Diffusive-thermal instability of counterflow flames at low Lewis number

The effects of hydrodynamic strain on diffusivethermal instabilities at low Lewis number were studied experimentally using a counterflow slot-jet apparatus with H2-O2-N2 mixtures. Three configurations were examined: single premixed, twin premixed and nonpremixed flames. For all three configurations a wide variety of nonplanar flame structures were observed. Two extinction limits, one at very high strain (corresponding to a residence time limitation) and one at low strain (corresponding to a heat loss extinction) were found. For premixed flames, nonplanar structures occurred for a wide range of mixtures. In many cases, near extinction the flame structures reduced to a single "flame tube" elongated in the direction of extensional strain. An examination of stoichiometry effects revealed a critical equivalence ratio for instability similar to that known to exist for flame balls. At low flow velocities a transition to flames attached to the jet exits in complicated ways was noted, and at high flow velocities transition to turbulent flow was noted. For nonpremixed flames, nonplanar flame structures were observed only for near-extinction conditions, but the resulting flame shapes were quite similar to those of premixed flames. These results are compared to recent theoretical predictions.