Effect of Fuel-Air Equivalence Ratio and Temperature on the Structure of Laminar Propane-Air Flames

Gas samples have been withdrawn from laminar atmospheric-pressure propane-air flat flames using an uncooled quartz probe with a 0.1 mm diameter orifice. The samples (60 torr total pressure) are analyzed by gas chromatography for all C11,-C44, hydrocarbons, CO1 C02, N2, and 02. The Quartz probe is movable vertically, allowing measurement of the concentrations of stable molecules as a function of height above the burner with vertical resolution better than 0.2 mm. During lean (Φ=0.6-0.9) operation, intermediate hydrocarbon products (C2H2, C2H4, C3H6, CH4) are seen in the luminous zone at concentrations of 200-2000 ppm. All hydrocarbons decrease to below I ppm within 0.25 mm above the top of the luminous zone. When the flame is rich (Φ= 1.2-1.4), a peak in the intermediate hydrocarbons is again seen in the luminous zone, but the decay rates of C2H2 and CH4 in the post-luminous zone are much slower. These species are present at concentrations of 100-1000 ppm several mm beyond the luminous zone for Φ>1.3. Their decay rates are critically dependent on both Φ and the temperature of the flame, which is controlled by the cold gas flow rate. These results are important in gaining a better understanding of pollutant generation in premixed flames such as those found in internal combustion engines.