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

摘要

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 CO_2. 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 CO_2 dilution than NPFs. In contrast, with air-stream dilution, NPFs move to a higher liftoff height and blow out at a lower CO_2 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 CO_2 mole fraction required for the extinction of both counterflow and coflow flames increases as φ is increased, i.e., as the level of partial premixing is reduced. Conversely, the critical air-stream CO_2 mole fraction decreases as φ is increased. Results also indicate a crossover value of φ ≈ 2.0, corresponding to the stoichiometric mixture fraction of f_s = 0.5, such that flames (including NPFs) with f_s < 0.5 are more difficult to extinguish with fuel-stream dilution, since oxygen is the deficient re-actant, whereas flames with f_s > 0.5 are more difficult to extinguish with air-stream dilution, since fuel is the deficient reactant for these flames.