THE ROLE OF HUMIDITY AND GRAIN-SIZE DISTRIBUTION IN VOLCANIC ASH AGGREGATION IN A PARTICLE-LADEN TURBULENT JET FLOW

Brandon Aspinwall

Volcanic ash transport and deposition (VATD) represents a major hazard associated with explosive volcanic eruptions, affecting everything from local infrastructure to global aviation systems and public health. Fine ash particles can remain airborne for extended periods and be transported over thousands of kilometers, posing risks far beyond the eruption source. One of the primary processes controlling this behavior is particle aggregation, where collisions between ash particles result in the formation of larger aggregates with higher settling velocities. The driving mechanisms behind this aggregation is turbulence, humidity, and electrostatic forces. However, despite its critical role, aggregation is still not well constrained and is frequently simplified or omitted in many volcanic ash transport and dispersion models (VATDMs), reducing their ability to accurately predict ash transport and fallout.This study focuses on the role of humidity on particle aggregation, applying humidification conditions to a particle-laden compressed-air jet. Utilizing multiple particle types (hollow glass spheres, nickel spheres, and volcanic ash particles), tests are designed to identify changes in the particle behavior throughout the process of moving through the jet and resulting fallout. Maintaining turbulent conditions, the particles are tested using parameters to help isolate the effects of humidity, such as controlling mass loading ratios and Reynolds numbers. Humidity is applied to the ambient air within the workspace, allowing for moisture to interact with particles as they left the jet exit. Humidity is also entrained into the jet itself, allowing particles to interact with moisture before exiting the jet. Particle image velocimetry is implemented to identify and quantify the particle responses to the flow of the air within the jet plume. These images are then processed to characterize local velocity profiles, centerline profiles, and turbulence data. Microscope slides are used to collect samples above the plume of the jet, as well as near the jet exit after the particles have time to fallout and settle. These samples are then taken to a microscope lab for imaging, and these images are post-processed with custom algorithms to help quantify particle and aggregate data. The overall results showed that for all cases, the addition of high ambient humidity levels showed minimal changes to the velocity field of the three particle cases. The jet humidification showed some modest increase in the effective inertia of the hollow glass particles, but additional testing will be needed to confirm this effect. In the aggregation analysis, the monodisperse hollow glass particles and the polydisperse ash particles showed very little response to the addition of humidity. The hollow glass and nickel mixture (HG-Ni), however, had a much more prominent response to the addition of ambient humidity, with the effective particle count per aggregate doubling from the control case to the highest humidity level. This study discusses the potential role of humidity in aggregation based on these results. Future potential work is also discussed, addressing challenges in the analysis or testing processes and indicating possible solutions for future endeavors.

THE ROLE OF HUMIDITY AND GRAIN-SIZE DISTRIBUTION IN VOLCANIC ASH AGGREGATION IN A PARTICLE-LADEN TURBULENT JET FLOW

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