Study of the combustion characteristics of micro-mixing flame by diffusion number

Micro-mixing combustion is a promising hydrogen combustion technology, micro-diffusion combustion characteristics are similar to those of premixed combustion under extreme conditions. However, there is lack of a unified evaluation method for the nozzles of diverse structures. This paper presents an investigation of a micro-diffusion burner, and proposes a characteristic parameter (diffusion number N_dif) to evaluate the degree of dispersion of one larger nozzle. The study examines flame stability, temperature, OH radicals and pollutant emissions at three different N_dif values. The results demonstrated that as the nozzle diameter decreased and the number of nozzles increased, the N_dif gradually decreased, accompanied by an increase in diffusivity and mixing degree of fuel and air. Additionally, the combustion characteristics of the micro-mixing flame exhibited greater similarities to those of premixed flame. At N_dif = 0.083, the flame exhibited a uniform temperature field and OH radical distribution. At Φ = 0.87, the NO_x emission reached its maximum. When Φ is less than 0.67, the NO_x emission is less than 10 mg/Nm³. As N_dif decreases, the combustion characteristics of the micro-mixing flame tend to be similar to those of the planar premixed flame. The results of this study may provide valuable guidance into the assessment of the degree of air–fuel mixing at the nozzle outlet and serve as a criterion for future micro-mixing flame research.