SOOT VOLUME FRACTION AND TEMPERATURE-MEASUREMENTS IN LAMINAR NONPREMIXED FLAMES USING THERMOCOUPLES

Thermocouple particle densitometry (TPD), a new method for measuring a bsolute soot volume fraction in flames which was suggested by Eisner a nd Rosner, has been successfully implemented in several laminar nonpre mixed flames. This diagnostic relies on measuring the junction tempera ture history of a thermocouple rapidly inserted into a soot-containing flame region, then optimizing the fit between this history and one ca lculated from the principles of thermophoretic mass transfer. The TPD method is very simple to implement experimentally, yields spatially re solved volume fractions directly, can easily measure small volume frac tions, and does not depend on the prevailing soot particle size, morph ology, or optical characteristics. In a series of methane and ethylene counterflow flames whose soot volume fractions varied by more than an order of magnitude, the TPD results agreed to within experimental err or with our own laser extinction measurements. In axisymmetric methane and ethylene co-flowing flames, the shape of TPD profiles agreed well with published laser extinction measurements, but the TPD concentrati ons were significantly larger in the early regions of the ethylene fla me and throughout the methane flame; these discrepancies are probably attributable to visible light-transparent particles that are detectabl e with TPD but not with laser extinction. The TPD method is not applic able to the upper regions of these co-flowing flames since OH concentr ations there suffice to rapidly oxidize any soot particles that deposi t. Gas temperatures were obtained simultaneously with volume fraction by averaging the junction temperature history shortly after insertion. The error in these temperatures due to soot deposition-imposed change s in the junction diameter and emissivity were assessed and found to b e moderate, e.g., less than 60 K near the centerline of the ethylene c oflowing flame where the volume fraction was 6 ppm and the gas tempera ture was 1550 K. (C) 1997 by The Combustion Institute.