Stabilization of a thermoacoustically unstable sequential combustor using non-equilibrium plasma: Large eddy simulation and experiments

Plasma-assisted combustion using Nanosecond Repetitively Pulsed Discharges (NRPDs) is an emerging technology that enhances the reactivity of fuel–air mixtures, offering significant improvements in operational and fuel flexibility—two crucial features for future sustainable gas turbines. The mechanisms that enable the stabilization of thermoacoustically unstable burners, however, remain unclear. Thus, to investigate the physical phenomena involved, we performed a massively parallel Large Eddy Simulation (LES) of the stabilization of a thermoacoustically unstable sequential combustor by NRPDs at atmospheric pressure. LES is combined with an accurate description of the combustion chemistry and a state-of-the-art phenomenological model for the non-equilibrium plasma effects. In this work, we have validated the simulation framework by comparison with experimental data including acoustic pressure and Heat Release Rate (HRR) signals in both stages of the sequential combustor, and OH-planar laser-induced fluorescence images in the second stage combustion chamber. Hence, this study provides a robust LES framework to study the effects of NRPDs on Thermoacoustic Instabilities (TIs). In addition, the analysis of the LES data reveals a significant decrease of the acoustic energy production in the sequential combustor thanks to the NRPDs. Surprisingly, the steady NRPD actuation generates HRR fluctuations upstream of the combustion chamber, which are in phase opposition to the acoustic pressure, inducing locally a sink term in the acoustic energy balance equation. Moreover, an analysis of the acoustic energy production during the onset of the TI reveals the predominant role of the second stage in developing and sustaining the self-excited TI. The effect of plasma is therefore very effective in stabilizing the system by reducing the acoustic energy production in the sequential stage.