Species structures in preheated ammonia micro flames

Abstract

Species structure of ammonia (NH3) micro flames stabilized on a preheated 2-mm diameter tube is investigated using laser-induced fluorescence (LIF) measurements and numerical simulations with detailed reaction mechanisms. Nitric oxide (NO) and hydroxyl (OH) radicals are measured using single-photon absorption laser-induced fluorescence (LIF), while hydrogen atoms (H) are measured using two-photon absorption LIF (TPLIF). NO and OH are measured using nanosecond excitation pulses while both nanosecond and femtosecond methods are employed for TPLIF of H. 1-D and 2-D flame simulations are performed using Cantera and ANSYS FLUENT, separately, and the results are compared with the experimental data. It was observed that experimental H-atom signals increased with increasing equivalence ratio (ϕ), while the model predicted an opposite trend. Compared to the model predicted a rapid drop of OH as a function of ϕ, experimentally measured OH profiles exhibit opposite trends in different height locations of the flame. It is conjectured that the thermal dissociation of NH3 to generate reactive radicals such as H and OH is enhanced with preheating rich mixtures. Measured NO-LIF signal decreases with increasing ϕ, which agrees with the predicted trend by the simulation. NO reduction in fuel-rich flames is expected to be due to NHi radicals generated from excess NH3. A negative correlation is observed between NO and H/OH as a function of ϕ, which contradicts with the simulation that shows a positive relationship. This discrepancy implies that the effect of NH3 thermal dissociation has to be considered in the simulation of rich NH3 flames. This study helps to fulfill the fundamental knowledge gap of NH3 dissociation kinetics and the role of reactive intermediates in oxidation and NO formation pathways, hence an important step towards realizing practical combustion devices operating of NH3 as an energy carrier.