Effects of Chemical Kinetics and Heat Loss on Near-LBO Combustion Dynamics - Stability Analysis

Abstract

*† Both the linearized and the nonlinear WSR (well-stirred-reactor) models are employed to examine the effects of chemical kinetics and heat loss on near-LBO (lean blowout) combustion dynamics. Please note that, in the present paper, the combustion dynamics simply refers to the temporal evolution of pressure or the heat release rate (quantified by OH* chemiluminescence), instead of the combustion instability. This study is mainly motivated by LBO prediction and control in DLE combustion systems. Eigenvalues analysis of the linearized third-order WSR models shows that, with decreasing equivalence ratios, a complex conjugate pair of eigenvalues emerges from three negative real ones, moves left towards the right half phase plane, and finally crosses the imaginary axis. A pair of complex eigenvalues corresponds to an oscillating mode, whose damping ratio consistently decreases at the approach of LBO. A lower preheat temperature, a higher percentage of incomplete combustion, and more heat loss exacerbate near-LBO combustion stability. The predicted near-LBO oscillating frequency is typically below 25 Hz, and decreases with incomplete combustion and increases with heat loss. Model predictions qualitatively and even quantitatively agree with the experiments. Numerical simulation of the normalized, nonlinear, unsteady WSR models is performed to examine the combustor’s responses to large external disturbances. “Triggered instability” is observed, i.e. the combustor may remain stable in the presence of small external disturbances, but may undergo subcritical bifurcations to flame quenching when the external disturbances exceed certain thresholds. A higher equivalence ratio, a higher preheat temperature, less heat loss, and a smaller percentage of incomplete combustion are very effective in strengthening the flame’s robustness to external disturbances. It is numerically demonstrated that that zero-mean small-amplitude fuel modulations, based on the modern feedback control principles, can be very useful in enhancing the lean combustion stability without exacerbating the overall emissions.