Experimental and kinetic modeling studies of quenching diameters of laminar premixed flames of NH3/air mixtures

Abstract

The quenching diameters of laminar premixed flames of NH₃/air mixtures under ambient temperature and pressure were investigated via experimentation and kinetic modeling. In the experiments, a premixed Busen flame was at first established on the exit of a cylindrical quartz tube, and then the gas flow was suddenly stopped to test whether it incurs quasi-steady state flame propagation or flame quenching in the tube. The tube diameter was progressively decreased from 15.0 to 8.0 mm and the quenching diameter is the diameter that just prevents flame propagation (flashback) and incurs flame quenching. Quenching diameters were measured at varied equivalence ratios of 0.6 - 1.4. A transient two-dimensional axisymmetric numerical model was applied to simulate the flame structure and the propagation and quenching processes. Heat exchange between the flame and the conductive wall was considered. Three detailed reaction mechanisms were employed for chemical kinetics, namely, the Glarborg et al., Okafor et al., and Zhang et al. mechanisms. The Glarborg et al. mechanism shows a better agreement with the experimental results in predicting the quenching diameters, while all three mechanisms over predicted the quenching diameter at the lean limit. Kinetic analysis suggests that the H involved reactions, H + O₂ = O + OH and H + NO (+M) = HNO (+M), are the most contributive exothermic reactions. The H₂NN related reactions that included in the Glarborg et al. mechanism shows a minor impact on the prediction of the quenching diameter, while influences the NO_x formation significantly.