Analysis of Hydrodynamics and Microstructure in a Bubble Column by Planar Shadow Image Velocimetry

Abstract

The hydrodynamics and bubble behavior in a laboratory-scale bubble column (diameter 140 mm) was analyzed using planar shadow image velocimetry. Different air flow rates were considered by using capillary aerators with different capillary diameter. This implies that a gas hold-up between 1 and 5% and a bubble number mean diameter between about 2 and 4 mm was realized. The imaging system consists of a background illumination utilizing a LED-array and a single CCD-camera which records simultaneously bubble and tracer images. The demarcation of the thickness of the imaging plane was realized by using a macrolens adjusted to small depth of field. To discard out-of-focus images of bubbles and tracer particles and to discriminate between both phases different gradient filters were applied. A Sobel filter was used to evaluate the bubble contours in order to obtain the area equivalent diameter, the eccentricity, and the bubble orientation. The velocity fields of both phases and horizontal profiles along the bubble column were determined by applying PTV (particle tracking velocimetry) for the bubbles and PIV (particle imaging velocimetry) for the tracer particles. For both phases axial and radial mean velocities as well as their fluctuating components were determined by averaging a sufficient number of double images. From these results also cross-sectional averages and global averages of turbulent kinetic energy and fluctuation energy of the bubbles were determined. It was found that the bubble fluctuation in the radial direction was higher than in the axial one for bubble sizes in the range between about 2.0 and 3.8 mm which is a result of the zigzag or helical motion of the bubbles. Finally also the bubble behavior was further analyzed by determining bubble eccentricity and orientation of the bubbles in the flow. These data are especially useful for modeling bubble oscillation and tumbling motion.