Chemical response of methane air diffusion flames to unsteady strain rate

Effects of unsteady strain rate on the response of methane/air diffusion flames are studied numerically. The numerical simulations are carried out for the finite-domain opposed flow configuration in which the nozzle exit velocities are prescribed as a function of time. The chemical kinetics is computed with the GRI mechanism v2.11 including NO, in methane/air combustion. The response of individual species to monochromatic oscillation in strain rate with various frequencies reveals that the fluctuation of slow species, such as CO and NO,, is quickly suppressed as the flow time scale decreases. Furthermore, it is observed that the maximum CO concentration is very insensitive to the variation in the scalar dissipation rate. It is also demonstrated that, for high-frequency oscillations, the scalar dissipation rate is a more appropriate parameter than strain rate to characterize the unsteady flame behavior. An extinction event due to an abrupt imposition of high strain rates is simulated by an impulsive velocity with various frequencies. For a fast impulse, a substantial overshoot in NO, concentration is observed immediately after extinction. The overall fuel burning rate shows a weak response to the variation in characteristic unsteady time scale, while the emission indices for NO, show a monotonic decay in response as the impulse frequency increases. (C) 1999 by The Combustion Institute.