Prediction model for the transition between oil–water two-phase separation and dispersed flows in horizontal and inclined pipes

There are two important flow patterns associated with horizontal and deviated wells: separation flows, which include stratified and annular flows, and dispersed flows. Because various types of flow patterns often exchange with each other under certain conditions, it is of great significance for improving the efficiency of oil transportation to develop prediction models describing the transition between oil–water stratified and annular or dispersed flows. Owing to the variable flow rate, water content, and pipeline inclination, the transition characteristics and prediction models for the transition from stratified to annular or dispersed flows are not clear. Therefore, in this study, relevant experiments were carried out and the transition characteristics were investigated to establish a new prediction model. In the transition from a stratified to annular flow, the prediction model for the transition boundaries and criteria is based on mixed Froude numbers. In contrast to previous studies, it was found in these experiments that the mixed Froude number is not constant when conversion between stratified flow and annular flow occurs. Instead, the mixed Froude number is related to the inlet water content and the pipeline inclination. The transition boundaries predicted by the proposed model are in good agreement with the experimental results. Based on the existing two-dimensional interfacial wave shearing mechanism, the three-dimensional interfacial wave shear deformation mechanism is elucidated. In the transition from stratified to dispersed flows, the external force on the flow direction of the interfacial wave, which includes the drag force, interfacial tension, gravity, and buoyancy, is theoretically derived by analysing the force situation before and after deformation of the two-phase interfacial wave. A prediction model for the deformation of the interfacial wave is developed, and the relationships between the external force and wave amplitude or wavelength are analysed with the prediction model. The results indicate that deformation is more likely to occur as the inclination angle of the well increases, the interfacial wave becomes unstable with increasing amplitude, and droplet formation occurs with long waves.