Effect of buoyancy on the radiative extinction limit of low-strain-rate nonpremixed methane-air flames

摘要

The structure and extinction of nonpremixed flames were investigated through comparison of experiments and calculations using a counterflow configuration. Experiments were conducted at the NASA Glenn Research Center's 2.2-s drop tower to attain suppression and temperature measurements in low-strain nonpremixed methane-air microgravity flames. Suppression measurements using nitrogen added to the fuel stream were performed for global strain rates from 7 to 50 s~(-1). Judicious hardware selection and an optimized experimental procedure facilitated rapid, controllable, and repeatable flame extinction measurements. The minimum nitrogen volume fraction in the fuel stream needed to ensure suppression for all strain rates in microgravity was measured to be 0.855 ± 0.016, associated with the turning point, which occurred at a global strain rate of 15 s~(-1) . This value was higher than the analogous value in normal gravity. Flame temperature measurements were attained in the high-temperature region of the flame (T > 1200 K) using visible emission from a SiC filament positioned axially along the burner center-line. The suppression and temperature measurements were used to validate a two-dimensional flame simulation developed here, which included buoyancy effects and finite-rate kinetics. The simulations yielded insight into the differences between microgravity and normal gravity suppression results and also explained the inadequacy of the one-dimensional model results to explain the microgravity suppression results.