Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.573419
Title: Exchange flow experiments and implications for degasing processes at basaltic volcanoes
Author: Beckett, F. M.
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2011
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Abstract:
Persistently active basaltic volcanoes are characterised by Strombolian eruptions that punctuate continuous passive degassing. Persistent passive degassing can be driven by an exchange flow of magma within the conduit. To interpret persistent gas emissions and changes in activity requires some knowledge of the flow regime and an understanding of how its volume flux depends on the physical properties of the magma and the conduit geometry. This thesis presents an experimental study of buoyancy-driven, low Reynolds number, (Re < 6) exchange flow of two Newtonian fluids in a vertical cylindrical pipe over long time durations. Two distinct flow regimes were observed~~ymmetric core annular flow (CAF) and side-by-side (SBS) flow. The flow regime formed was found. to be dependent on the viscosity ratio of the two fluids ((3); CAF occur at (3 2: 100, SBS flow occur at (3 ;S 100. It is shown that fluids are not arranged in the regime that maximises volume flux (e.g. SBS or CAF), nor do they adopt the geometry that maximises volume flux within that particular regime. When the pipe is inclined from the vertical at e > 5° the flow regime is always SBS flow, regardless of (3. The physical and chemical processes driving an exchange flow at Stromboli are con- strained by degassing and crystallisation paths to produce a 3-phase (melt, crystals, bubbles) model of ascending and descending magma driving persistent gas fluxes at Stromboli, assuming a cylindrical pipe geometry. Combining modelled magma prop- erties with the analogue exchange flow experiments the flow regime of magma driving persistent degassing is defined as a function of pressure. At pressures 2: 90 MPa the flow regime is CAF and at pressures ;S 90 MPa the flow regime is SBS flow. Strombolian eruptions are posited to be driven by the ascent and bursting of large gas slugs. The influence of an exchange flow on the rise velocity of a gas slug is considered, and that of the gas slug on the geometry of an exchange flow. Analogue experiments are presented of gas slugs ascending through exchange flow with (3 > 1, scaled to basaltic volcanoes using the Reynolds, Eotvos, Froude and Strouhal numbers. Gas slugs rise through, and are constrained by the less viscous ascending fluid. Their rise velocities are shown to be a linear combination of the liquid velocity ahead of the gas slug and the rise velocity of the slug due to buoyancy. Its buoyant rise velocity is a function of the liquid viscosity, density and surface tension, and the lateral dimension of the less viscous fluid which it is constrained by. Given the calculated magma properties for ascending and descending magma at Stromboli gas slugs ascending through an exchange flow have velocities ranging from 1 - 3 m s-l. Further, experiments conducted in inclined pipes suggest that velocities may be increased by up to 56% in a conduit inclined from the vertical.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.573419  DOI: Not available
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