Unlike hydrocarbon fuel, ammonia is a carbon-free and renewable energy source. It is also regarded as one of the potential energy carriers. However, ammonia combustion for power generation is not well studied under micro-scale conditions, especially concerning nitrogen oxides (NO x ) emission. For this, thermal performances and NO emission characteristics of premixed ammonia/oxygen combustion are numerically investigated on a micro-planar combustor. The effects of 1) the equivalence ratio ϕ, 2) inlet temperature T in and 3) inlet pressure P in are examined. The outer wall mean temperature (OWMT) is found to vary non-monotonically as the mixture varies from lean to rich conditions, with the peak occurring at ϕ = 0.9 mainly due to the optimal heat transfer performance. However, a low ϕ could lead to the high nitric oxides (NO) concentrations because of the high flame temperature as well as O atom concentrations. Up to 75.5% of NO reduction could be achieved, as ϕ is optimized. Furthermore, increasing T in is shown to be associated with a low OWMT and NO concentration. In addition, varying P in is shown to lead to not only OWMT being changed but also NO formation being mitigated. The decrease in NO concentration for high P in is mostly attributable to the short residence time of high flame temperature in the channel and low OH concentrations. This work reveals that optimizing the operating thermodynamic parameters is an effective means to reduce NO emissions and improve thermal performances.

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The present work examines the NO x emission characteristics of a premixed micro-combustion system with a perforated plate implemented. For this, a three-dimensional (3D) computational model involving a detailed chemical-kinetic mechanism for ammonia-oxygen combustion in the micro-combustor is developed. The model is first validated with the experimental measurements available in the literature before conducting comprehensive analyses. It is found that implementing a perforated plate in the micro-combustion system creates a flow recirculation zone downstream characterized by a low flame temperature and combustion speed. Meanwhile, the conjugate heat transfer between the combustion products and the inner combustor walls is shown to play a key role in the NO generation by relocating the flame in the axial direction and thus changing the chemical reaction rate. Furthermore, the preferential diffusion caused by the variation in the mass diffusivity of different species and the two-dimensionality flow is identified to vary significantly in comparison with the case in the absence of the perforated plate, especially in the vicinity of the recirculation zone. This diffusion effect results in the considerable drop in the N/O atomic ratio, primarily due to the reduction and increase of O 2 and H 2 O, together with less available N 2 , and consequently affecting the NO generation rate. This work confirms that the flow field, the conjugate heat transfer as well as the preferential diffusion effect could be regarded as the potential mechanisms leading to the NO x emission variation in the recirculation zones. Nomenclature 3Dthree-dimensional specific heat of species, CO carbon monoxide CO 2 carbon dioxide CH 4

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