Dynamics, frequency and oscillation characteristics of near-blowout plasma stabilized flames in a swirled burner using high-speed chemiluminescence and spectral mode decomposition methods

This experimental study investigates the influence of sinusoidal plasma discharges on low-swirled methane/air flames, exploring conditions ranging from stable operation to near-lean blowout. A ring-needle plasma actuator operating at 20 kHz near the nozzle exit is used, and high-speed chemiluminescence diagnostics track the spatial and temporal flame dynamics. Plasma actuation significantly reduces the flame lift-off height by approximately 20 mm and decreases the variance in OH* chemiluminescence intensity by 30%, enhancing flame stability. Flame morphology changes are quantified using image processing techniques, including Zernike coefficients, Hu moments, curvature, perimeter, area, and centroid coordinates. These analyses provide a detailed description of flame shape variations under different operating conditions. Advanced decomposition techniques, such as Proper Orthogonal Decomposition (POD) and Spectral Proper Orthogonal Decomposition (SPOD), reveal dominant flame structures and frequency fluctuations. Phase Portrait POD and Autocorrelation SPOD Modes show a marked reduction in oscillatory amplitudes, highlighting the plasma's stabilizing effects. Plasma discharges redistribute energy from the dominant first mode to higher modes, reducing heat fluctuations and suppressing instabilities. This leads to a more uniform energy distribution across the flame, improving combustion efficiency. Acoustic measurements, analyzed through Fast Fourier Transform (FFT) and Wavelet Decomposition, correlate with imaging results, providing a comprehensive understanding of plasma's influence on both chemiluminescence and acoustic flame properties. These findings demonstrate that plasma effectively dampens high-frequency oscillations, redistributes energy, and enhances combustion stability, particularly near the lean blowout limit.