The structure of soot aggregates was investigated, emphasizing the fra
ctal properties as well as the relationships between the properties of
actual and projected soot images. This information was developed by c
onsidering numerically simulated soot aggregates based on cluster-clus
ter aggregation as well as measured soot aggregates based on thermopho
retic sampling and analysis by transmission electron microscopy (TEM)
of soot for a variety of fuels (acetylene, propylene, ethylene, and pr
opane) and both laminar and turbulent diffusion flame conditions. It w
as found that soot aggregate fractal properties are relatively indepen
dent of fuel type and flame condition, yielding a fractal dimension of
1.82 and a fractal prefactor of 8.5, with experimental uncertainties
(95% confidence) of 0.08 and 0.5, respectively. Relationships between
the actual and projected structure properties of soot, e.g., between t
he number of primary particles and the projected area and between the
radius of gyration of an aggregate and its projected image, also are r
elatively independent of fuel type and flame condition.