Optical diagnostics of low-temperature lean combustion characteristics of premixed ammonia-hydrogen mixtures

The unfavorable combustion properties of ammonia pose significant challenges for its direct use as a fuel, and hydrogen-assisted combustion has emerged as a promising solution. In this study, an experimental investigation was conducted on the low-temperature lean combustion characteristics and radical distributions of premixed ammonia-hydrogen flames in a constant volume combustion chamber (CVCC). The results show that increasing the hydrogen blending ratio (αH) from 0 to 30 vol% and the equivalence ratio (Φ) from 0.5 to 0.9 shortens the flame initiation period by 62.1% and 79.5%, respectively, and the total combustion duration by 61.2% and 78.9%, respectively; however, further increases in either parameter yield diminishing returns. While hydrogen addition improves low-temperature combustion of ammonia-hydrogen mixtures, its effect is limited under ultra-lean conditions. Compared with pure ammonia, the addition of 10% hydrogen significantly enhances the overall NO∗ formation activity and accelerates the NO∗ formation rate. However, further increasing the hydrogen blending ratio from 10% to 40% produces no appreciable change in the overall NO∗ formation activity. As Φ increases, NO∗ formation activity rises, with a pronounced jump at Φ = 0.9. Although hydrogen promotes NO∗ formation activity, its levels remain relatively low under lean conditions.