Global quenching of premixed CH₄/air flames: Effects of turbulent straining, equivalence ratio, and radiative heat loss

Global quenching of premixed CH4/air flames with turbulent straining, equivalence ratio, and radiative heat loss effects is explored in a cruciform burner. The burner equipped with a pair of counter-rotating high-speed fans and perforated plates provides downward propagating flames through near-isotropic intense turbulence, where flame-turbulence interactions are not influenced by ignition. Several CH₄/air flames with different degrees of radiative heat loss, from small (N₂-diluted) to large (CO₂-diluted), are investigated. Each case covers a range of the equivalence ratio (φ) with turbulent intensities (u'/S_L) as much as 100, where S_L is the laminar burning velocity, in which high rates of strain are achieved until, ultimately, global quenching of flames occurs. A Bradley's Karlovitz number, defined as K = 0.157(u'/S_L)² = 0.157Ka, is used to quantify global quenching boundaries of these turbulent flames, where Re_TRe_T^-0.5 and Ka are the turbulent Reynolds and Karlovitz numbers, respectively. For pure CH₄/air flames, the critical value of K for global quenching of rich/lean CH₄ flames must be greater than 1.0/6.2. Values of Kc are very sensitive to φ, because Kc increases significantly as φ gradually approaches 1 from either lean or rich sides, with the maximum Kc occurring possibly near φ = 1. By comparing N₂- and CO₂-diluted flames of the same S_L, it is found that global quenching of lean/rich CH₄ flames is/is not influenced by the radiatvie heat loss, respectively. The larger the radiative heat loss, the smaller the value of K_c for lean CH₄ flames, in which values of K_c decrease from 4 (N₂-diluted) to 3 (CO₂-diluted) where S_L ≈ 10 cm/s and φ = 0.62. On the other hand, K_c ≈ 1.3 for both N₂- and CO₂-diluted rich CH₄ flames where S_L ≈ 10 cm/s and φ = 1.20-1.45. These experimental results are important to the understanding of global quenching processes for turbulent premixed combustion.