Study On The Flow Regime Transition In Horizontal Gas-Oil Flow And The Characteristics Of Gas-Liquid Interface

Oil-gas mixing transportation technology plays an important role with its characteristics of high efficiency, low cost and wide applicability in oil industry. The system of the oil-gas transportation, which is affected by the flux, the fluid physical properties, the geometry of the pipeline, the inclination angle and so on, makes the change of the void fraction, and shows different kinds of flow pattern. As a typical two-phase flow problem, the nature of horizontal oil-gas transient flow in pipes is studied from a flow pattern perspective.By detailed analysis and using a methodology developed specifically for this study, the observed flow patterns were classified into stratified smooth, stratified wavy, bubbly, slug, and annular flow regimes. The physical mechanisms that govern the transition between stratified and slug flow were identified and discussed. Based on the experimental data obtained in this study and on comparison with published flow regime maps, a flow regime map was presented to predict the prevailing flow regime for oil-gas flows in small diameter horizontal pipe at near-atmospheric conditions. The boundaries of the regimes were represented by transition bands.A theoretical approach which is based on one-dimensional wave model combined with slug stability model was presented to predict transition from the stratified patterns to slug flow. Data confirming the validity of the approach were generated from a self-built 25.4 mm diameter horizontal oil-gas pipeline in SJTU. The experimental data obtained in a 50 mm diameter horizontal oil-gas pipeline in Zhejiang Univ. were also analyzed. The comparison showed that these two models are applicable for different ranges of gas flows respectively. At low gas velocities, one-dimensional wave model works well, but is not suitable for high gas flows in which slug stability model provides better prediction of the critical liquid heights and the critical superficial velocities for the transition to a slug flow.Based on the observed disposition of the phases in each flow regime, simplified two-fluid models were used to predict flow regime transition or critical parameters such as superficial velocities and liquid holdup. Results clearly showed that the interactions between the phases at the interface have considerable effects on the velocity profiles within the phases such that, in general, the conventional definitions of the friction factors are invalidated. The measured liquid holdup and pressure gradients were used to obtain the interfacial friction factors for corresponding flow regime. A framework for the correlation of the deduced interfacial shear stress was presented from the experimental measurement. The uncertainty analysis was used to show that the measured liquid holdup and the calculated gas-wall shear stress by Blasius equation did not significantly influence the overall results, while the existing correlations might lead to large uncertainties, irrespective of accuracy of the experimental data or the appropriateness of the correlating technique.To evaluate the interactions between the phases at the interface, a numerical modeling of fully developed stratified wavy gas-liquid pipe flow in circular cross-section pipe was presented. Two-dimensional, steady-state axial momentum equations were solved with a two-layer turbulence model. The governing equations were discretized using a finite volume method on a bipolar coordinate system. Given longitudinal velocity profiles in the gas and assuming a logarithmic law above the waves, the simulations validated the concept of interfacial roughness to account for gas-liquid interactions.The region finally investigated was that of flows with relatively low liquid loading. Such flows are typically experienced in gas-condensate lines in oil industry. At low gas velocities, the interface curvature is dominated by the secondary flows, which is attributed to the non-uniformity of the interfacial roughness experienced by the flow of the turbulent gas-phase, at high gas velocities, the process of entrainment and deposition is the dominant contributor to the liquid phase transfer to the upper part of the pipe wall. A two-fluid model was employed for low liquid loading flows. New correlations were proposed for the interfacial curvature and the interfacial friction factor. The effect of droplet entrainment on the interfacial friction factor was also accounted for. The predictions of liquid holdup and pressure gradient from the new correlations matched well with the reported experimental data.