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'/SL) as much as 100, where SLis 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'/SL)2= 0.157Ka, is used to quantify global quenching boundaries of these turbulent flames, where ReTReT-0.5and 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 CH4flames 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 SL, it is found that global quenching of lean/rich CH4flames is/is not influenced by the radiatvie heat loss, respectively. The larger the radiative heat loss, the smaller the value of Kcfor lean CH4flames, in which values of Kcdecrease from 4 (N2-diluted) to 3 (CO2-diluted) where SL≈ 10 cm/s and φ = 0.62. On the other hand, Kc≈ 1.3 for both N2- and CO2-diluted rich CH4flames where SL≈ 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.
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