Enhancement of hot and cool flame propagation in lean DME/air mixture by non-equilibrium plasma

Plasma-assisted ignition (PAI) offers a promising route to extend the lean flammability limits of dimethyl ether (DME), a clean alternative fuel hampered by low volumetric energy density. However, the multi-dimensional flame dynamics governing PAI-induced cool-to-hot flame transition in lean DME/air mixtures remain poorly understood due to the limitations of zero-dimensional (0-D) or one-dimensional (1-D) models. In this work, two-dimensional (2-D) direct numerical simulation (DNS) with adaptive mesh refinement (AMR) is performed to investigate spherical flame initiation in lean DME/air ( φ = 0.5, T 0 = 600 K, P 0 = 1 atm) under non-equilibrium plasma deposition. The results show that PAI promotes low-temperature chemistry (LTC) ahead of the main high-temperature chemistry (HTC) zone through electron-impact dissociation of DME, yielding additional CH 3 OCH 2 (R). This enhances the chain-propagating reaction R157: CH 2 OCH 2 OOH (R’OOH) = OH + 2CH 2 O, thereby establishing an OH barrier that suppresses OH back-diffusion into the LTC region and stabilizes hot flame propagation, whereas spark ignition (SI) at the same energy fails to sustain HTC and only supports cool flame development. Consequently, PAI achieves self-sustained hot flame with one-third lower discharge energy and 30.9% higher propagation speed compared to SI. At 0.2 mJ, both ignition modes exhibit similar cool flame speeds due to transient extinction and re-ignition events that cause the mixture to exhibit negative temperature coefficient (NTC) regime; however, PAI establishes a stable cool flame 1.4 ms earlier. With the relatively low discharge energy (i.e. 0.3 mJ), conventional cool flames are constrained by the NTC regime; however, plasma-induced cool flames overcome this constraint, achieving accelerated propagation via CH 2 O accumulation, which drives conditions favourable for subsequent OH production from H 2 O 2 decomposition R16: H 2 O 2 (+M) = 2OH (+M). This facilitates the acceleration of R40: CH 2 O + OH = HCO + H 2 O and triggers a centrally located hot flame, resulting in a dual-flame structure. These findings highlight the critical role of non-equilibrium plasma chemistry in modulating low-temperature pathways and enabling robust ignition under fuel-lean conditions.