Experimental Study of Particle Aggregation in a Humid and Turbulent Environment

Abstract:

Buoyant plumes generated through volcanic eruptions transport fine ash into the upper atmosphere where it can find its way hundreds of kilometers from the vent before settling to the ground. Fine ash can have detrimental effects on air and ground transportation, urban infrastructure, and respiratory health so it is important to predict the extent of its dispersion and implement mitigation strategies accordingly. Numerous well-documented eruptions have shown that much of the fine ash falls prematurely in the form of aggregates. Although the exact detail of the aggregation process can only be inferred from the deposits, a number of factors may lead to enhanced aggregate growth, including (but not limited to) initial polydispersity of ash, relative humidity, and turbulence. We have developed a laboratory-scale experiment to probe these three effects on aggregate growth rate. The relevant parameters can be controlled within the range expected in the buoyant region of a typical plume. The nominal characteristics of the gas phase include a Taylor Reynolds number of 200, a dissipation rate of 7.2 m²/s³, a Kolmogorov length scale of 148 µm, and relative humidity of 50-100%. Experiments have been conducted with various particles having Stokes number of 4-12 based on the Kolmogorov time scale and particle volume fractions of 5x10^-6-5x10^-5. Particle-pair statistics have been measured using stereoscopic particle tracking velocimetry (SPTV), which can be used to estimate particle collision rates. In addition, the aggregate size distribution exiting the turbulence section is measured for a range of humidity levels using an aggregate sampling device. The results will be used to provide constraints for collision rates and sticking efficiencies found in ash forecast models.