UV and Visible Radiation Characteristics of Thermoacoustic Instabilities in an Ammonia–Methane Premixed Swirl-Stabilized Combustor

Ammonia (NH₃) is a promising carbon-free energy carrier for low-carbon power generation. However, in turbulent ammonia–methane (NH₃-CH₄) premixed swirling flames, operating at lean conditions to limit NO_X, emissions can trigger strong thermoacoustic oscillations. This study investigates thermoacoustic oscillatory instability in an NH₃-CH₄ swirl-stabilized combustor using the chemiluminescence of CH*, OH*, and NH* over a wide range of ammonia fuel fraction (X_NH3). Combined spectral measurements and 2D chemiluminescence imaging are employed to obtain the global emission characteristics and spatial distributions of OH* and NH* in the UV band and CH* in the visible band. A custom-designed intensified CMOS (ICMOS) camera based on a high-gain UV–visible image intensifier with direct coupling is developed to enable sensitive OH* and NH* imaging (gain > 10⁴). Frequency analysis of continuous CH* imaging, together with morphology-based principal component analysis and k-means clustering of 46 image features, shows that oscillatory combustion occurs for X_NH3 < 0.40, whereas X_NH3 ≥ 0.40 leads to multimode, stable combustion. As X_NH3 increases, OH* and NH* fields progressively decouple from CH*, becoming more elongated and shifting downstream. These results demonstrate that UV radical chemiluminescence provides indispensable information on NH₃ reaction zones and should be combined with CH* diagnostics for reliable thermoacoustic analysis and control in practical NH₃-fueled combustion systems.