Stability Limits and Critical Size of Structures in Premixed Flames

Abstract An apparatus for the study of the instabilities of a premixed flame in a laminar flow is described. In this burner, the gas flow is laminar, uniform, and homogeneous. The flame front is 9 cm in diameter and, below the instability threshold, is flat to within 1%. This work concerns a propane/air mixture diluted with nitrogen. The flame speeds are measured by laser anemometry close to the front and range from the extinction limit (6 cm/s) to the autoturbulent regime (∼11 cm/s). The dilution ranges from 12% to 16% and the equivalence ratio from 0.8 to 1.2. The isodilution curves and the stability limits are presented and compared with theoretical predictions. The critical size of the cellular structures is obtained from an appropriate analysis of photographic images of the flame front. The experimental results agree fairly well with the relation: Λ c = π U fc 2 / ( 1 − γ ) g where Λ c is the critical wavelength, U fc is the flame velocity, γ is the thermal expansion ratio, and g is the acceleration of gravity.