Magnetic resonance study of two-phase gas-liquid systems :

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.