High-energy gamma-ray computed tomography (CT scanner system) has been used to study, for the first time, the structure of the pebble bed in terms of cross-sectional time-averaged void and solids distributions, and their radial profiles along the bed height. The pebble beds were packed with pebbles of glass spheres of diameter of d 1 = 0.0127 m, d 2 = 0.0254 m and d 3 = 0.05 m in a Plexiglas cylinder with diameter of D = 0.3048 m, with D/d 1 = 24, D/d 2 = 12 and D/d 3 = 6, respectively. The average void fraction of these beds were = 0.389, = 0.40 and respectively. It was found that the void and solids distribution depends on pebble size, the bed diameter and the packing method. The void radial profiles vary with the bed height for larger pebbles at the measured axial levels (0.0762, 0.1524, and 0.2286 m from the bed base). However, adding additional bed height on the top of the original one did not show any influence on the average voids and their radial profiles at the same measured axial levels. The experimental results indicated that void variations are normally distributed in the bulk region of the packed bed.

The accurate identi cation of the boundaries of the main ow regimes in uidized beds is very important since the degrees of mixing and mass and heat transfer depend on the prevailing ow regime. Both the minimum uidization velocity U mf (0.103 m•s 1 ) and the transition velocity U trans (0.12 m•s 1 ) to bubbling uidization regime have been successfully identi ed based on a newly developed maximum information entropy (IE max ) algorithm applied to gamma photon time series. The latter are recorded by means of gamma-ray densitometry scans performed in an air-polyethylene uidized bed (0.438 m in ID) operated at ambient conditions. A comparison with the Kolmogorov entropy (KE) algorithm has found that the new approach yielded more accurate transition velocities. The value of U mf is also validated by means of the pro les of both bed pressure drop and bed height, respectively. It has been found, however, that these parameters are not capable of identifying the second transition velocity U trans . It is demonstrated that the U mf value identi ed on the basis of IE max is theoretically predictable. In the bubbling uidization regime, a simple correlation between both the KE and IE max values is developed.

Two-dimensional (2D) bubble columns have been used in the literature to study the detailed flow field for evaluation and validation of computational fluid dynamics (CFD) and for developing the needed closures and interfacial forces models for gas-water systems. Hence, this work focuses on investigating the bubble dynamics (i.e., local gas holdup, bubble velocity, bubble chord length, and frequency) in a 2D bubble column. Two advanced measurement techniques were implemented in this work, high-speed video camera imaging and a four-point optical probe. A new method for analysis of video camera imaging was employed. The results of both techniques were compared. The results show good overall agreement between the two methods for the horizontal profile of both bubble size and gas holdup. The findings revealed a strong positive correlation between bubble size and gas holdup. It was found that video image analysis could become an accurate, noninvasive alternative to fiber optical probes, at least in 2D systems, although the technique needs more refinement.