(150c) Magnetic Resonance Imaging of Injected Bubble and Jet Dynamics in Fluidized Beds

The behavior of bubbles and jets in fluidized beds is key to gas-solid contact and solids mixing in industrial systems. The behavior or these voids has fascinated scientists and engineers for decades because of their similarities to voids seen in gas-liquid systems (1–3). These similarities exist despite a lack of surface tension in gas-solid systems and the ability for gas to flow through the “interface” between a bubble and the surrounding particulate phase.

Due to scientific and industrial interest, a variety of theoretical, experimental and computational studies have been conducted to better understand bubbles and jetting behavior in fluidized beds. A frequent difficulty in understanding bubble behavior is the difficulty in visualizing dynamics inside 3D opaque systems coupled with the fact that bubbles behave differently is pseudo-2D fluidized beds than 3D beds. For this reason, tomographic imaging has been used to visualize bubble dynamics using techniques such as electrical capacitance tomography (4), X-ray (5–7) and magnetic resonance imaging (MRI) (8–11). An advantage of MRI over other tomographic imaging techniques is that it can image the flow field of particles in addition to the local concentration of particles; however, MRI has traditionally been limited by low temporal resolution and small fluidized bed sizes as compared to other tomographic imaging techniques. Recently, multichannel MRI measurements conducted in medical MRI scanners have been introduced (12), enabling imaging of particle concentration and flow field in a fluidized bed 190 mm in diameter with a temporal resolution of 18 ms.

In this study, we have utilized this rapid, large-scale MRI capability to image the dynamics of bubbles and jets injected into an incipiently fluidized bed. The imaging capabilities have enabled new insights into the evolution of bubbles, the interaction of multiple bubbles and how the flow of particles is affected by these interactions. This data has been used to quantify the leakage of air from injected bubbles into interstitial flow as well as the volume of particles carried up in the wake of bubbles under different particle and injection conditions. Additionally, anomalous flow phenomena have been uncovered, such as the “disappearance” of bubbles with a neighboring bubble taking up the air from a disintegrated bubble as well as a “zipper-like” instability forming between two interacting jets. The flow phenomena and quantitative data obtained serve as important challenge problems for computational models.

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