Effects of preferential diffusion on the burning intensity of curved flames

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 have been experimentally studied. Their results substantiate theoretical predictions and quantify previous experimental observations that, for mixtures whose effective Lewis numbers (Le) are less than unity, the flame temperature (T_f) is less than the adiabatic flame temperature (T_ad). This temperature also decreases towards the flame tip, which has the largest curvature and therefore may locally extinguish. For mixtures with Le>1, T_f appear to be greater than T_ad and the burning intensity increases towards the flame tip. For the Le ≈ 1 mixtures the behavior is somewhat neutral. The dominance of diffusional transport in influencing the intensity of curved flames is further 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.