Fluid dynamics in a two dimensional fluidized bed of large particles

Observation shows that bubbles in large particle fluidized beds behave quite differently from beds of fine particles. They expand and can grow faster than they rise. Bubbles in fine particle beds seem to grow primarily by coalescence, while in large particle beds they are observed to grow mainly without coalescence. The aim of this study was to gain a better insight into bubble behavior in fluidized beds of intermediate and large particles (particle diameter greater than 0.4 mm), and understand the cause of the observed bubble growth. In the theoretical part of this work it is shown that emulsion voidage invariability, incompressible viscous gas flow and potential solids flow assumed by existing theoretical two-phase models cannot account for the observed gas accumulation within bubbles. A simplified theoretical form for the gas accumulation rate in a cylindrical bubble is developed. It is based on different areas of gas inflow and outflow at the bubble boundary, and on the fact that the emulsion voidages above and below the bubble are different. It was shown that voidage variations in the emulsion phase around a bubble are important to bubble growth considerations. The experimental cases analyzed for single growing two dimensional bubbles indicate a higher voidage at the wake region, than the voidage at the nose of the bubble. It was found that a relatively small difference in emulsion voidage above and below the bubble can cause significant gas accumulation within the bubble reaching values comparable to the gas throughflow. Finally the movement of bubbles in the vicinity of an immersed tube and pressure-time curves representing the slow bubble and rapid bubble growth regime, and the transition range are analyzed in order to identify important bubble behavior characteristics. An experimental set up was developed making possible simultaneous permanent recording of the two-dimensional bubble behavior and instantaneous pressure data acquisition at multiple locations in an area under study. Lapsed cinematography and a special circuit activating the data acquisition system was used. The experimental set up enabled the collection of a large amount of data which gave valuable information about fluid dynamic behavior in the vicinity of an immersed tube. Experiments were performed at atmospheric pressure and room temperature in a two-dimensional fluidized bed with an immersed 50.8.mm (2 in) in diameter internal located at a height 0.35 m above the distributor plate. Glass, sand and dolomite particles with mean surface particle diameters of 0.65 to 2.6 mm were used. Superficial air velocities used spanned a region from well below minimum fluidization conditions to 2.82 m/s