Magnetic resonance study of two-phase gas-liquid systems :: acoustic cavitation and vertical bubbly flow
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Date
2015
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University of New Brunswick
Abstract
We performed magnetic resonance studies of gas-liquid systems: acoustic cavitation
and vertical bubbly flow to obtain information about the water molecules mobility,
bubble size, and void fraction, critical for a better understanding of these complex
dynamic systems.
A varying magnetic susceptibility between the phases in gas-liquid systems
fundamentally limits the employment of conventional magnetic resonance imaging
methods for quantitative measurements. Susceptibility effects can be removed with the
addition of paramagnetic salts to the solution. We, however, decided to use the
susceptibility difference to extract useful information about bubble size, provided its
image distortion effects are overcome. Pure phase encode imaging methods are suitable
means for the quantitative measurement of these heterogeneous systems, as they
incorporate fixed encoding times.
A significant modulation of the magnetic resonance parameters, as driven by
susceptibility, necessitates short signal acquisition times. The fast dispersion of water
molecules in the acoustically cavitating liquid permitted the employment of pure phase
encode imaging methods with short phase encoding times to obtain 3D voidage and
velocity maps. Compressibility, divergence, and vorticity maps of the cavitating medium
were also created.
The average bubble size, both bulk and spatially resolved, was extracted for the vertical
bubbly flow from the susceptibility-modulated Ti. For that, an analytical approach wit}:i
two significantly different measurement regimes was employed. The fast mobility of
water spins with respect to the short encoding times in the employed pure phase encode methods satisfied the fast diffusion regime requirements. A good agreement was
observed between magnetic resonance and optics-based estimates of bubble sizes for the
slower airflow rates.
Bulk measurements of vertical bubbly flow were also performed at low magnetic field.
We analyzed the susceptibility-induced effects in the Carr-Purcell-Meiboom-Gill signal
using new analytical approaches, and derived two simplified equations for voidage and
inclusion size estimation with basic relaxometry. The approach introduced was validated
through control measurements on silica bead solutions with known bead sizes, and
through comparison with measurements performed using an optics-based technique. The
developed approach worked well for both control samples and bubbly flows.
Our approach can also be used to study other gas-liquid systems with spherical non-interacting
inclusions, like gas hydrates and sprays, provided all the theoretical
requirements are met.