Experimental study on chemiluminescence characteristics of ammonia/methane partially premixed swirling flames

Understanding the complex chemiluminescence characteristics of ammonia/methane partially premixed flames is crucial for developing optical diagnostic sensors, yet it remains insufficiently investigated. This study experimentally examines the combustion characteristics of ammonia/methane partially premixed swirling flames with spectral analysis and key excited radical imaging of flame chemiluminescence signals. The effects of both the global equivalence ratio (φ) and the ammonia blending ratio (X_NH3) on flame chemiluminescence characteristics were explored. The results demonstrate that in weak turbulent swirling flames, the flame chemiluminescence spectra exhibit pronounced distinctions between the 280–400 nm ultraviolet (UV) band and the 400–800 nm visible band. With increasing X_NH3, the chemiluminescence intensity decreases in the UV range while concurrently enhancing in the visible spectrum. Notably, the flame background radiation displays a similar trend. The integrated chemiluminescence intensities of key radicals (OH∗, NH∗, CN∗, CH∗, and NH₂∗) exhibit a non-monotonic trend with φ, namely initially increasing before peak value and subsequently decreasing. Additionally, OH∗, CN∗, and CH∗ chemiluminescence intensities diminish with the increase of X_NH3, whereas NH₂∗ displays an opposing trend of intensity enhancement under the same conditions. The intensity ratios of CH∗/OH∗, NH₂∗/OH∗, and NH₂∗/CH∗ are sensitive to changes in φ and X_NH3. Specifically, the NH₂∗/CH∗ can be the markers of φ and X_NH3 due to the monotonically varying trend. The flame dynamic characteristics can be represented by the fluctuated chemiluminescence intensity of excited radicals such as OH∗ and NH∗. It demonstrates a significantly quantitative correlation between NO emission and normalized chemiluminescence intensities (OH∗/NH∗/CN∗/CH∗/NH₂∗) in present experimental conditions. The findings provide critical insights for developing chemiluminescence diagnostic strategies in ammonia/methane combustion applications.