Forced ignition in turbulent mixtures of sufficiently large Lewis numbers

• Applicant: Professor Dr. Christian Hasse
• Subject Area: Energy Process Engineering ; Technical Thermodynamics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 411275182

Forced ignition in premixed mixtures is significantly affected by flame stretch (flow-induced strain and curvature). Experimental studies have shown that turbulence can lower the minimum ignition energy (MIE) for mixtures with large Lewis numbers, resulting in the phenomenon of turbulence facilitated ignition (TFI). This unexpected experimental observation was the starting point for the first funding period (FP1), which focused on the numerical analysis of stretch effects and the modeling of facilitated ignition. The project goals were achieved, and our analyses emphasized the significance of prefential diffusion, curvature-induced transport effects, and heat loss to cold electrodes. In addition, a new flamelet model was developed that can accurately predict ignition and subsequent ignition kernel evolution (IKE) in hydrogen-air mixtures. The model was extensively validated and subsequently extended to include stretch effects. To the best of our knowledge, we have published the first simulations demonstrating facilitated ignition. Convective effects transport the ignition kernel away from the electrodes, minimizing both flame stretch and heat losses, thereby facilitating ignition. The analysis reveals consistent trends and limiting conditions for facilitating effects compared to the TFI experiments. However, discrepancies persist between experiments and simulations, particularly the interaction of turbulence and heat loss to cold electrodes are not yet fully understood. This constitutes the basis and the motivation for the second funding period (FP2). In FP2, we aim to close this scientific gap by performing and analyzing turbulent direct numerical simulations (DNS) of IKE with heat losses. A comprehensive DNS database will be generated by performing parameter variations based on the results from FP1. The focus will be expanded to include DME-air mixtures with pronounced low-temperature chemistry and two-stage ignition characteristics in FP2, in addition to fuel-rich H2-air mixtures (FP1 and 2). Based on our previous work we expect that stretch effects during IKE of DME-air mixtures lead to a complex interaction of cool, warm, and hot flames. The insights from DNS will be leveraged for flamelet modeling. The model from FP1 will be extended and validated for turbulent IKE in H2-air and DME-air mixtures. In summary, FP2 aims at a detailed understanding of IKE in turbulent mixtures with heat losses and the development of advanced flamelet models. Both the insights and the models are key for the development of the next generation of combustion systems for renewable fuels.

• DFG Programme: Research Grants