The plasma-assisted ignition and stabilisation of premixed methane-air flames in moderate to high-velocity bulk flows is investigated using direct numerical simulation. The approach accounts for the primary thermal and kinetic effects of non-equilibrium plasmas sustained by nanosecond repetitively pulsed discharges. The numerical framework is firstly validated against several experimental and numerical test cases from the literature. It is then applied to study the spatiotemporal evolution of the flame kernel ignition and growth in a flowing methane-air mixture with an equivalence ratio of 0.9. For this purpose, a parametric study is performed based on a custom test case at ambient conditions using discrete discharge energies of 2.8 and 3.2 mJ per pulse, pulse repetition frequencies in the range of 20 to 50 kHz, and bulk flow velocities between 0 and 100 m/s. It is shown that the inter-pulse coupling, governed by the relationship between the bulk flow velocity and the pulse repetition frequency, plays a crucial role in the mixture ignition process. Moreover, for a given discharge energy per pulse, a critical discharge kernel convection distance can be determined to distinguish between three different operating regimes: (i) no ignition, (ii) unstable operation, characterised by an oscillating behaviour with periodic flame kernel extinction and re-ignition, and (iii) a stabilised flame. Simulations in three-dimensional space are performed to support the findings and further analyse the dynamics and interaction of consecutive discharge kernels. Finally, the possibility of extending the flame stabilisation limit by means of mixture preheating is demonstrated.
Fredrich et al. (Thu,) studied this question.
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