Role of pumping and wrinkle propagation mechanisms in exciting different acoustic-modes in turbulent syngas combustion

Abstract

The stability map of a non-premixed syngas turbulent combustor is obtained by varying its Reynolds number (Re). The map shifts from very high-amplitude, low-frequency instability at low Re to high-amplitude, high-frequency instability at higher Re. Significant distortion is observed in the former, as a result of self-excited higher harmonics. Simultaneous time-resolved OH∗ chemiluminescence reveals significant flame length variations for the condition of low-frequency instability, while the other condition displays serrated flame. The axial Rayleigh index phase is positive across the flame, with sinusoidal modulation for low-frequency instability. The high-frequency instability displays a sinusoidal trend in the axial Rayleigh index phase, implying alternating acoustic driving and damping regions, at a wavelength much smaller than that of the acoustic variables. The findings illustrate the presence of two different mechanisms for the excitation of different acoustic modes in the non-premixed syngas combustor— (1) a pumping mechanism imposed by the acoustic velocity, resulting in flame elongation and contraction at the condition of low-frequency instability and (2) wrinkle propagation at the local flow-convective velocity for the condition of high-frequency instability. The two mechanisms are distinguished based on the time-lag of the flame response. A modified two-parameter model is employed to allow contributions from individual mechanisms to the overall heat release rate fluctuations. The results of the two-parameter model match with the experimental observations, in determining the most unstable modes, and with the generation of harmonics in the low-frequency instability seen to be a result of wrinkle propagation, which is overwhelmed by the pumping mechanism.