Investigating the bubble dynamics in fluidized bed by CFD-DEM

Abstract

Fluidized beds are of great importance in various industries and presence of bubbles significantly influence their efficiency. In order to gain a more accurate vision on the dynamics of a single bubble in fluidized beds, formation, rising, and eruption of a single bubble in a rectangular fluidized bed were studied by CFD-DEM and validated by experimental data. Single bubble in a 2-D rectangular fluidized bed at different stages, from forming to erupting, was modeled by the CFD-DEM technique. Investigating the pressure fluctuations and bubble dynamics were the main focus of this study. Pressure fluctuations were measured using a pressure transducer and a high-speed digital camera was used to record the whole bubbling process, from bubble formation on the distributor to its eruption at the surface. Validation was performed by comparing experimental and simulated bubble diameter and pressure fluctuations. CFD-DEM results such as particles and gas velocity vectors, bed voidage and pressure contours revealed new information on characteristics of pressure fluctuations which was not possible to gain by experimental studies. In the formation stage, due to the compression wave from injection, particles accelerate and bed reaches to a packed state. This causes a pressure peak in the pressure. At the beginning of bubble formation on the distributor, pressure decreases and when the bubble detaches from the distributor, pressure reaches a minimum. While bubble is rising through the bed, pressure increases again, however, it remains below zero because of the lower voidage behind the bubble in comparison to pre-injection state and the greater velocity of the gas above and below the bubble. By bubble eruption, another small compression wave is seen in the pressure.