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 (N-2-diluted) to large (CO2-diluted), are investigated. Each case covers a range of the equivalence ratio (0) with turbulent intensities (mu'/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(mu'/S-L)(2) Re-T(-0.5) = 0.157Ka, is used to quantify global quenching boundaries of these turbulent flames, where Re-T and Ka are the turbulent Reynolds and Karlovitz numbers, respectively. For pure CH4/air flames, the critical value of K-c for global quenching of rich/lean CH4 flames must be greater than 1.0/6.2. Values of K-c are very sensitive to phi, because K-c increases significantly as phi gradually approaches I from either lean or rich sides, with the maximum K-c occurring possibly near phi = 1. By comparing N-2- and CO2-diluted flames of the same SL, it is found that global quenching of lean/rich CH4 flames is/is not influenced by the radiatvie beat loss, respectively. The larger the radiative heat loss, the smaller the value of K, for lean CH4 flames, in which values of K, decrease from 4 (N-2-diluted) to 3 (CO2-diluted) where S-L approximate to 10 cm/s and phi = 0.62. On the other hand, K-c approximate to 1.3 for both N-2- and CO2-diluted rich CH4 flames where S-L approximate to 10 cm/s and phi = 1.20-1.45. These experimental results are important to the understanding of global quenching processes for turbulent premixed combustion.