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Original Articles

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

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Pages 226-244 | Received 07 Apr 2008, Accepted 15 Aug 2008, Published online: 19 Feb 2009
 

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.

This work is supported by U.S. Department of Navy under grants N00014-02-1-0756 and N00014-02-1-0837, and is also supported by NASA under grants RF 970493 and 745431.

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