Direct numerical simulations with multi-step chemistry of liquid water interaction with laminar spherically expanding premixed hydrogen/air flames

In the present study, the behavior of laminar spherically expanding premixed hydrogen/air flame interacting with liquid water droplets is investigated through direct numerical simulations. A hybrid Eulerian–Lagrangian approach with two-way coupling is employed for the numerical representation of the problem, and the droplets are modeled as point sources. The effects of equivalence ratio, initial droplet diameter, and multi-species diffusion model variation are analyzed. Results indicate that an accurate description of the diffusion process is crucial at low equivalence ratios due to the strong influence of thermodiffusive properties, but becomes less significant at higher equivalence ratios. Additionally, the Soret effect enhances flame speed and susceptibility to intrinsic flame instabilities onset at low equivalence ratios, while contributing to flame stability in higher equivalence ratio conditions. However, the introduction of water droplets significantly alters the Soret effect’s influence by modifying H radical production and distribution. Furthermore, water addition is found to increase flame area generation, yet simultaneously reduces the burning rate per unit area of the flame. Lastly, the impact of water addition on emissions is assessed, revealing that, in the configuration considered, water enhances nitrogen oxide production by influencing the N 2 O pathway, which is triggered by the presence of H and O radicals. This pathway becomes more prominent at lower temperatures due to its insensitivity to temperature. In this context, nitrogen monoxide production is promoted in the presence of strongly localized “cold spots”, aided by water evaporation, which increase radical concentration, as well as by the high collision efficiency of water, which is important for initiating the N 2 O pathway. • The effects of different multi-species diffusion models on flame propagation are assessed. • The Soret effect alters flame propagation depending on the mixture composition. • Water addition enhances area generation and decreases the burning rate per unit area. • Water droplets can increase NO x emission via the N 2 O pathway at the “cold spots”.