Phase doppler anemometry measurements of multiphase bubble column

• Main Author: Gan, Zhiwen
• Other Authors: Law Wing-Keung, Adrian
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Chemical engineering::Biochemical engineering
• Online Access: http://hdl.handle.net/10356/51240
• Institution: Nanyang Technological University

Bubble column is widely used in both industrial and environmental applications. In this study, the flow dynamics and stability of both two-phase and three-phase bubble columns were examined using Phase Doppler Anemometry (PDA). To model the bubble column, experiments were first performed to select the optimal bubble generator for the PDA measurements. Three different type spargers that are most commonly used in the literature, i.e. air stones, needles, and membranes, were investigated. The bubble characteristics generated by these three types of spargers were examined with both flow visualization and PDA measurements. After the comparison, a 30mm diameter, perforated air stone was finally selected as the sparger for the model bubble column. With the air stone, the generated bubbles had a steady size range and the largest value of both standard deviation and coefficient of variation which would create spatial contrasts in the bubble concentrations and flow gradients in the bubble column for our investigation. The detailed information obtained in this study using these three spargers can provide a reference set of information for the investigation of bubble column reactors. The model two-phase bubble column was cylindrical with an inner diameter of 152 mm, and filled with the liquid to about 1 m height above the point air source. A customized setup was developed for accurate PDA measurements of the two phases of bubbles and liquid, and detailed turbulent characteristics of the liquid phase velocity, bubble diameter, bubble velocity as well as the slip velocity were quantified throughout the column. The comprehensiveness of the data set enabled a close examination of the hydrodynamic stability inside the column. The results illustrated a large-scale coherent liquid circulation pattern inside the column. The circulation pattern in the upper column was relatively steady, while the pattern in the lower column was strongly unsteady with the probability density functions (pdf) for both the liquid and bubble velocities showing distinct twin peaks. An analysis based on the determination of the bubble drag forces and transversal lift forces was performed by decomposing the twin-peaked pdfs into two separated Gaussian distributions, one for the upward flow due to the bubble rises and the other for the downward flow due to the circulation. Through the decomposition, a stability criterion was established by choosing the local bubble size as the representative length scale for the turbulent eddies inside the column. The analysis with the proposed criterion illustrated why a steady circulation pattern was achieved in the upper column, and also how the instability at the bottom column was induced by the low frequency meandering of the bubble swarm. With added solid particles of acrylic beads of a nominal diameter 3 mm, the model bubble column became a three-phase bubble column. Phase Doppler anemometry (PDA) was again used to quantify the flow characteristics of the three phases (liquid, solid and bubbles). The flow field was found to be relatively steady compared to the two-phase column, referencing the probability density functions for both the liquid and bubble velocities. An analysis with an established criterion based on the determination of the bubble drag forces and transversal lift forces was also performed to examine the flow stability in the three-phase bubble column, and a new stability criterion was proposed for the three-phase bubble columns by applying the automatic control theory. The analysis illustrated that how the added solid phase effectively stabilized the flow field to achieve a steady circulation in the entire bubble column. The minimum amount of the added solid phase required to achieve the steady circulation was also determined. The results of this study can further improve the design of the bubble column with a point air source at the bottom for environmental applications. The detailed knowledge of the spatial characteristics of the circulation pattern can further optimize the bubble column design for waste water treatment. The quantitative stability criterion can also provide the guideline for the prototype reactor designs.