Combustion and ignition characteristics of ammonia/air mixtures in a micro flow reactor with a controlled temperature profile

Combustion and ignition characteristics of a stoichiometric ammonia/air mixture were investigated by a micro flow reactor with a controlled temperature profile from ambient temperature to 1300 K at atmospheric pressure. Three kinds of flame dynamics depending on inlet mean flow velocities, namely, normal flames in the high velocity regime, flames with repetitive extinction and ignition (FREI) in the intermediate velocity regime, and weak flames in the low velocity regime, which have also been observed for hydrocarbons in previous studies, were observed for ammonia by the direct observation with a digital still camera in this study. The existence of ammonia weak flames at 1270 K was confirmed. Special attention was paid to weak flames to examine ignition characteristics of an ammonia/air mixture at low temperature because ignition experiments of ammonia have been conducted only at very high temperatures (around 2000 K). Species measurement for a stoichiometric ammonia/air weak flame at 10 cm/s was made using and a mass spectrometer and a T-shaped reactor fused with a micro-probe. The present species measurement elucidated the structure of the ammonia/air weak flame and complete combustion was confirmed at 1290 K. Five reaction mechanisms were used for computations of weak flames and the four of them did not predict complete combustion within the given computational domain. One reaction mechanism predicted complete combustion but the weak flame position in computation was located in a temperature region lower than that in the experiment. The order of reactivity evaluations among the five mechanisms using weak flames in the micro flow reactor agreed with that using ignition delay times at low temperatures which are generally difficult to be taken by ignition experiments. The capability of weak flame methodology to validate ignition properties of reaction mechanisms for low reactivity fuels like ammonia was successfully demonstrated in this study.