Low Fractal Dimension Cluster-Dilute Soot Aggregates from a Premixed Flame

Using a novel morphology segregation technique, we observed minority populations ($\approx 3\%$) of submicron-sized, cluster-dilute fractal-like aggregates, formed in the soot-formation window (fuel-to-air equivalence ratio of 2.0–3.5) of a premixed flame, to have mass fractal dimensions between 1.2 and 1.51. Our observations disagree with previous observations of a universal mass fractal dimension of $\approx 1.8$ for fractal-like aerosol aggregates formed in the dilute-limit via three-dimensional diffusion-limited cluster aggregation processes. A hypothesis is presented to explain this observation. Subject to verification of this hypothesis, it may be possible to control the fractal dimension and associated properties of aggregates in the cluster-dilute limit through application of a static electric field during the aggregation process.

Figure 1

Schematic diagram of the experimental setup used for soot aggregate generation and characterization.

Figure 2

Mass fractal dimension $D_f$ of morphology segregated, $D_m=460\mathrm{nm}$ soot aggregates generated with fuel-to-air equivalence ratios $\phi$ of (a) $\phi =2.3$, (b) $\phi =2.8$, and (c) $\phi =3.5$.

Figure 3

(a) A commonly observed soot particle with $D_f\approx 1.8$ formed by 3D DLCA (not from this study), (b) a typical soot particle observed with $D_f=1.2$ ($\phi =3.5$) in this study, and (c) soot particles with $D_f=1.51$ ($\phi =2.3$). These aggregates look elongated and “stringy” in comparison to the more common aggregate in (a). (d) At high $\phi \approx 5.0$ soot particles in our premixed flame lose their fractal nature and become near-spherical particles.