Qualitative comparison of bubble evolution in a two dimensional gas-solid fluidized bed using image analysis and CFD model

Abstract

Gas-solid fluidized beds are widely used in industries like chemical, metallurgical, environmental, and pharmaceutical sectors for a variety of heat and mass transfer applications. Bubbling beds are the most prevalent type due to excellent solid-gas interfacial contact. Bubble characteristics such as shape, size, and trajectory control the hydrodynamics and therefore heat and mass transfer in fluidized bed reactors. Thus understanding these characteristics is very important for the design and scale-up of fluidized beds. In this work, a pseudo two-dimensional fluidized bed made of polycarbonate material has been used. Single bubbles are injected separately at the center and near the wall by offsetting the jet position, thus presenting the effect wall has on asymmetrical injection as compared to symmetrical injection. The bubble behavior is characterized using an in-house developed digital image analysis technique. The experimental data is compared with an earlier developed Eulerian-Eulerian two-fluid model for simulating dense gas–solid two-phase flow. The high-speed photography reveals an asymmetric wake formation during bubble detachment indicating an early onset of the mixing process. The wall forces act tangentially on the bubble and has a significant impact on the bubble shape, neck formation during detachment and its trajectory through the bed. Larger bubbles are found to drift away from the center with longer paths while the smaller ones found to oscillate. Whereas, centrally injected bubbles found to be more circular and progressing sooner to the surface. This qualitative behavior is well predicted by CFD modelling. The longer bubble hold up in asymmetric injection creates more particle mixing that may improve the fluidized bed heat and mass transfer characteristics.