STRUCTURE AND EXTINCTION OF METHANE-AIR FLAMELET WITH RADIATION AND DETAILED CHEMICAL KINETIC MECHANISM

The methane-air flamelet was studied with the effect of thermal radiat ion considered in this paper. The chemical reaction mechanism used is the latest detailed mechanism of GRI-Mech 2.11 which contains 49 speci es and 279 elementary reactions including C-2 chemistry of methane oxi dation and chemistry of thermal NO and prompt NO. The numerical result s show that flame radiation can induce another extinction limit at a l ow scalar dissipation rate (0.029 s(-1)), which is in addition to the well-known extinction limit caused by the overstretching of flame at a high scalar dissipation rate (18.4 s(-1)). The influence of thermal r adiation on flamelet temperature and NOx concentration is found to be significant enough in the small to moderate range of the scalar dissip ation rate to change the trend of the temperature and species mass fra ction profiles in the direction opposite to that when the thermal radi ation is ignored. NOx production is shown to be strongly dependent upo n the flamelet temperature. In the methane-air flamelet structure, it is found that the major species, H2O, can be well predicted by the equ ilibrium state relationship for the scalar dissipation rates between t he two extinction limits, but not CO2 or CO since they are affected no t only by the mixture fraction, but also by the scalar dissipation rat e. Also shown is that preheating the air side has a more profound effe ct on flamelet temperature than preheating the fuel side, and that rai sing either air or fuel temperature increases NOx and CO production. F inally, the effect of nonunity Lewis numbers of hydrogen (H-2 and H) o n the flamelet structure is examined and shown to increase CO2, but de crease the flamelet temperature and mass fractions of H2O, CO, and NOx . The Lewis number is also found to have an important effect on the ex tinction limits of the scalar dissipation rate for methane-air flames. (C) 1998 by The Combustion Institute.