Jetting Visualization in an Acoustic Fluidized Bed Using X - Ray Computed Tomography

Introduction Understanding the jetting phenomena near the gas distributor plate in a fluidized bed is important to gassolids mixing, heat and mass transfer, and erosion on any bed internals, which can all affect the performance of the bed. Moreover, acoustic vibration in a fluidized bed can be used to enhance the fluidization quality of particulate matter. Visualizing the jetting structure using X-ray computed tomography in a 3D fluidized bed, with and without acoustic intervention, is completed in this study. A 10.2 cm ID fluidized bed filled with glass beads, with material density of 2500 kg/m 3 and particles sizes ranging between 212-600 μm, is used in these experiments. X-ray computed tomography (CT) imaging is used to determine local time-average gas holdup. From this information, qualitative characteristics of the hydrodynamic structure of the multiphase flow system are visualized. Jetting has been studied extensively in the gas-solid fluidized bed literature [1-3]. However, most of the studies focused on jetting produced by using a central vertical nozzle or focused on jetting produced by a horizontal nozzle penetrating a lightly fluidized bed. Sound assisted fluidized beds have been studied in the literature because it has been shown that the inclusion of sound vibrations can help overcome fluidization problems in certain material types. Different behaviors under sound vibrations have been observed [4, 5]. Noninvasive techniques have also been used to investigate the effects of acoustic waves on the void fraction distribution in fluidized beds. Escudero and Heindel [6] used X-ray computed tomography imaging to characterize the gas distribution in an acoustic gas-solid fluidized bed. The goal of this study is to visualize the jetting phenomena created by the distributor plate of an acoustic fluidized bed using images obtained from X-ray computed tomography. Experimental Setup The material used in these experiments is glass beads (ρglass = 2500 kg/m 3 ), with particles sizes that range from 212 – 600 μm. The material is sifted several times using a mechanical shaker and sieves with different mesh sizes, to assure the material falls in the desired particle size range. The bed bulk density was determined using the material mass and the static bed volume. X-ray computed tomography (CT) scans are captured with and without acoustic intervention at different H/D ratios (0.5, 1, 1.5) and different superficial gas velocities Ug = 1.5 Umf and 3 Umf, where Umf is the minimum fluidization velocity previously determined [5]. The minimum fluidization velocity differs between the acoustic and the no acoustic fluidized bed. Table 1 summarizes the general bed characteristics.