Gas rising through a large diameter column of very viscous liquid

Significance 

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.

[youtube https://youtu.be/KQQPNVx8g58&w=1019&h=573] [youtube https://youtu.be/mRAcSbVeCHc&w=1019&h=573]
Gas rising through a large diameter column of very viscous liquid: Flow patterns and their dynamic characteristics - Advances in Engineering
Figure 1: Time series of void fraction for silicone oil-air experiments. Numbers refer to gas superficial velocity (m/s). Photos (right) are for the large bubbles rising upward in the column at different gas superficial velocities (Hasan et al., 2019).
Gas rising through a large diameter column of very viscous liquid: Flow patterns and their dynamic characteristics - Advances in Engineering
Figure 2: Schematic drawing showing the mechanism of large bubbles bursting in columns of viscous liquids at high gas flowrates. The arrows in the figure correspond to the direction of the liquid flow, the numbers at the top section of the column corresponding to the liquid levels in the column. D and E are the more common structure for this flow regime. A–C occur when the liquid accumulates at the bottom of the column and the gas build up and rise as one long bubble and carry the whole liquid up to drain again as a falling film. The gas superficial velocity is 0.566 m/s (Mohammed et al., 2018).
Gas rising through a large diameter column of very viscous liquid: Flow patterns and their dynamic characteristics - Advances in Engineering
Figure 3: Mechanism of the Taylor bubble bursting and rupturing/retracting of the liquid at the top surface; (a) bubble is just covered by a thin film of liquid (Taylor bubble just to burst), (b) bursting of a Taylor bubble, (c) falling down of the liquid film entrapping gas bubbles, (d) retracting of the liquid, (e) next Taylor bubble to arrive, (f) liquid level is rising up again (milkiness is obvious) (Hasan et al., 2019).

Gas rising through a large diameter column of very viscous liquid: Flow patterns and their dynamic characteristics - Advances in Engineering

Gas rising through a large diameter column of very viscous liquid: Flow patterns and their dynamic characteristics - Advances in Engineering
Figure 4: Time series of film thickness from both ECT measurement planes showing waves on film around Taylor bubbles, churn regions and liquid slugs. ▬▬ Lower plane; ••••• upper plane. Gas superficial velocity = 0.223 m/s. (Hasan et al., 2019).
Gas rising through a large diameter column of very viscous liquid: Flow patterns and their dynamic characteristics - Advances in Engineering
Figure 5: Mean lengths of Taylor bubbles and liquid slugs in the slug flow region.

About the author

Dr Abbas H Hasan
Lecturer in Chemical Engineering
University of Hull, UK

Dr Abbas Hasan joined the University of Hull (UK) in 2018, as a lecturer at Chemical Engineering Department. He received his BSc degree in Process Control & Instrumentation from Bahrain University. He got his MSc with Distinction in Control and Instrumentation in 2005 and his PhD in multiphase flow modelling, measurement and metering in 2010, from the University of Huddersfield.

His areas of research interests include; Multiphase flow metering, development & measurement; fluid dynamics of complex two phase flows & viscous fluids; Process control & Instrumentation and process tomography particularly, for oil/gas industry and environmental applications (e.g. Marine, Volcano).

He developed, at the University of Huddersfield, a novel Conductance Multiphase Venturi flow meter in conjunction with a new flow model which is capable of measuring the phase volume fractions and the phase flow rates of gas-liquid two phase flows

Reference

Hasan, A., Mohammed, S., Pioli, L., Hewakandamby, B., & Azzopardi, B. (2019). Gas rising through a large diameter column of very viscous liquid: Flow patterns and their dynamic characteristics. International Journal of Multiphase Flow, 116, 1-14.

Go To International Journal of Multiphase Flow

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