(83b) CFD-DEM Study of Bubble Properties in a Single Jet Fluidized Beds

Fluidized beds are widely applied in many industrial processes such as pharmaceuticals, food, petroleum, environmental and power generation. They have many inherent features like good gas-solid mixing, excellent heat and mass transfer which are directly related to and are immensely affected by the presence of bubbles. Therefore, understanding and predicting the bubble properties are of practical importance. The objective of the present work is to study the bubble properties mainly bubble size, bubble size distribution and bubble shape (aspect ratio and shape factor) in a bubbling gas solid fluidized bed operated with a continuous central jet by using CFD-DEM. Fluidization of 200,000 Geldart group B particles in a quasi-two dimensional fluidized bed of height 0.08 m, width 0.02 m and depth 0.0004 m has been simulated at five central jet velocities 0.15, 0.20, 0.25, 0.30 and 0.35 m/s while maintaining the background air velocity at incipient fluidization condition. The results show that generally, the continuous injection of a central air jet to the bed leads to the formation of series of bubbles which rises through the bed and then finally bursts at the top of the bed. Initially, after the introduction of a central air jet to the bed, the first bubble was formed at a certain height above the distributor. Then there were several successive bubbles formed one after another. The average bubble diameters increase with the increase of jet velocity. The bubble size distribution in the overall bed follow gamma distribution function. A mono-peak positively skewed distribution was found for all the jet velocities. As the jet velocity increased the long tail end pointing towards the right expands. At the upper part of the bed, the distribution is more scattered signifying the presence of small and large bubbles. The distributions of bubble shape in terms of aspect ratio are also examined, showing a normal distribution. The bubble aspect ratio has a very high scattered distribution at lower part of the bed compared to that at the upper part. The distribution of bubble aspect ratio shifts towards the left and the mode increases as the height of the bed increases. The analysis of the bubble size and bubble shape provides useful information for understanding the gas-solid contact and mixing in gas fluidisation.