EXTINCTION OF NEAR-LIMIT PREMIXED FLAMES IN MICROGRAVITY

摘要

The extinction characteristics of near-limit premixed flames were studied both experimentally and numerically under microgravity conditions in the opposed-jet counterflow configuration. The study assessed the synergistic effects of heat loss and strain rate on flame extinction and their implications on the existence of fundamental flammability limits. An experimental methodology was established appropriate for extinction studies of near-limit, weakly strained, premixed flames in microgravity. A novel approach was developed for the accurate determination of the mixture composition and the determination of extinction conditions. Experiments were conducted for CH_4/air and C_3H_8/air mixtures, in order to assess Le number effects. Global extinction strain rates were determined for large nozzle separation distances, in order to minimize upstream heat losses. The numerical simulations of the experiments were conducted along the stagnation streamline and included the use of detailed description of chemical kinetics, molecular transport, and different models of thermal radiation. The original one-dimensional PREMIX and stagnation flow codes were modified to allow for one-point continuation, so that turning point, extinction behavior could be calculated. The numerical results showed that for Le < 1, near-limit mixtures exhibit a C-shape extinction response, while mixtures with Le > 1 exhibit a monotonic response, in agreement with previous studies. It was also found that as the nozzle separation distance is reduced, the extinction behavior is complicated by upstream heat losses, and the extinction diagram is of no fundamental value. Furthermore, when radiative heat reabsorption is included in the simulations, the structure of the postflame region is modified, and the maximum flame temperature is noticeably affected. However, the effect of reabsorption on the numerically determined flammability limits of CH_4/air flames was found to be minor. The experimental data are also in good agreement with the simulations.