Soot volume fraction and temperature measurements in laminar nonpremixed flames using thermocouples

Thermocouple particle densitometry (TPD), a new method for measuring absolute soot volume fraction in flames which was suggested by Eisner and Rosner, has been successfully implemented in several laminar nonpremixed flames. This diagnostic relies on measuring the junction temperature history of a thermocouple rapidly inserted into a soot-containing flame region, then optimizing the fit between this history and one calculated from the principles of thermophoretic mass transfer. The TPD method is very simple to implement experimentally, yields spatially resolved volume fractions directly, can easily measure small volume fractions, and does not depend on the prevailing soot particle size, morphology, 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 error 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 concentrations were significantly larger in the early regions of the ethylene flame and throughout the methane flame; these discrepancies are probably attributable to visible light-transparent particles that are detectable with TPD but not with laser extinction. The TPD method is not applicable to the upper regions of these co-flowing flames since OH concentrations there suffice to rapidly oxidize any soot particles that deposit. 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 changes in the junction diameter and emissivity were assessed and found to be moderate, e.g., less than 60 K near the centerline of the ethylene coflowing flame where the volume fraction was 6 ppm and the gas temperature was 1550 K. (C) 1997 by The Combustion Institute.