AEROSOL DYNAMIC PROCESSES OF SOOT AGGREGATES IN A LAMINAR ETHENE DIFFUSION FLAME

Laser scattering/extinction tests on a coannular ethene diffusion flame were analyzed using cross sections for polydisperse aggregates. Using an improved experimental arrangement that allowed simultaneous measurement of light scattering at multiple angles, it was possible to determine the fractal dimension of the aggregates in the flame. The analysis also yields the mean-square radius of gyration, the aggregate number concentration, the average number of primary particles per aggregate, as well as the volume average of the volume-mean diameter as a function of height or residence time along the particle path of maximum soot concentration in this flame. These results lead to the conclusion that soot aerosol dynamic processes in the laminar ethene flame are partitioned into four regions. Low in the diffusion flame there is a region of particle inception that establishes the number of primary particles per unit volume that remains constant along a prescribed soot pathline. In the second region, there is sustained particle growth through the combined action of cluster-cluster aggregation (CCA) accompanied by heterogeneous reactions contributing to monomer-cluster growth. Oxidation processes occur in the third region where CCA continues. If aggregate burnout is not complete in the oxidation region, then smoke is released to the surroundings in the fourth region where reactions cease but clusters continue to grow by CCA. The experiments yield the CCA growth rate within the flame which compares favorably with the theoretical value. The similarities and differences between this data reduction and the traditional analysis based on the use of cross sections for Rayleigh spheres and Mie theory spheres is discussed.