Extinction of turbulent counterflow flames under periodic strain

Abstract

The effects of imposed oscillations on the extinction of turbulent nonpremixed and premixed counterflow flames have been quantified as a function of amplitude and frequency of the oscillation. Forced flame extinction was shown to depend on the total duration of pulsation and ranged from a few milliseconds to almost a second, depending on the amplitude and the frequency of the oscillation. Thus extinction times increased quasi-exponentially with decreasing amplitude and increasing frequency of oscillation for nonpremixed flames but were a nonmonotonic function of frequency in premixed flames, with the longest extinction time corresponding to higher frequencies as the flame tended to stoichiometric. Premixing of the fuel or the air stream increased the stability of forced flames, with extinction times reaching their maximum values for twin stoichiometric flames and decreasing towards leaner or richer mixtures. Velocity measurements revealed that the rms of the velocity fluctuations due to the imposed forcing was comparable to, or larger than, the bulk velocity so that oscillated flames were subjected to and survived instantaneous strain rates which were higher than those which caused extinction of the corresponding unforced flames. Visualization showed that the instantaneous strain rate was up four times that of the bulk flow for about half the oscillation period and promoted flame extinction during this part of the oscillation cycle. However, if the duration of the oscillation was smaller than a critical time scale, extinction did not occur, revealing that there was weakening of the flame over several cycles during which the temperature was reduced.