Experimental and Numerical Investigation on Combustion Characteristics of Cracked Ammonia Flames

The combustion characteristics of cracked NH3 flames are investigated experimentally and numerically. The one-dimensional (1D) calculations are performed to validate the accuracy of the reaction mechanisms in cracking conditions and to understand the fundamental characteristics of the cracked NH3 flames. In the experiments, the local distributions of OH and NO and the species emissions of NO, NO2, N2O, and NH3 are measured using hydroxyl radical planar laser-induced fluorescence (OH-PLIF), NO* chemiluminescence, and Fourier transform infrared (FTIR) spectroscopy, respectively. The effects of partial cracking on the flame structure and emission characteristics are investigated. In addition, the large eddy simulations (LESs) with flamelet-generated manifold (FGM) methods, which consider the preferential diffusion effect, are conducted to understand the validity of the LES and to further elucidate the effects of the cracking ratio (Cr) on cracked NH3 flames. The 1D calculation results show that reaction mechanisms by Mei et al. (Mei, B.; Zhang, J.; Shi, X.; Xi, Z.; Li, Y. Enhancement of ammonia combustion with partial fuel cracking strategy: Laminar flame propagation and kinetic modeling investigation of NH3/H2/N2/air mixtures up to 10 atm. Combust. Flame 2021, 231, 111472, 10.1016/j.combustflame.2021.111472), Shrestha et al. (Shrestha, K. P.; Lhuillier, C.; Barbosa, A. A.; Brequigny, P.; Contino, F.; Mounaïm-Rousselle, C.; Seidel, L.; Mauss, F. An experimental and modeling study of ammonia with enriched oxygen content and ammonia/hydrogen laminar flame speed at elevated pressure and temperature. Proc. Combust. Inst. 2021, 38, 2163−2174, 10.1016/j.proci.2020.06.197), and Otomo et al. (Otomo, J.; Koshi, M.; Mitsumori, T.; Iwasaki, H.; Yamada, K. Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion. Int. J. Hydrogen Energy 2018, 43, 3004−3014, 10.1016/j.ijhydene.2017.12.066) have acceptable accuracies for predicting combustion characteristics of cracked NH3 flames. As the cracking ratio increases, the ignition delay time (tig) and laminar flame speed (Sl) are shortened and accelerated, respectively, and the effect of Cr on Sl becomes evident in higher ϕ conditions. The NO emissions exhibit a tendency to achieve peaks around Cr = 0.6–0.9, irrespective of ϕ. The experimental results show that the partial cracking significantly changes the NH3 swirl flame structure. As Cr increases, the flames changed from "V" to "M" shape and the OH and NO* signals are enhanced. While OH and NO* signals show a strong positive correlation in a pure NH3 flame, the correlation gradually weakens when Cr increases. As Cr increases, the NO and NO2 emissions increase, whereas the NH3 emissions decrease. As a result, the optimal condition for minimizing global emissions is considered to be Cr = 0.2. By comparison of the velocity and OH fields to the experiments, it is verified that the present LESs coupled with the FGM combustion model, considering the preferential diffusion effect, capture the general feature of the cracked NH3 swirl flames well. The heat release rate (HRR) is enhanced as a result of partial cracking. A higher Cr enables the flame to stabilize even in highly strained areas of the flame front. As Cr increases, the amount of both fuel NO and thermal NO increases as a result of higher O and OH and higher temperature. However, these are reduced by increasing ϕ and enhancing heat loss through walls.