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.