Prediction of vertical flows in large diameter pipes

Abstract

There is an increasing interest in multiphase flows in large diameter vertical pipes (typically with diameters greater that 100 mm) in the context of hydrocarbon production systems. There are strong indications that flows in such pipes differ greatly from those in smaller diameter pipes on which most of the prediction methodologies are based. In small diameter pipes, an important mechanism for the bubble flow to slug flow transition is the formation of void waves. This research reveal this wave growth and also predict the breakdown points from bubble-to-slug flow transition using Biesheuvel and Gorissen (1990) approximate void wave model based on Harwell small tube bubble flow experiments. As the gas velocity is further increased, the slug flow itself breaks down into churn flow by a process of flooding in the Taylor bubbles. In large diameter pipes, it appears that conventional slug flow does not occur; this is probably due to the fact that there is a size limit on spherical cap bubbles. Thus, this study reviews most of literatures in terms of bubble coalescence and breakup kernels in order to evaluate dynamic bubble size changes by applying population balance model. Unfortunately, these kernels have their own problems to be solved. Therefore we establish a simplified two-group bubble interaction model by taking into account mechanisms of large bubble shearing-off breakup and small bubble coalescence in large bubble wakes, respectively, assuming small bubbles do not coalesce to each other. In large diameter pipes, the bubble/slug and slug/churn transitions appear to be by-passed in favour of a direct transition from bubble to churn flow with increasing gas mass flux. Note that the churn flow studied here is emphasized by a continuous path for the gas phase. This study also describes work aimed at developing a phenomenological understanding of the bubble/churn and churn/annular transition regions in large diameter pipes. Investigation of the liquid transport mechanisms has led to the definition of two new flow regime transition criteria, namely liquid upflow potential and minimum entrained fraction. To estimate the bubble-to-churn flow transition, the liquid upflow potential of a churn flow at the particular local set of gas and liquid flow rates is estimated by using axial view experiments and the existing adiabatic equilibrium data. In churn flow, liquid upflow is achieved by the net upward flow in the film (bearing in mind that both upflow and downflow are occurring in the film, though the net value must be positive) and by droplet transport in the gas core. Once the Kutateladse flooding is reached, suggested by Pushkina and Sorokin (1969), then it is postulated that the transition to churn flow occurs. As the gas velocity is further increased, the flow rate of entrained drops in the gas core decreases to a minimum and then rises again. This minimum is observed to occur at a dimensionless gas velocity approximately equal to one and this serves as a possible criterion for the churn-to-annular flow transition. As a framework for prediction, an existing one-dimensional steady state modelling code (GRAMP2) has been selected. This code takes account of regime changes and predicts void fraction and pressure gradient using phenomenological models. Work on connecting the void wave growth, bubble size evaluation and GRAMP2 code for large diameter pipes will be the main target for the nearly future. In the meantime, CFD simulation is also being undertaken using a finite volume method based the STARCD software in order to numerically predict the evaluations of dynamic bubble size and flow regime changes in large diameter pipes.