Applications of ultra-fast MRI to high voidage bubbly flow: Measurement of bubble size distributions, interfacial area and hydrodynamics

Abstract

Ultrafast magnetic resonance imaging (MRI) has been applied to high voidage bubbly flow systems for the first time. Using an MRI protocol selected for fast acquisition (15.3ms per image) and robustness to fluid shear, images were obtained of bubbly flow in a vertical pipe of internal diameter 31mm for systems up to a gas voidage of 41%. From these images, two measurements of bubble size were extracted (one on the basis of volume and one on the basis of projected radius), which allowed the quantification of both the bubble size distribution (BSD) and bubble shape (and hence interfacial area). These measurements were found to be accurate to within 5.6% for voidages up to 22%. Measurements were acquired at 10 positions along a column for an electrolyte stabilised system to show the evolution of the BSD. Additionally, using a velocimetric variant of the MRI technique, maps of the fluid velocity in the vertical direction were obtained for all voidages. High temporal resolution series of MRI images were also acquired, from which it is possible to track the motion of individual bubbles. These measurements are non-invasive and applicable to optically opaque systems. The MRI technique should prove useful both for phenomenological studies of high voidage bubbly flow, and for the validation of models of multiphase flow systems.