Numerical investigation of gas-liquid hydrodynamics during trapped-liquid displacement from low sections of high-pressure gas pipelines

Assuring the displacement of trapped liquids from low sections by gas flow is essential for a proper operation of high-pressure natural gas pipelines. Numerical investigation of the associated transient turbulent high-pressure gas-liquid flow can provide understanding of the involved two-phase flow phenomena. To this aim, the numerical tool OpenFOAM has been used for simulating the gas-liquid flow encountered in a pipeline of a long shallow upward inclined section connected to a horizontal section by an elbow. Effects of various system parameters (e.g., gas pressure, liquid properties, up-comer inclination, elbow radius of curvature) on the gas-liquid flow dynamics are studied. Since 3D transient numerical simulations are more challenging, results obtained by 2D and 3D numerical simulations are compared to identify the similarities, sources of discrepancies, and their significance. Quantitative integral flow characteristics and the associated local flow phenomena are examined. 3D simulations verify the main findings of 2D simulations concerning the flow pattern, the associated critical gas velocity for purging the liquid and liquid propagation rate at supercritical gas velocities. The critical gas flow rate required to purge out the accumulated liquid is associated with that required to mobilize the liquid film tail along the up-comer. The main differences between 3D and 2D simulation results were captured near the elbows. The 3D effects, which result from the centrifugal force acting on the gas during its flow through the elbow, affect the downstream axial gas velocity profile and the secondary flow pattern (i.e., Dean vortices). The resulting higher interfacial shear exerted by the gas on the liquid film tail is the source of the deviations between the 3D and 2D simulation results. However, 2D simulations are sufficient for representing the gas-liquid dynamics out of the elbows' influence region, where their effects on the flow fade out.