture composition is introduced and used along with the Damköhler number to map the DTIs observed. At lower $Le_{F,eff}$ and $D_a$, the cellular attributes of the superimposed instabilities dominate, while at larger Lewis numbers and the near marginal stability state, pulsations prevail.