Large eddy simulation of plasma assisted ignition: Effects of pulse repetition frequency, number of pulses, and pulse energy

The impacts of the pulse repetition frequency (PRF), number of pulses, and energy per pulse in a train of nanosecond discharge pulses on the ignition of a flowing lean premixed methane–air mixture are investigated using numerical simulations. A phenomenological plasma model coupled with a compressible reacting flow solver is used for these simulations. The simulation strategy has been well validated by comparing the experimental schlieren and OH planar laser induced fluorescence (PLIF) results with the numerical schlieren (i.e., density gradient) and OH density profiles, respectively. The characteristics of the ignition kernels produced by each discharge pulse and their interaction with each other as functions of the PRF are investigated. Three regimes were defined in the literature based on this interaction of the ignition kernels — fully coupled, partially coupled, and decoupled. This study uses numerical simulations to probe into the constructive and destructive effects, that ultimately determine ignition success, in these different regimes. The complete overlap of kernels and the complete lack of synergy between kernels produced by consecutive pulses are attributed to the success and failure of ignition and flame propagation in the fully coupled and decoupled regimes, respectively. In the partially coupled regime, the convection heat loss driven by the shock-turned-acoustic wave of the next discharge pulse, on the kernel produced by the previous discharge pulse, in addition to diffusion losses, contribute to ignition failure. However, the expansion of the next kernel in a region of higher average temperature and radical concentration created by the previous kernel could help to bridge the gap between the two kernels and result in successful ignition. The important parameters of energy per pulse, number of pulses, and equivalence ratio affect the competition between these constructive and destructive effects, which eventually determines the ignition success in this regime. Finally, the change in the nature of interaction between consecutive kernels from decoupled to partially coupled, at the same frequency but with different energies per pulse, is also shown. Novelty and significance statement This study presents large eddy simulation (LES)-based results on the impact of the pulse repetition frequency (PRF), number of pulses, and energy per pulse, on the success of plasma assisted ignition of a flowing lean premixed methane–air mixture. This is the first simulation work to show direct validation based on both schlieren and OH density from the experiments of Lefkowitz et al. (2021). This is also the first work which identifies and explains the constructive and destructive effects to explain the reduced ignition probability in the partially coupled regime observed in Lefkowitz and Ombrello (2017). The role of the shock-turned-acoustic wave produced by every subsequent discharge pulse, on the previous kernel in a pulse train (destructive); and the assistance provided by the previous kernel to the next kernel (constructive), has been shown quantitatively. The use of a compressible solver is imperative to identify this destructive effect. The change in the regime boundaries defined by the PRF, by changing the energy deposition, is also shown.