Numerical and experimental investigation of the flame kernel growth in a methane/air mixture near the lean flammability limit

Journal Article · Sun Nov 13 00:00:00 EST 2022 · Combustion and Flame

DOI:https://doi.org/10.1016/j.combustflame.2022.112463· OSTI ID:2404313

Kim, Joohan ^[1]; Scarcelli, Riccardo ^[1]; Biswas, Sayan ^[2]; Ekoto, Isaac ^[3]

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1. Argonne National Laboratory (ANL), Argonne, IL (United States)
2. Sandia National Laboratories (SNL-CA), Livermore, CA (United States); Univ. of Minnesota, Minneapolis, MN (United States)
3. Sandia National Laboratories (SNL-CA), Livermore, CA (United States)

Lean combustion has the potential to improve the thermal efficiency of spark-ignition engines, but it faces the significant challenge of increased cycle-to-cycle variation due to low mixture reactivity and unstable flame dynamics. Computational fluid dynamics (CFD) employing predictive models can guide engine design and optimize operating strategies for lean combustion. However, ignition and combustion models have rarely been validated at fuel-lean conditions, and a fundamental understanding of the early flame kernel growth process is also lacking for a successful sub-model development. Here, the present study develops a numerical simulation framework used to investigate early flame kernel growth in methane/air mixtures. A nanosecond-pulsed discharge (NPD) approach is employed to effectively decouple the flame kernel growth from the electrical discharge due to their difference in timescales, and equivalence ratios near the experimentally measured lean flammability limit (LFL) are selected to focus on challenging mixture conditions. Three numerical investigations, such as the choice of turbulence modeling, grid size, and grid control strategies, are examined to match both LFL and flame kernel structure measured from experiments. It is demonstrated that a quasi-direct numerical simulation (QDNS) with a fixed grid embedding of 10 μm can predict the LFL as φ_CFD=0.61 and match the displacement speed of the kernel’s boundary marked in schlieren images. To predict the LFL and flame kernel shape, a fine grid (Δ≤12.5 μm) is needed to capture the consumption of formaldehyde (CH₂O) in kernel’s reaction branches attached to the anode, and adaptive mesh refinement is replaced with the fixed embedding due to loss of simulation accuracy. Also, it is found that a large-eddy simulation (LES) using the Dynamic Structure model is not suitable for the NPD-induced flame kernel simulation because artificial sub-grid turbulent kinetic energy induced by shock dynamics alters the flow velocity calculation, resulting in divergence of LES from QDNS. Lastly, the simulation well matches the experimental data for the flame kernel evolution in three mixture conditions (φ = 0.7, 0.61, 0.55), showing toroidal flame kernel expansion and flame kernel growth/extinction.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States); Sandia National Laboratories (SNL-CA), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA); USDOE Office of Energy Efficiency and Renewable Energy (EERE), Office of Sustainable Transportation. Vehicle Technologies Office (VTO); USDOE Office of Science (SC)
• Grant/Contract Number: AC02-06CH11357; NA0003525
• OSTI ID: 2404313
• Alternate ID(s): OSTI ID: 1961895
• Journal Information: Combustion and Flame, Journal Name: Combustion and Flame Journal Issue: 112463 Vol. 247; ISSN 0010-2180
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

33 ADVANCED PROPULSION SYSTEMS
computational fluid dynamics
flame kernel evolution
lean flammability limit
nanosecond-pulsed discharge
spark ignition processes