SPECTROSCOPIC AND NH2* CHEMILUMINESCENCE MEASUREMENTS OF SPHERICALLY PROPAGATING AMMONIA FLAMES

Abstract

Ammonia (NH3) has received significant attention recently as a promising carbon-free, low-cost, and safe candidate as a chemical energy storage element and/or fuel alternative. As for any new fuel, fundamental aspects of the combustion process, such as the detailed chemical kinetic pathways of NH3 oxidation, flame propagation and stability, and emissions, are to be understood to predict its combustion behavior in practical systems. One aspect of improving our understanding of the combustion of NH3 is to explore the emission spectra of flames under controlled laboratory conditions. In this work, flame spectra from NH3-containing mixtures were obtained utilizing two different experimental flame configurations, namely a spherically propagating flame and a modified Mckenna burner flame. Several excited-state species within the UV-visible spectral region were identified such as NO*, OH*, NH*, and NH2*. The effect of the mixture on the spectra was investigated by examining two different mixtures utilizing the same experimental apparatus. The first mixture was an oxy-ammonia mixture consisting of NH3 + (25% O2 + 75% N2) with the goal of studying neat ammonia. The second mixture consisted of H2:NH3:N2 with a ratio of 45:40:15 by volume. Additionally, the effects of the flame configuration—spherical flame versus burner—were also investigated by studying the same mixture, and it was revealed that the spectra produced using the modified Mckenna burner had less noise, and all features were easily identified. Another way to increase the knowledge of ammonia combustion is by investigating the flame zone of a laminar flame. Using a high-spatial-resolution flame zone measurement technique, the flame zone of NH3-containing mixtures was measured experi¬mentally. A high-speed chemiluminescence imaging setup with variable magnification was used to directly resolve NH2* over the flame zone. Measurements were repeated while varying the pixel density of the imaging system to get the true NH2* profile (or flame) thickness independent of the imaging system. The study revealed that improving the pixel density of the chemiluminescence setup plays a significant role in producing accurate and reliable NH2* thickness measurements. The effect of the fuel mixture, initial temperature, and initial pressure on NH2* profile shape was investigated. Kinetics predictions of NH2* profiles, described in terms of thicknesses, were obtained using three different previously published models. One of these kinetics’ models produced predictions in relatively close agreement with experimentally measured NH2* thickness, while the other ones under-predicted the measured values by up to 60%. In closing, some work on improving the current, in-progress chemical kinetics modeling of NH2* can be performed using species measurements through the flame zone as a metric, as demonstrated in this thesis.