Churn flow is an important intermediate flow regime occurring in between slug and annular flow patterns in two-phase flow, with profound implications in chemical and petroleum industry. The majority of studies to date in churn flow have been carried out mainly using water or liquids of low viscosities and limited information exists regarding the behaviour of high viscosity liquids which resemble realistic process conditions. In this paper, a study that investigated churn flow and its characteristics in high viscosity oils (360 and 330 Pa.s) and large diameter columns (240 and 290mm) is presented for a first time. Transition to churn flow regime starts when the structure velocity, length and frequency of the liquid bridges, which appear at the end of slug flow, increase. In churn flow, gas flows at the core of the oil column with a wavy passage, leaving the top surface open to atmosphere with a possibility of creating a very long bubble. The average length of the bubbles seen to decrease with increasing the gas flow rate. While, no considerable change is observed in void fraction, structure velocity and film thickness at this flow pattern.

Gas-liquid flows are affected strongly by both the liquid and gas properties and the pipe diameter, which control features and the stability of flow patterns and their transitions. For this reason, empirical models describing the flow dynamics can be applied only to limited range of conditions. Experiments were carried out to study the behaviour of air passing through silicone oil (360 Pa.s) in 240 mm diameter bubble column using Electrical Capacitance Tomography and pressure transducers mounted on the wall. These experiments are aimed at reproducing expected conditions for flows including (but not limited to) crude oils, bitumen, and magmatic flows in volcanic conduits. The paper presents observation and quantification of the flow patterns present. It particularly provides the characteristics of gas-liquid slug flows such as: void fraction; Taylor bubble velocity; frequency of periodic structures; lengths of liquid slugs and Taylor bubbles. An additional flow pattern, churn flow, has been identified. The transition between slug and churn has been quantified and the mechanism causing it are elucidated with the assistance of a model for the draining of the liquid film surrounding the Taylor bubble once this has burst through the top surface of the aerated column of gas-liquid mixture. It is noted that the transition from slug to churn is gradual.

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The dynamic behaviour of the gas-liquid two phase flows and in particular the flow pattern stability and transition between the flow regimes are influenced significantly by both the properties of the liquid and gas as well as the pipe diameter. The majority of the studies reported in the literature on the dynamics of gas-liquid flow transitions focus only on low viscosity liquids (e.g. water) and small diameter pipes. In the present work a series of experiments were carried out to study the dynamics of flow transitions (bubbly to slug and slug to churn) of gas rising through very viscous oils (330 Pa.s and 360 Pa.s) in two large diameter columns (290 and 240 mm respectively), using Electrical Capacitance Tomography (ECT) and pressure sensors. The experiments aimed to imitate a number of realistic flow conditions that might be encountered, for examples, in; bitumen, crude oil, viscous liquids in food processing and volcanic magmatic flows. Observation and quantification of bubbly to slug and slug to churn flow transitions for gas-high viscous liquids in large pipe diameters are presented for the first time. Flow parameters and characteristics including; void fraction, pressure gradient, Probability Density Function, structure velocity, lengths of large/Taylor bubbles and liquid slugs and the effect of liquid temperature on the void fraction and Taylor bubble lengths, were measured and analysed. It was found that transition to slug and churn flow occurs gradually. Transition to slug flow occurs at a gas superficial velocity of 0.011 m/s to 0.016 m/s, while transition to churn appears in the range of 0.127 to 0.243 m/s in both columns.