Effects of unsteady strain rate on the response of methane/air diffusion fl
ames are studied numerically. The numerical simulations are carried out for
the finite-domain opposed flow configuration in which the nozzle exit velo
cities are prescribed as a function of time. The chemical kinetics is compu
ted with the GRI mechanism v2.11 including NO, in methane/air combustion. T
he response of individual species to monochromatic oscillation in strain ra
te with various frequencies reveals that the fluctuation of slow species, s
uch as CO and NO,, is quickly suppressed as the flow time scale decreases.
Furthermore, it is observed that the maximum CO concentration is very insen
sitive to the variation in the scalar dissipation rate. It is also demonstr
ated 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 str
ain rates is simulated by an impulsive velocity with various frequencies. F
or a fast impulse, a substantial overshoot in NO, concentration is observed
immediately after extinction. The overall fuel burning rate shows a weak r
esponse to the variation in characteristic unsteady time scale, while the e
mission indices for NO, show a monotonic decay in response as the impulse f
requency increases. (C) 1999 by The Combustion Institute.