Effects of the Lewis number and radiative heat loss on the bifurcation andextinction of CH4/O-2-N-2-He flames

Effects of the Lewis number and radiative heat loss on flame bifurcations a nd extinction of CH4/O-2-N-2-He flames are investigated numerically with de tailed chemistry. Attention is paid to the interaction between radiation he at loss and the Lewis number effect, The Planck mean absorption coefficient s of CO, CO2, and H2O are calculated using the statistical narrow-band mode l and compared with the data given by Tien, The use of Tien's Planck mean a bsorption coefficients overpredicts radiative heat loss by nearly 30 % in a counterflow configuration. The new Planck mean absorption coefficients are then used to calculate the extinction limits of the planar propagating fla me and the counterflow flame when the Lewis number changes from 0.967 to 1. 8, The interaction between radiation heat loss and the Lewis number effect greatly enriches the phenomenon of flame bifurcation. The existence of mult iple flames is shown to be a physically intrinsic phenomenon of radiating c ounterflow flames. Eight kinds of typical patterns of flame bifurcation are identified. The competition between radiation heat loss and the Lewis numb er effect results in two distinct phenomena, depending on if the Lewis numb er is greater or less than a critical value. Comparisons between the standa rd limits of the unstrained flames and the flammability limits of the count erflow flames indicate that the flammability limit of the counterflow flame is lower than the standard limit when the Lewis number is less than the cr itical value and is equal to the standard limit when the Lewis number is hi gher than this critical value. Finally, a G-shaped curve and a K-shaped cur ve which respectively represent the flammable regions of the multiple flame s for Lewis numbers lower and higher than the critical value are obtained. The G- and K-shaped curves show a clear relationship between the stretched counterflow flame and the unstrained planar flame. The present results prov ide a good explanation of the physics revealed experimentally in microgravi ty.