Gas rising through a large diameter column of very viscous liquid

Understanding the gas-liquid flow dynamics is highly desirable for the design of safe and environmentally-friendly systems. Most studies on the gas-liquid flows involve liquids with relatively low viscosity and small pipe diameters. Flow patterns are generally used to describe the steady-state and dynamic behavior of flows. For instance, various flows including bubbly, slug, annular and churn have been observed in vertical pipes. Apart from the liquid and gas properties, the dynamics of liquid-gas flows are also affected by the pipe diameter. This influences the specific features, stability and transition mechanisms of various types of flows.

With the increasing evidence on the existence of broad flow transition regions,: Dr. Abbas Hasan at University of Hull together with Dr. Shara Mohammed, Dr. Laura Pioli, Dr. Buddhika Hewakandamby and Dr. Barry Azzopardi from the University of Nottingham investigated the flow patterns and dynamic characteristics of a gas rising through a large diameter column of a very viscous liquid. The main objective was to enhance the understanding of the flow pattern stability in vertical gas-liquid flows. The work is published in International Journal of Multiphase Flow.

In particular, an experiment comprising of silicone oil in a 240 mm diameter bubble column was conducted. Air was passed through the silicone oil with a viscosity of 360 Pa.s. Electrical capacitance tomography and pressure transducers mounted on the walls were used to capture the dynamic features of the flow including; mean void fraction, structure velocity, frequency of flow structures, lengths of Taylor bubbles and liquid slugs, film thickness and flow transition. Ideally, the experiments were designed to reproduce desirable flow patterns in crude oil, bitumen and magmatic flows in volcanic conduits. Finally, they identified the various flow patterns and determined their flow characteristics, stability, and transition mechanisms.

The high viscous silicone oil recorded viscosity of 360 Pa.s that was significantly higher than that of the liquids initially used in describing flow pattern transitions. This allowed quantification and monitoring of the flow patterns for a larger range of gas viscosity. For instance, three distinct flow patterns were observed. At the lowest gas flow rate, the bubbly flow pattern was observed even though they were fewer and large as compared to those reported in low viscous liquids. An increase in the flow rate resulted in slug flow while further increase introduced a transition region comprising of a combination of alternating churn and slug flows. They, however, exhibited different structural velocities and mean void fractions.

Based on the equations obtained from the previous works, the bubble velocities for bubbly and slug flows were predicted. Additionally, for bubbly flow, the dominant frequency was observed to rise with the increase in the gas superficial velocity. On the other hand, the frequency decreased with an increase in the gas superficial velocity in the slug flow region due to the coalescence between Taylor bubbles. The up and down movement of the large waves was characterized by the flooding of the film around the Taylor bubbles that initiated the transition to churn flow. The wavy arrangement formed a cycle where the liquid falls to the bottom forming slug and the collected gas formed Taylor bubble which pushed it to the top of the column when it burst and the cycle repeated. Altogether, the study will enhance understanding of the flow pattern stability and their characteristics in vertical gas-liquid flows.