Cellular structures of laminar lean premixed H2/CH4/air polyhedral flames

Fundamental studies on the effects of differential diffusion of hydrogen (H2) on flame structure are motivated as the high diffusivity of H2 presents challenges for the modeling and optimization of combustion systems. Polyhedral Bunsen flames are examples of cellular flames mainly induced by the thermal-diffusive and hydrodynamic instabilities, which are characterized by periodic positively curved troughs and negatively curved cusps. Stationary laminar premixed fuel-lean H2/CH4/air polyhedral flames, with 50%, 68% and 79% H2 (by volume) and Lewis number (Le) less than unity, are investigated in this study. The internal scalar structures of cellular troughs and cusps in target flames are measured with a high-spatial-resolution 1D Raman/Rayleigh scattering system, combined with planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF) and chemiluminescence imaging measurements to quantify the cell number and local flame curvature. The performance of the 1D Raman/Rayleigh imaging system is first assessed by comparing measurements of temperature and major species in a laminar premixed counterflow H2/CH4/air twin flame with a corresponding simulation. The results reveal significant combined effects of differential diffusion and curvature on flame structures with differences between trough and cusp regions in the measured mole fractions, equivalence ratio, temperature, and C/H-atom ratio. The positively curved troughs have significantly higher H2 mole fraction compared to the negatively curved cusps, due to the respective focusing/defocusing effect of curvature on highly diffusive H2. Consequently, the local equivalence ratio and temperature in trough regions are higher than those of cusps. With the increase of H2 content in the reactant mixture, the scalar differences between trough and cusp regions are enlarged due to the enhanced effects of curvature and differential diffusion. Near-vertical initial trajectories in H2 mole fraction, equivalence ratio, and C/H-atom ratio plotted against temperature showed that differential diffusion of H2 alters the species mole fractions in the cold reactants (≤ 350 K).