A phenomenological understanding of the multimodal low-frequency oscillating combustion of ammonia induced by filamentary plasma discharge

This work finds that with the assistance of filamentary plasma discharge, the ammonia combustion becomes very complicated with multiple modes, especially including a low-frequency (<2 Hz) oscillating combustion mode. This flame oscillation, which is sensitive to the discharge characteristics and the flow/fuel/boundary conditions, can be further categorized as three sub-modes denoted as Mode C1, C2 and C3. Their transitions occur depending on the flow rate (Q_tot), the equivalence ratio (ϕ) and the temperature of the chamber wall. Detailed dynamic characteristics of flame kernels and flow fields during the oscillating process are further acquired to confirm the changing recirculation zone and the Karlovitz (Ka) number. A phenomenological mechanism based on recirculated energy feedback is thus proposed to explain the oscillating flame. In addition, the S-curve is used to elucidate the plasma impacts on the complicated combustion characteristics, especially the oscillating combustion modes and their transitions. It is found that the folded S-curve can be stretched with the help of plasma discharge. By considering the energy feedback effects, the S-curve can become more stretched with the Da number for extinction (i.e. $D{a}_{E^{\prime }}$) being larger than that for ignition (i.e. $D{a}_{I^{\prime }}$). Meanwhile, the branch with the Da number between $D{a}_{I^{\prime }}$ and $D{a}_{E^{\prime }}$ is unstable and the multiple oscillating combustion modes (i.e. Mode C1-C3) can be categorized on the unstable branch of the stretched S-curve.