Numerical Investigations of Extinction/Reignition of Laminar Diffusion Flamelets under Oscillating Scalar Dissipation Rates

摘要

In this work, we perform numerical investigations of unsteady extinction/reignition of laminar diffusion flamelets subjected to oscillating scalar dissipation rates. The unsteady flamelet equations are solved in mixture fraction space with unity species Lewis numbers. The pressures and temperatures simulated are representative of those in practical combustors such as diesel chambers. N-heptane is chosen as the fuel, and its oxidation chemistry described by a kinetic mechanism incorporating 159 species among 1540 steps. A range of oscillation amplitudes and frequencies are considered. Results show that the flame response settles into an oscillatory extinction/reignition limit cycle for relatively low frequencies. At relatively high frequencies, the flame fails to respond to the imposed scalar dissipation oscillation due to reduced residence times. Moreover, the tendency for reignition decreases as the amplitude of the imposed oscillation increases. The unsteady responses of the flame temperature, major products CO{sub}2 and H{sub}2O, and the pollutants NO and unburned hydrocarbons (UHC) are investigated. These investigations reveal that while the temperature and major products show good recovery during reignition, NO shows poor recovery due to a significant phase lag resulting from relatively slow chemical response times. In addition, the results indicate that such benefits in flame stability and NO emission-control during extinction/reignition may be offset by an associated increase in UHC emissions. Conditions favorable for extinction followed by reignition are identified in terms of non-dimensional numbers based on the amplitude and frequency of the imposed perturbation.