Pressure distribution and vortical structure in the wake behind gas bubbles in liquid and liquid-solid systems

Abstract

An important characteristic of large bubbles rising in liquid columns and liquid-solid fluidized beds is the near wake. Unsteady wakes are typified by the stable liquid layer, and the cyclic sequence of formation, growth and shedding of vortices near the bubble base. The pressure distribution in the near wake is a critical variable which determines the fluid motion, solids concentration and size of the wake, and is closely related to the wake structure. The static pressure traces in the near wake are examined with varying vertical distance from the bubble nose and lateral distance from the bubble axis of symmetry. Single bubbles are injected into a two-dimensional liquid column and a liquid-solid fluidized bed, and static pressure is measured by means of a transducer mounted on the column wall and interfaced with a high-speed computer data acquisition system. The bubble-wake is visualized using a video camera system synchronized with the data acquisition system. The geometry of the primary wake is shown to be uniquely characterized by the pressure trace, and a one-to-one correspondence between the pressure distribution and vortical structure is established. The effects of particle size and solids holdup are investigated, and compared with stationary water. Pressures vs time traces are normalized as modified pressure coefficient vs dimensionless time. These traces exhibit similar trends in stationary water and in a liquid-solid fluidized bed. However, results show that the presence of solids affects the quantitative nature of the pressure distribution in the near wake behind the bubble.