Applications of ultra-fast MRI to high voidage bubbly flow: Measurement of bubble size distributions, interfacial area and hydrodynamics Alexander B. Tayler, Daniel J. Holland, Andrew J. Sederman n , Lynn F. Gladden Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, United Kingdom article info Article history: Received 31 August 2011 Received in revised form 2 November 2011 Accepted 10 November 2011 Available online 19 November 2011 Keywords: Bubble columns Multiphase flow Hydrodynamics Visualisation Magnetic resonance imaging Tomography 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 (15.3 ms per image) and robustness to fluid shear, images were obtained of bubbly flow in a vertical pipe of internal diameter 31 mm 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. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction High voidage bubbly flow systems have historically been very difficult to characterise experimentally. This is despite the tremendous importance of understanding bubbly flow for pro- cesses such as oil transportation, nuclear cooling systems, mineral separation, metal purification, liquefaction of solid fuels and gas– liquid chemical reactors (Deckwer, 1985). Accurate experimental measurements of bubbly flow systems are needed both to con- tribute to a fundamental understanding of these systems and for the validation of increasingly prevalent multiphase computa- tional fluid dynamics codes. The challenges associated with experiments on high voidage systems essentially stem from three aspects of the nature of bubbly flow: it is opaque, which restricts optical measurements to boundary flows; the gas–liquid interface cannot support significant stress, which fundamentally under- mines the accuracy of intrusive measurements; and the system structure is highly dynamic, which imposes a challengingly short time scale to obtain measurements by tomographic means. In the present work these obstacles are overcome by using ultra-fast magnetic resonance imaging (MRI), with the specific goals of measuring bubble size distributions, interfacial area and liquid phase hydrodynamics for high voidage bubbly flow. Experimental investigations into bubbly flow tend to focus upon either the characterisation of the bubbles themselves or of the liquid phase hydrodynamics. For the former, it is highly desirable to obtain accurate measurements of the bubble size distribution, which governs the bubble rise velocity and the residence time in a given unit operation. Many techniques have been developed for the measurement of bubble size. Most prominent are photographic techniques, which are reviewed by Tayali and Bates (1990). This approach involves obtaining photo- graphs of bubbly flow through a transparent section of the column. Several improvements to the basic technique have been suggested, such as shadowgraphy, which removes the influence of the position of bubbles within the column by projecting bubble shapes onto an opaque medium, such that the focal length of the camera is the same for all bubbles. Bubble sizes were measured in this way by van den Hengel (2004) and Majumder et al. (2006). Due to the occlusion of the dispersed phase in the bulk flow, however, these optical techniques are typically limited to rela- tively low voidages (in general less than 5%). Also commonly used are acoustic techniques, wherein the frequency of pressure variations in the column (caused by either passive or driven bubble shape oscillations) are measured and correlated with bubble size. The acoustic measurement of bubble size is reviewed Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.11.014 n Corresponding author. E-mail address: ajs40@cam.ac.uk (A.J. Sederman). Chemical Engineering Science 71 (2012) 468–483