Evidence of soot superaggregates in a turbulent pool fire

We report experimental observations of extremely large, 10–100 μm, soot aggregates in a blended methanol/toluene fueled turbulent pool fire, which are believed to be the first observation of “superaggregates” in a turbulent flame. Laser-induced incandescence images of soot volume concentration, at the center of the fire plume and at a height within the active flaming region, reveal the appearance of large-scale particle-like features across a broad range of apparent volume fraction, which emit at an intensity that is comparable with that of the laser-heated soot particles. We argue that the features in the incandescence images result from very large soot aggregates. This observation is supported by scanning electron microscope imaging of extracted soot that reveals large soot structures composed of much smaller chains of individual primary particles. Analysis of the soot aggregate structure from the electron-microscope images reveals a 1.8 fractal dimension at micron scales, comparable with commonly reported soot aggregate sizes from hydrocarbon flames. At larger scales of 10s of microns, comparable with the total aggregate size, a larger volume-filling fractal dimension of 2.5–2.6 is observed. This type of fractal structure is consistent with reported, but apparently rare, observations of soot superaggregates in heavily sooting laboratory laminar diffusion flames, but is encountered in the much larger meter-scale pool fire at much lower soot volume concentrations.