Flame Radiation and Soot Emission From Partially Premixed Methane Counterflow Flames

Motivated by heat transfer and environmental concerns, a study of flame radiation and soot particulate emission is reported for partial premixing in low strain-rate $(<20s−1)$ methane counterflow flames. Temperature, $OH$ concentration, and soot volume fraction distributions were measured along the stagnation streamline for progressive addition of oxygen to methane. These measurements along with an optically thin model for soot and gas radiation were used to study the effect of partial premixing on flame radiation and soot emission. It was found that with progressive partial premixing, the peak soot loading and the thickness of the soot zone first decreased and then increased, and while the gas radiation was enhanced, the gas radiative fraction (gas radiation per unit chemical energy release) showed a systematic decrease. The net radiative fraction $(soot+gas)$⁠, however, first decreased and then increased. A configuration with the soot zone spatially entrapped between the premixed and non-premixed reaction zones was experimentally found. This flame configuration has the potential to enhance radiative heat transfer while simultaneously reducing soot and $NOx$ emissions.