CFD Prediction of Stratified and Intermittent Gas-Liquid Two-Phase Turbulent Pipe Flow Using RANS

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Abstract

The transport of multi-phase flow in pipelines can be met in a wide range of industrial applications, including the oil and gas industry, showing great savings in developments. In addition, as the exploration of new fields in oil and gas expands to harsh environments, such as ocean or polar, the multi-phase flow transport sometimes becomes the only feasible option. The important features of such multi-phase flow applications include flow regimes, pressure drop and liquid holdup. The precise estimation of these parameters has significant technical and economical impacts on the design and operation of an oil and gas pipelines. Many prediction correlations and methods have been developed; computational fluid dynamics (CFD) being one of them. This type of modelling approach has many advantages over the conventional approaches such as its ability to solve 3D transient problems; offering access to a wealth of information which with conventional techniques is extremely difficult to obtain. Therefore, interest in applying CFD for multi-phase flow transport in pipelines has been on the rise.This thesis is aimed at presenting CFD simulations based on the use of the Volume of Fluid model (VOF) approach for various conditions of gas-liquid turbulent flow in a horizontal circular pipe. In the current VOF formulation in addition to the secondary phase transport equation, a geometric reconstruction technique based on a piecewise-linear interface construction approach is used for reconstructing the interface.A number of multi-phase studies using different turbulence models to the current one have recently appeared in the open literature for simple flow geometries such as rectangular channels. However, most of them assume specific boundary conditions (such as fully-separated phases for stratified flows, the use of square wave at the inlet to represent slug flow or imposing an interfacial disturbance to initiate slugging). These require case-by-case empirical information such as, interfacial roughness for stratified- or slug frequency for intermittent-flow. However, most of them have not presented any detailed validation of their results. The former two points are very crucial for the design of transport pipelines as a pre-knowledge of the operative flow regime and empirical information are not available at the design stage.The predictive accuracy of the present simulations is tested against most common mechanistic approaches and detailed measurements of stratified two-phase flow in a horizontal pipe of Strand (1993) and have been found to be in reasonable quantitative agreement. For the intermittent flow type cases, the numerical results are qualitatively compared against experiments in a horizontal pipe of Al-alweet (2008). The computed flow data of intermittent flow type are further tested against some empirical and mechanistic correlations; the numerical results are qualitatively in a reasonable agreement. Gas compressibility effects on the simulations of slug flow are also explored and are found to bring about some positive benefit. Overall, the predictive accuracy of the present approach is reasonable and promising, demonstrating the ability of the model to predict different types of flow regimes found in two-phase pipe flows. Furthermore, the proposed model shows potential for general applicability to the design of two-phase pipeline systems as it does not require pre-knowledge of the flow regime or any case-by-case empirical information.

Keyword(s)

Multi-phase Flow, Pipelines, CFD, VOF

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