Stability and Control of Lean Blowout in Chemical Kinetics-Controlled Combustion Systems

Abstract

This study is motivated by lean-blowout (LBO) detection and control in dry-low-emission (DLE) combustion systems. However, this analysis is confined to chemical kinetics-controlled combustion. Despite its simplicity, some useful insight may still be shed on near-LBO combustion systems, as the chemical reaction rates are rather low near LBO. A third-order linear well-stirred reactor (WSR) model is derived to examine a combustor's responses to small deviations from equilibrium points or small external disturbances. Numerical simulation of the normalized, nonlinear, unsteady WSR model is performed to examine a combustor's responses to large deviations from equilibrium points or large external disturbances. Eigenvalue analysis shows that, with decreasing equivalence ratio, two real negative eigenvalues will merge and bifurcate into a complex conjugate pair, and will finally cross the imaginary axis and move into the right-half-phase plane. Complex eigenvalues imply the existence of an oscillating mode for which the damping ratio is found to consistently decrease 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-extinction oscillating frequency is typically below 25 Hz, and decreases with a larger percentage of incomplete combustion. Comparisons between linear predictions and experiments, where appropriate, are made. Triggered instability is observed (i.e., a WSR may remain stable in the presence of small external disturbances, but will undergo a subcritical bifurcation to complete flame quenching if external disturbances exceed certain thresholds). A slight increase in equivalence ratio, a higher preheat temperature, less heat loss, and a smaller percentage of incomplete combustion are effective in strengthening a WSR's resistance to LBO. This paper numerically demonstrates that zero-mean, small-amplitude fuel modulations based on modern control strategies can be very useful to enhance lean combustion stability and mitigate the danger of LBO.