Time Series and Spectral Analysis of Thermoacoustic Oscillations for Propane-Oxyfuel Combustion in a...

In this study, the time series data for sound pressure and heat release fluctuations were analyzed to establish the stability window of the propane-oxyflames under different operating conditions. The CO₂ dilution level and the combustor power density were varied at a fixed global equivalence ratio to investigate the thermoacoustic instability of the flames. The phase difference between the fluctuations and the instantaneous Rayleigh index reveals whether the heat release and pressure fluctuations are coupled, which can result in the amplification of the sound pressure fluctuations. Results showed a negative Rayleigh index and uncoupled fluctuations at low (<40%) and high (>60%) CO₂ dilution with coupled fluctuations and sound pressure amplification at intermediate CO₂ dilution levels. A peak of varying magnitude appeared in the frequency domain at 465 Hz for both the heat release and pressure fluctuations in the coupled mode. The Strouhal number at different CO₂ concentrations revealed a range of vortex-shedding frequencies (300-1000 Hz), suggesting that the coupled mode is vortex-induced. Phase space reconstruction for the sound pressure fluctuations was carried out and it is observed that although the pressure fluctuations are amplified in the coupled mode, limit cycle amplitudes have not been reached. The recorded coupling and uncoupling of the oscillations associated with flame-vortex interactions at certain CO₂ concentrations provides a valuable insight on the combustor's dynamics and toward the development of nonpremixed-oxy-flames combustors.