Local Rayleigh index reconstruction: Application to plasma-assisted sequential combustion under varying pulse repetition frequency

This study investigates the influence of Nanosecond Repetitively Pulsed Discharges (NRPDs) on the acoustic response of the second stage of a Constant Pressure Sequential Combustor (CPSC) operating at atmospheric pressure. NRPDs are applied upstream of the second-stage combustion chamber to modify the autoignition process, thereby altering the combustor’s acoustic scattering properties. Large Eddy Simulations (LES) combined with System Identification (SI) methods are employed to better understand the NRPD-flame-acoustic interactions in the sequential flame across three different Plasma Repetition Frequencies (PRF), namely 20, 40, and 60 kHz. Results show that, while NRPDs always improve the overall acoustic scattering properties of the system compared to the combustion without NRPDs, the improvement is non-monotonic with respect to PRF. The most favorable acoustic characteristics are observed at $\text{PRF}=\text{20}$ kHz. Analysis of local Rayleigh index fields, reconstructed from broadband-forced simulation data, reveals that variations in PRF alter the physical mechanism by which plasma discharges influence system acoustics. Plasma-generated kernels can either directly induce heat release rate fluctuations and act as acoustic energy sources or sinks, or indirectly affect the system’s acoustics by interacting with the main flame brush and modifying its response. The ability to influence the interaction between autoignition kernels and acoustics by simply adjusting the PRF underscores the potential of NRPDs as a versatile tool for controlling the acoustic behavior of sequential combustors, enabling adaptation to the varying operational needs of real gas turbines.