Effects of preferential diffusion on the burning intensity of curved flames

Abstract

Using the Bunsen flame as a model curved flame, the coupled influence of preferential diffusion and flame stretch on the burning intensity of lean and rich mixtures of methane, propane, butane, ethylene, and hydrogen with air has been experimentally studied. The results substantiate theoretical predictions and quantify previous experimental observations that, for mixtures whose effective Lewis numbers (Le) are less than unity, the flame temperature is less than the adiabatic flame temperature. This temperature also decreases towards the flame tip, which has the largest curvature and therefore may locally extinguish. The dominance of diffusional transport in influencing the intensity of curved flames is demonstrated by showing that the tip opening of the highly diffusive hydrogen/air flames occurs at constant hydrogen equivalence ratios of about 1.15 to 1.20, being almost independent of the flow intensity and uniformity.