Nonlinear Dynamic Analysis of the Transition from MILD Regime to Thermoacoustic Instability in a Reverse Flow Combustor

In the present study, the stability limit of the MILD regime of combustion has been studied in a reverse flow combustor using syngas (20% H-2, 20% CO, 60% N-2) as fuel. Pressure oscillations obtained from the combustor are analyzed using regular approaches from the dynamical systems theory as the oxidizer preheat temperature varies from 772 K to 424 K. Stable MILD combustion is obtained at preheat temperatures >= 724 K; at lower preheat temperatures combustion becomes unstable, leading to high amplitude periodic oscillations. Permutation spectrum test and recurrence plot of the MILD condition unravels the deterministic nature of the pressure oscillations. The correlation dimension confirms that the system has high-dimensional chaotic characteristics. At temperatures <724 K, the correlation dimension becomes close to 1 as the system transitions to combustion instability. In these conditions, the permutation spectrum test shows close to zero standard deviation in all ordinal indices, the phase-space shows doughnut-shaped structures, and recurrence plot consists of long diagonal lines, all confirming the periodic nature of the system. The dominating frequency during combustion instability is attributed to the second axisymmetric mode of the combustor. Finally, three recurrence quantification criteria, namely, Recurrence Rate, Determinism, and Laminarity, and K-value obtained from 0 to 1 test, are tested as indicators for the onset of combustion instability. Determinism and K-value are recommended for capturing the transition in the underlying dynamics.