Acoustic scattering of a sequential combustor controlled with non-equilibrium plasma: A numerical study

Abstract

This study aims to investigate the impact of Nanosecond Repetitively Pulsed Discharges (NRPDs) on the acoustic scattering properties of the second stage of a Constant Pressure Sequential Combustor (CPSC). Despite the proven capability of NRPDs to stabilize such systems, a comprehensive understanding of the NRPDs-flame-acoustic interaction is lacking. To address this knowledge gap, Large Eddy Simulations (LESs) with state-of-the-art plasma modeling are combined with methods from system identification to characterize the system’s acoustic response both in the absence and presence of NRPDs. The results demonstrate that NRPDs initiate reacting kernels upstream of the second stage combustion chamber, which interact with the acoustic field and with the main flame brush, thereby significantly impacting the scattering matrix coefficients. An analysis of the system’s acoustic power amplification characteristics in absence of NRPDs underscores the system’s capability to amplify the incident acoustic power between 300 and 450 Hz, up to a maximum of +160%. This highlights the potential of the second stage to drive system destabilization. In contrast, with NRPDs, the system’s response is more balanced, with maximum amplification factor consistently below +25% across the entire spectrum. To shed light on this behavior, the relation between heat release rate and pressure fluctuations is examined at 327 Hz. As opposed to the main flame brush, plasma-induced kernels generate heat release fluctuations that are out-of-phase with the pressure fluctuations. Hence, NRPDs induce a drastic reduction in the component of the sequential stage heat release fluctuations that is coherent with the acoustic field and participates to the acoustic power amplification, thanks to the change in the flame morphology they induce.