A Novel Simulation of Hydrodynamic Mixing and Heat Transfer During Molten FuelCoolant Interaction

Hazard assessments of volcanoes at risk of hydrovolcanic eruptions would benefit from a better quantitative understanding of magmawater interactions. One of the most energetic explosive mechanisms that can occur as a result of magmawater interactions is Molten FuelCoolant Interaction (MFCI) where the molten fuel is magma, lava, or analog melt, and the coolant is external water. Much has been learned about MFCI through laboratory experiments and tephra analysis, but the important energetic processes at the magmawater interface are still largely inferred. Therefore, as an allied method to understanding this complex mechanism, were developing the first MFCI simulation for volcanic systems, using a suite of ANSYS fluid dynamics and solid mechanics software. For the purpose of validation, the simulation is set up similarly to MFCI laboratory experiments and olivine-melilitite experimental melt properties were acquired by previous workers or obtained using RhyoliteMELTS. We focus in this presentation on the initial, pre-explosion stage of MFCI (i.e., Phase 1), which is characterized by the production of a vapor film between the melt and water, and hydrodynamic mixing of the two fluids. We observed processes consistent with MFCI laboratory experiments, such as the Leidenfrost effect, which confirms appropriate scaling parameters, and a stable vapor film, which provides validation of the simulation stability. We observed that fluid instabilities developed as expected, dominated by RayleighTaylor instabilities. As these instabilities developed, hydrodynamically fragmented fine ash formed at the interface, which may be more likely to form from the very low-viscosity experimental melt, than basaltic magma or lava. As Phase 1 progressed, vapor film turbulence increased and dynamic movement of air, both down and up through the melt, was observed. This production of air bubbles can influence hydrodynamic fragmentation and ejection force during the subsequent explosion. This potential additional source of energy is not discussed in MFCI theory, nor is it a process that could be observed during MFCI laboratory experiments. Therefore, this simulation provides a novel opportunity to observe and quantify this effect among others fundamental to hydrovolcanic processes.