[SVC45-P03] Model experiments of degassing process in a crystal bearing magma
Keywords:magma, degassing, shear-thinning, model experiment, volumetric fraction, bubble
A transparent model fluids are made by mixing a silicone powder (particles) and a silicone oil which has the same refractive index. The volumetric fraction phi of the particles range from 0 to 0.5. From the rheology measurements, we find that the viscosity and the yield stress increases with phi. In addition the fluid becomes increasingly shear thinning with phi.
In this study, we conduct two types of experiments. Experiment 1 is conducted to study how the bubble ascent velocity (U ) depend on its volume (V ). Experiment 2 is conducted to observe the pattern of the bubbly flow and the fluctuations of differential air pressure at several flow rates Q. From experiment 1 (phi = 0-0.4), we find that U decreases with phi. We fit the data to a power-law relation (U proportional to Vn ) and obtain the power-law exponents n, for each fluids with different phi. From the Stokes' law, n in a Newtonian fluid is n = 2/3 ~ 0.67. At phi = 0, 0.1 we find that U is smaller than the Stokes' velocity and that n < 0.67. Reynolds number (Re ) in these experiments are Re ~ 100 which is much larger than 1, and we consider that the turbulent drag is the cause of this deviation. On the other hand for phi = 0.3-0.4, U becomes sufficiently small such that Re < 1. The value of the exponent n is n > 0.67 and the measured U agree well with the Stokes' velocity calculated using the shear-thinning viscosity. Here we note that n increasing with phi implies that the coalescence of bubbles with different sizes are enhanced, which we confirmed in our experiments. For fluid with phi = 0.5, we find that the bubble ascent is strongly inhibited because the yield stress becomes comparable to the bubble buoyancy. From experiment 2 (phi = 0.4-0.5), we observe that the generation and coalescence of the bubbles occur continuously. As Q increases, the bubbles become larger and vertically elongated (slugs). Furthermore, at phi = 0.5, narrow cracks form near the orifice where the bubbles form. The style of the differential pressure fluctuations also change with phi. At phi = 0.4, the pressure fluctuates regularly having a short period. However at phi = 0.5, the fluctuation becomes irregular with a longer period.
Our experiments suggest that as the magma cools and phi increases, the bubble ascent velocity becomes slower, such that the bubbles may even become trapped. However if the bubble size exceeds a critical value such that they can ascend, at large phi coalescence of bubbles with different sizes are enhanced which promotes degassing. Our experiments also show that whenever a bubbly flow occurs, an increase in phi results in an irregular, long period pressure fluctuations, which may excite volcanic tremors having similar temporal features.