Climate change and energy crisis have motivated researchers and engineers to seek renewable and sustainable carbon-free fuels that could be potentially applied to industrial heating, propulsion, and power generation sectors. Ammonia, as an alternative fuel with a relatively higher hydrogen content, has been of emerging interest in the combustion community, thus contributing to the net zero coalition. However, the burning of such a renewably-sourced fuel is challenging as a result of the low flame speed and high NOx emissions arising from the fuel N. Meanwhile, understanding the fundamental combustion process is not an easy task due to the complexity of the reaction mechanisms.

The objectives of this thesis are to conduct systematic investigations on flame propagation, reaction mechanism simplification, combustion, and emission performances in ammonia-fueled premixed micro-combustors. For this, the fundamental laminar burning velocity of ammonia/air blended with dimethyl ether is numerically examined first. Then, the discussion on the combustion and emission performances is provided, as the operating conditions are varied. The presence of dimethyl ether is found to significantly enhance the laminar burning velocity of ammonia/air flames due to the coupled thermal and chemical effects. Elevating inlet temperature is beneficial to promoting the flame speed as a result of the enhanced reaction rate while increasing inlet pressure exhibits a negative impact mainly due to the enhanced three-body termination reactions.

By utilizing a three-dimensional computational model, NOx emissions and combustion performance of premixed ammonia-oxygen micro-combustors are examined and discussed in detail. This is followed by an evaluation of the response of NO formation to the secondary fuel injection strategy. For ammonia-fueled combustors, operating under slightly fuel-lean conditions is desirable in terms of increased outer wall mean temperature, whereas operating under slightly fuel-rich conditions is attractive from the standpoint of reduced NOx emissions. This is also the case for blending ammonia with hydrogen on a molar basis. On the contrary, a high inlet pressure is found to give rise to enhanced thermal performance and a low pollutant formation. Additionally, the implementation of such a means is shown to lead to an approximately 28% decrease in NOx emissions without a penalty of increased NH3 leakage in comparison to the conventional combustor. This reduction is primarily achieved through the selective non-catalytic reduction process that converts NO into N2 by reacting with the reducing agent NH3. Meanwhile, both the secondary fuel injection location and the injection ratio are confirmed to play a key role in determining pollutant formation.

Finally, the detailed chemical kinetic mechanism in ammonia-fueled premixed mixtures is simplified with the aid of a direct relation graph with error propagation (DRGEP) method. This leads to 4 types of reduced mechanisms for ammonia and ammonia/air flames being developed. After the validation in terms of laminar burning velocity and ignition delay time over a wide range of operating conditions relevant to the micro-combustion, internal combustion engines, and gas turbine engines, these reaction mechanisms are generally demonstrated to well capture the experimental phenomena. All of these findings obtained could be helpful in developing high-efficient and low-emission ammonia combustion techniques.