MEAN SOOT VOLUME FRACTIONS IN TURBULENT HYDROCARBON FLAMES: A COMPARISON OF SAMPLING AND LASER MEASUREMENTS

ABSTRACT

Thermophoretic sampling (TS) followed by transmission electron microscope (TEM) image analysis has recently been developed to obtain local particle volume fraction in addition to size distribution and fractal morphology in a laminar flame. This intrusive, visual TEM observations have the key advantage over laser-based diagnostics of direct characterization of particulates without requiring complicated interpretations and optical properties. This novel experimental technique was implemented here to measure mean soot volume fractions for the first time within two turbulent nonpremixed flames burning ethylene and acetylene in air. Laser extinction (LE) measurements were also conducted at the same axial and radial flame locations and compared to the independent TS/TEM data for assessing the uncertainties associated with the experimental determinations of soot volume fractions. After the initial soot inception regions of both flames, where soot volume fractions varied more than an order of magnitude, the TS and LE results agreed to within the combined experimental uncertainties. This consistency not only demonstrated the ability of the TS/TEM method to yield spatially resolved soot volume fractions but also favored the complex refractive index used in the present LE analysis (1.62 + 0.66i) for mature soot aggregates existing at upper flame regions. However, the soot volume fractions from TS/TEM were about a factor of 2 to 3 higher than those from LE in the lower portions of mainly the ethylene flame. This significant difference was evidently due to the presence of young soot particles that are essentially transparent to the visible light. The present experiments which capture carbonization of these precursor particles in turbulent flames, complemented similar past observations in laminar flames.

Keywords:

• soot diagnostics
• turbulent flames
• thermophoretic sampling