Flame merging and NOx control in scaled-up hydrogen micromix combustors

Reducing the number of fuel injectors in hydrogen micromix combustors increases the loading of individual elements, creating challenges for flame separation and NOx control. This study numerically investigates two design approaches: air channel modification for elements widened from 3 to 4.5 mm, and a Micromix Side-Cooling Concept (MSC) with alternating fuel-fed and air-only channels. The numerical approach is assessed against published NOx measurements for a baseline configuration. Within the investigated range, air channel modifications alter mixing, recirculation, and lateral flame spreading. The optimised enlarged-element configuration predicts 1.40 ppm thermal NO at a global equivalence ratio of 0.54. At the same global equivalence ratio, MSC predicts 3.2 ppm thermal NO compared with 12.43 ppm for its reference configuration, both using 0.5 mm hydrogen injection holes. The MSC results show spatially separated OH reaction zones while retaining the same total air and hydrogen supply per unit frontal area and requiring 50% fewer hydrogen injection holes. These numerical findings indicate the potential of air channel modification and spatial fuel redistribution to reduce injector count while maintaining flame separation, with implications for NOx control.