Taylor Bubbles of Viscous Slug Flow in Inclined Pipes

Unique structures are formed when gases and liquids flow simultaneously in pipelines. The geometric characteristics of these structures are fundamental parameters in intermittent flow regimes. The length of liquid slugs and Taylor bubbles are inputs to mechanistic and empirical models for pressure drop estimation, slug catcher sizing and determination of the periods of no or low liquid in pipelines. Although slug flow has been studied for decades, there still exists a lack of comprehensive understanding of flow structures dynamics due to the complex interactions between the gas and liquid phases in two-phase flow. This study investigates the influence of pipe inclination on the length and hydrodynamics of large gas structures in intermittent flows, particularly, ‘Taylor bubbles’ in slug flow regime. An experimental study was conducted in a 67 mm ID pipe to estimate the bubble lengths of an air-silicone oil mixture from void fraction measurement using a twin-plane Electrical Capacitance Tomography (ECT) tool. The results show that the pipe inclination, gas and liquid flow rates have a substantial effect on the length of large bubbles in slug flow. Taylor bubbles get longer when the void fraction increases, or the pipe inclination deviates towards the horizontal pipe orientation. The influence of pipe inclination on bubble length is quite significant; this variation in bubble length with pipe inclination is attributed to the expansion or compression of large gas structures when there is an alteration on the forces acting on the bubble nose. The weight of the liquid column above the bubble nose which has been often neglected in earlier models was identified to have a notable effect on the volume occupied by the large bubbles and consequently, its length. A semi-mechanistic model is proposed based on the analysis of forces acting on the Taylor bubble nose in a quiescence liquid phase. A comparative analysis of the model and previous models shows that the proposed model outperforms existing mechanistic and empirical models across all pipe inclinations. This study gives an insight into the effect of pipe inclination on the length of large bubbles during slugging in pipes, as these bubbles can be up to 10 times longer in horizontal pipes compared to vertical pipes at the same flow conditions. The proposed model has the potential of estimating the length of large bubbles across all pipe inclinations in upward slug flow with acceptable accuracy, particularly for pipelines installed in undulating terrains.

DEVELOPED LIQUID FILMS FALLING AROUND TAYLOR BUBBLES INSIDE VERTICAL STAGNANT COLUMNS

The present work reports an experimental study of developed liquid films falling around single Taylor bubbles inside vertical tubes containing stagnant liquids. Experiments were carried out in acrylic tubes with 2.0 m length and inner diameters of 0.019, 0.024 and 0.034 m. Five water-glycerin mixtures were used, corresponding to film Reynolds number（Ref）ranging from 2 to 7650. A pulse-echo ultrasonic technique was applied to measure the rise velocity of the bubble and the equilibrium thickness of the liquid film. These parameters together with the calculated standard deviation of the equilibrium film thickness provided information about the development of waves on the gas-liquid interfaces, which could be related with the laminar-turbulent transition of liquid films falling around Taylor bubbles. The results indicated that the wave amplitudes increased sharply for Ref> 1000. This value of Ref is in agreement with literature concerning the laminar-turbulent transition for free falling films on vertical surfaces.

Computational Fluid Dynamics (CFD) Simulations of Taylor Bubbles in Vertical and Inclined Pipes with Upward and Downward Liquid Flow

Summary Two-phase flow is a common occurrence in pipes of oil and gas developments. Current predictive tools are based on the mechanistic two-fluid model, which requires the use of closure relations to predict integral flow parameters such as liquid holdup (or void fraction) and pressure gradient. However, these closure relations carry the highest uncertainties in the model. In particular, significant discrepancies have been found between experimental data and closure relations for the Taylor bubble velocity in slug flow, which has been determined to strongly affect the mechanistic model predictions (Lizarraga-García 2016). In this work, we study the behavior of Taylor bubbles in vertical and inclined pipes with upward and downward flow using a validated 3D computational fluid dynamics (CFD) approach with level set method implemented in a commercial code. A total of 56 cases are simulated, covering a wide range of fluid properties, pipe diameters, and inclination angles: Eo ∈ [10, 700]; Mo ∈ [1×10–6, 5×103]; ReSL ∈ [–40, 10]; θ ∈ [5°, 90°]. For bubbles in vertical upward flows, the simulated distribution parameter, C0, is successfully compared with an existing model. However, the C0 values of downward and inclined slug flows where the bubble becomes asymmetric are shown to be significantly different from their respective vertical upward flow values, and no current model exists for the fluids simulated here. The main contributions of this work are (1) the relatively large 3D numerical database generated for this type of flow, (2) the study of the asymmetric nature of inclined and some vertical downward slug flows, and (3) the analysis of its impact on the distribution parameter, C0.