Gas slug ascent in bidirectional conduit flow

Abstract:

Persistently active, basaltic volcanoes display a wide range of eruptive activity from continuous passive degassing to intermittent explosive bursts. Buoyancy-driven exchange flow provides a framework for understanding the imbalance between magma erupted and gas emitted and for the persistence of activity over long periods but it does not explain why or how eruptions occur. It is hence not surprising that Strombolian-type eruptions, which are discrete bursts that eject pyroclasts to a height of more than ten meters, are usually conceptualized very differently than the steady state by relating them to the ascent and burst to conduit-filling gas slugs.

This study investigates the interaction between exchange flow and slug ascent in a vertical conduit using analogue laboratory experiments and direct numerical simulations. We initialize core-annular-geometry exchange flow in a vertical pipe connecting two reservoirs by removing a barrier between two gravitationally unstable liquids with different viscosities. We then inject a gas slug at the base of the pipe with a thin syringe, which always rises through the core liquid. As the slug ascends much faster than the background core flow, the two flows are largely decoupled. However, in some experiments the injected gas slug perturbs the exchange flow and breaks up during ascent. To better understand and generalize these observations, we use an axisymmetric, direct numerical model to resolve the interactions between the multiple phases at the scale of the gas-liquid and liquid-liquid interfaces.

Our simulations suggest that slug breakup observed in some experiments is related to the gas injection mechanism. Rapid injection leads to a rapid expansion of the gas volume in the domain, which interacts with the metastable interface separating the two liquids. The interaction between the gas-liquid and the liquid-liquid interfaces leads to build-up of a wave-like perturbation, which can lead to the break-up of the slug and the deformation of the liquid-liquid interface. When injection is stable, a single ascending slug only has a temporary effect on the exchange flow. However, a sequence of ascending slugs, as is expected in many volcanic systems, permanently alters the exchange flow because it disrupts the mass balance which is essential to the stability of the exchange flow.