Liftoff and extinction characteristics of fuel- and air-stream-diluted methane-air flames

Partial premixing of fuel and oxidizer is of common occurrence in fires. However, most previous studies dealing with flame extinction have focused on nonpremixed flames. In this experimental-numerical study, we examine the effectiveness of fuel-stream versus air-stream dilution for extinguishing laminar methane-air partially premixed (PPFs) and nonpremixed flames (NPF) using the chemically inert fire suppressant CO₂. Experimental measurements were made in lifted methane-air coflow flames, while both counterflow and coflow flames were simulated using a time-accurate implicit algorithm that incorporates detailed chemistry and includes radiation effects. Both measurements and simulations show that with fuel-stream dilution, PPFs stabilize at a higher liftoff height and blow out at a lower CO₂ dilution than NPFs. In contrast, with air-stream dilution, NPFs move to a higher liftoff height and blow out at a lower CO₂ dilution than PPFs. Despite different configurations, there is remarkable similarity in the extinction characteristics of coflow and counterflow flames with regard to the level of partial premixing and air- and fuel-stream dilution. The critical fuel-stream CO₂ mole fraction required for the extinction of both counterflow and coflow flames increases as ? is increased, i.e., as the level of partial premixing is reduced. Conversely, the critical air-stream CO₂ mole fraction decreases as ? is increased. Results also indicate a crossover value of ?≈2.0, corresponding to the stoichiometric mixture fraction of fs=0.5, such that flames (including NPFs) with fs<0.5 are more difficult to extinguish with fuel-stream dilution, since oxygen is the deficient reactant, whereas flames with fs>0.5 are more difficult to extinguish with air-stream dilution, since fuel is the deficient reactant for these flames.