Numerical analysis of very rich propagating spherical flames: Soot formation and its impact on the determination of laminar flame speed

Abstract Fuel-rich combustion is a promising technique to produce hydrogen by reforming hydrocarbons. Predictive modeling of the fuel reforming process needs a reliable chemical mechanism which is usually validated using laminar flame speed (LFS) data. While LFSs have been extensively employed to optimize mechanisms under fuel-lean, stoichiometric, and slightly fuel-rich conditions, it remains a formidable challenge to measure the LFSs of very rich mixtures (e.g., equivalence ratio ϕ ≥ 2 ) due to significant soot formation in such conditions, which leads to the LFS data under fuel-rich conditions being rather scarce. To overcome the challenge, a clear understanding of soot formation and its impact on LFS measurements is required. In this work, a series of one-dimensional outwardly propagating spherical flames (OPFs) with and without considering soot formation are simulated for rich ethylene/air mixtures ( 2.0 ≤ ϕ ≤ 3.5 ), in order to understand soot dynamics and morphology in the OPFs and to quantify the effects of soot formation on the determination of LFS using the OPF method under fuel-rich conditions. To this end, a detailed chemistry accounting for major pathways of PAH formation up to A7 (coronene, C 24 H 12 ) is employed and coupled with a state-of-the-art soot model considering nucleation, condensation, coagulation, surface growth, oxidation, and fragmentation. It is found that soot dynamics and morphology are very sensitive to the change in equivalence ratio and flame radius. Specifically, surface growth and oxidation play a dominant role in soot formation/evolution and can balance each other at ϕ = 2.0 , resulting in limited soot formation. However, the PAH-based condensation can dominate over other processes at ϕ ≥ 2.5 and hence a large amount of soot is observed. Moreover, the maximum value of primary particle diameters would tend to a constant as the flame radius increases. The results also show that the ratio between total soot mass and the mass of burned gas, m soot / m burned , follows an exponential law given by m soot / m burned = 10 0.47 ϕ − 3.76 , implying that soot formation can be exponentially enhanced by increasing equivalence ratio. Furthermore, it is found that under very rich conditions, the presence of soot has a significant impact on the determination of LFS, which leads to the measured LFS at ϕ = 3.5 being 22% lower than 1D planar adiabatic flame speed. This is mainly attributed to the thermal and flow effects of soot radiation, i.e., the reduction of flame temperature and negative flow speed of the burned gas. The above findings suggest that the impact of soot radiation should be carefully treated when determining the LFSs of rich premixed mixtures at ϕ ≥ 2.0 .