Cellular instability phenomenon in premixed tubular flames and non-premixed opposed tubular flames

Abstract

Instability phenomenon (cellular structure or multi-petal structure) and extinction behavior are studied in premixed tubular flames and non-premixed opposed tubular flames. A newly designed tubular burner that is capable of operating under a wide range of conditions allows more accurate and detailed study of tubular premixed and non-premixed flames.

Extinction and cellular instabilities are investigated in premixed tubular flames. Extinction measurements of premixed tubular flames are carried out through the entire equivalence ratio spectrum for various types of fuels without introducing serious turbulent effects in the new burner. In this study, the extinction conditions of premixed tubular flames generated by H2/O2 or CH4/O2 mixed with various inert diluents (Ar, CO2, N2) and enriched oxygen are measured. In addition, the onset conditions for flame instability at different cell numbers are reported. Petal shaped cellular flames with symmetric regions of local extinction are observed for lean premixed H2 tubular flames with low Lewis numbers (ratio of the bulk thermal diffusivity to the mass diffusivity of the deficient reactant). Wrinkled flames with no local extinction are observed for nearly equidiffusive lean CH4 tubular flames. The combined effects of pure curvature, thermal-diffusive instability and hydrodynamics are thought to be responsible for the cellular structure. The lack of extinction in the nearly equidiffusive lean CH4 tubular flames indicates that strong thermal-diffusive effects due to non-unity Lewis number are required to produce local extinction in the cellular structure of tubular flames.

Similarly, an experimental study regarding the cellular instability of non-premixed opposed tubular flames was conducted on the same tubular burner with the addition of a central porous fuel nozzle. In this study, the fuel mixture was H2 diluted with CO2, N2, Ar and He; the oxidizer was air only. The cellular instability phenomenon (multi-petal structure, which is similar to that observed in the premixed tubular flame) appeared in this experimental set up; the instability transition points were also carefully recorded and illustrated. Two different experimental procedures were applied and the variation in cell-transition process were compared and analyzed. Further, the effect caused by various diluents in the fuel confirmed that the fuel Lewis number and Damköhler number (the ratio of mixing time to the chemical reaction time) are related to this phenomenon. An attempt to investigate the dependency of cell number and cell length on stretch rate and initial mixture strength was made but more detailed work is needed to fully clarify this dependence.