Effects of Arrayed-Microtube Geometry on Hydrogen Micromix Combustion Characteristics

Abstract This study experimentally investigates the effects of microtube diameter and nozzle spacing on the combustion characteristics of hydrogen micro-mixing burners with arrayed microtubes. Three microtube diameters (14, 10, and 7 mm) and two nozzle spacings (1.5D and 2D) were examined under atmospheric pressure and room-temperature conditions with a 3% air pressure drop. Flame morphology, NOx emissions, pressure fluctuations, and flame dynamics based on proper orthogonal decomposition (POD) were analyzed over a range of equivalence ratios. The flame gradually transitions from a lifted to a stable state as the equivalence ratio increases. At high equivalence ratios, yellow-orange radiation appears because of locally non-uniform fuel-air mixing and partially premixed reaction zones. NOx emissions increase with equivalence ratio, while the 1.5D spacing produces higher NOx than the 2D spacing owing to stronger flame-flame interactions and enhanced local heat release. Pressure fluctuations become more pronounced when the equivalence ratio exceeds approximately 0.55, indicating stronger thermoacoustic coupling. POD analysis shows that flame fluctuations evolve from localized structures to organized axial coherent modes and finally to fragmented higher-order structures with increasing equivalence ratio. The findings provide new insights into the coupling between geometric configuration, flame interaction, and thermoacoustic dynamics, and offer guidance for the design of low-emission, combustion-stable hydrogen micro-mixing combustors.