Instabilities of propagating quasi-2D gaseous flames and chemical fronts in narrow channels

In order to examine systematically the effects of buoyancy and thermal expansion on propagating fronts, two types of fronts, one having small thermal expansion and one having large thermal expansion, were studied in two-dimensional Hele-Shaw cells (a narrow gap between two flat parallel plates). These cells were used because it is probably the simplest flow apparatus capable of exhibiting buoyancy effects. The small-expansion fronts employed were aqueous Autocatalytic Chemical Reaction fronts that produce propagating fronts, similar to premixed gas flame fronts. The aqueous systems have numerous advantages over gaseous fronts for obtaining fundamental data on the interaction between propagating fronts and buoyancy. These advantages include nearly constant density, nearly constant thermodynamic and transport properties, negligible influence of heat losses, and a high Schmidt number.; Only upward propagating fronts exhibited wrinkling, indicating that buoyancy effects induced wrinkling. A well-defined characteristic wrinkle wavelength was found that was practically independent of the cell thickness and the laminar propagation rate but inversely proportional to the square root of the cosine angle between gravity and the propagating front. The only mechanism consistent with these observations is the presence of an effective surface tension at the interface between product and reactant, which, according to the Saffman-Taylor analysis, will produce a wavelength selection mechanism for buoyantly unstable flows. Correlation between a buoyancy parameter and the wrinkled front speed matched the Yakhot model for the effect of turbulence intensity on turbulent flame speed reasonably well even with no adjustable parameters.; Experiments using gaseous flame fronts were also performed in order to compare with aqueous fronts to assess thermal expansion effects and heat losses. The gaseous flames were composed of CH₄ or C₃H ₈ as fuel, O₂ as oxidizer and N₂ or CO₂ as diluent. Wrinkled fronts were found for all orientations for the gaseous flames. Based on the Joulin-Sivashinsky model, some of the mechanisms that control wrinkling were identified, specifically thermal expansion, viscous fingering and diffusive-thermal effects, which apply only to gaseous fronts. The characteristics of small-scale wrinkling were similar for all orientations but varied with the Lewis number. The characteristics of large-scale wrinkling were buoyancy dominated.