SIMULTANEOUS MEASUREMENTS OF SOOT VOLUME FRACTION AND PARTICLE SIZE MICROSTRUCTURE IN FLAMES USING A THERMOPHORETIC SAMPLING TECHNIQUE/

A new particle volume fraction measurement technique was developed usi ng electron microscope analysis of thermophoretically sampled particle s/aggregates based on a theoretical treatment of particle deposition t o a cold surface immersed in a flame. This experimental method, referr ed to as the thermophoretic sampling particle diagnostic (TSPD), can y ield all particle parameters of principal interest (particle volume fr action, particle and aggregate sizes, and fractal properties) without requiring knowledge of particle bulk density and refractive index. To assess its reliability, the TSPD technique was implemented at various heights on the centerline of a soot-containing coflowing ethylene/air nonpremixed laminar flame. Inferred soot volume fractions agreed with previous laser extinction and thermocouple particle densitometry measu rements within experimental uncertainties at sampling positions where only aggregates of mature particles were present. However, TSPD-soot v olume fractions were about a factor of 3 higher than light extinction results in the lower part of the flame. This significant difference wa s evidently a result of the presence of translucent precursor soot par ticles, which do not absorb as much visible light as mature particles, but can be quantified with the electron microscope. Clearly, this abi lity of TSPD to separately measure the concentration and morphology of each type of soot is a significant advantage over other available dia gnostics, making it extremely valuable for studying particle formation in flames. (C) 1997 by The Combustion Institute.