Laminar burning velocity, radiative heat loss and cellular instability for NH3/biogas/H2/air premixed flame

Abstract

This study presents, for the first time, a comprehensive investigation into the laminar flame propagation of ammonia/biogas/hydrogen/air mixtures. The laminar burning velocities (LBVs) were determined using the outwardly propagating spherical flame method under atmospheric temperature and elevated pressures across various equivalence ratios. The Okafor and CEU-NH3-Mech-1.1 mechanisms showed the best agreement with flame speed measurements. The influence of equivalence ratio, hydrogen ratio, and initial pressure on LBV was examined. Results indicate that LBV varies non-monotonically with the equivalence ratio, decreases with increasing pressure, and increases with higher hydrogen ratios. The enhancement of LBV due to hydrogen addition is mainly attributed to chemical effects, with thermal effects also playing a role. Given the significant CO2 content in biogas, which is a key radiative species, the study also analyzed the radiative heat loss of ammonia/biogas/hydrogen mixtures. It was found that as the equivalence ratio nears the flammability limit, LBV is more affected by radiative heat loss, with a lower hydrogen ratio increasing this effect. Additionally, higher CO2 mole fractions in biogas increase radiative heat loss. Detailed analysis of cellular instability, considering factors such as thermal expansion ratio, flame thickness, effective Lewis number, the logarithmic growth rate of perturbations and in conjunction with schlieren images, revealed that flame instability is enhanced with higher initial pressure and hydrogen ratios, while higher equivalence ratios inhibit instability.