Direct numerical simulation of soot break-through in turbulent non-premixed flames

Soot break-through, or “leakage”, is the transport of soot particles into fuel-lean regions leading to smoke emissions even when the global equivalence ratio is below unity. Under turbulent conditions, this phenomenon is affected by small-scale mixing, and a better understanding of the non-linear interactions between soot and gas-phase is crucial to improving the prediction of the existing reduced-order model in case of soot break-through. To this end, three Direct Numerical Simulations (DNSs) of temporally evolving turbulent non-premixed jet flames have been performed to study the later stages of soot evolution in turbulent flames. Various degrees of soot break-through are obtained by enforcing three different levels of flame extinction by rescaling the geometry and flow conditions, resulting in a variation in Damköhler number, while the Reynolds number remains constant. The simulations employ a detailed chemical mechanism for the gas phase chemistry and a moment method for modeling the soot number density function evolution. Both soot evolution and turbulence-chemistry-soot interaction are discussed in mixture fraction space and along Lagrangian trajectories in physical space. The results reveal that soot break-through is strongly correlated with both high mixture fraction dissipation rates and strong soot transport in mixture fraction space. Due to the higher mixture fraction dissipation rate, the lowest Damköhler number case exhibits more significant extinction, allowing significant soot break-through. Additionally, soot particles in the lean regions are shown to be smaller than those inside the flame due to the smaller residence times in the fuel-rich growth regions, with a significant probability of leaked soot particles at incipient size. From the Lagrangian statistics, the ratio of the mixture fraction diffusion rate and soot oxidation rate constant is shown to be a suitable parameter to identify soot break-through events. Overall, soot break-through is found to be dominated by local flame extinctions and, while a fast drift of soot toward lean regions is required for such phenomena, an accurate prediction of local extinctions is also necessary to capture soot break-through occurrences. Novelty and Significance This study presents new insights into turbulent sooting flames through a novel dataset, obtained using direct numerical simulations (DNSs), to investigate the later stages of soot evolution, including incomplete oxidation phenomena leading to smoking behavior. In contrast to existing DNS datasets which have primarily focused only on the early stages of soot formation and growth, the dataset presented and analyzed in this study provides key new data on soot oxidation and break-through in turbulent non-premixed flames. As modeling turbulence-chemistry-soot interactions is still an open research question in the combustion community, this dataset contributes to advancing the current understanding of the underlying physics governing soot evolution in turbulent flames.