Effects of ammonia in-situ partial cracking on the structure of bluff-body non-premixed flames

Direct utilization of ammonia for energy generation saves the cost of the endothermic cracking process that consumes 15% of the total specific energy in ammonia. This combined experimental and computational study focuses on the direct use of ammonia and enhancing its burning rate through in-situ partial conversion into H₂ and N₂ within the recirculation zone. In this paper, an axisymmetric bluff-body burner, is employed to establish a self-sustained pure ammonia flame at jet Reynolds number, 9000, enabling stable turbulent non-premixed flames. Various diagnostic techniques, including planar laser-induced fluorescence (PLIF) of NH, NO & OH, Fourier-transformed infrared (FTIR), and gas chromatography (GC), are used to explore flame features, and concentrations of NO, N₂O, H₂, O₂, and NH₃ within the flame and at the exhaust. The RANS flamelet/progress variable (FPV) approach is utilized to qualitatively capture the flame features. The flame exhibits a broad and luminous recirculation region, followed by a narrower zone downstream, where the reaction continues in flameless mode. In the flameless region, reactant temperatures rise despite the absence of a visible flame. The flammable region is marked by high concentrations of NO, OH, NH, and the post-flame region witnesses rapid consumption of H₂, NO, and N₂O. Approximately 20% of NH₃ dissociates in the first zone, while more than 60% of NH₃ dissociated by the flame's maximum height. At the flame tip, most H₂ and NO are consumed, leaving around 30% NH₃ and 100 ppm of N₂O slip. Ammonia cracking is greater radially across the bluff-body surface, over 70% dissociating into H₂ and intermediates. Overall, this study provides insights into the concept of in-situ partial cracking at the flame base as a method to establish self-sustained ammonia combustion, allowing for on-site and direct ammonia fuel combustion and yielding low NO emissions and a promising optimization potential of ammonia utilization.