Pressure Effects on Low-Liquid-Loading Oil/Gas Flow in Slightly Upward Inclined Pipes: Flow Pattern, Pressure Gradient, and Liquid Holdup

This paper studied the effects of system pressure on oil/gas low-liquid-loading flow in a slightly upward inclined pipe configuration using new experimental data acquired in a high-pressure flow loop. Flow rates are representative of the flow in wet-gas transport pipelines. Results for flow pattern observations, pressure gradient, liquid holdup, and interfacial-roughness measurements were calculated and compared to available predictive models. The experiments were carried out at three system pressures (1.48, 2.17, and 2.86 MPa) in a 0.155-m-inside diameter (ID) pipe inclined at 2 degrees from the horizontal. Isopar (TM) L oil and nitrogen gas were the working fluids. Liquid superficial velocities ranged from 0.01 to 0.05 m/s, while gas superficial velocities ranged from 1.5 to 16 m/s. Measurements included pressure gradient and liquid holdup. Flow visualization and wire-mesh-sensor (WMS) data were used to identify the flow patterns. Interfacial roughness was obtained from the WMS data. Three flow patterns were observed: pseudo-slug, stratified, and annular. Pseudo-slug is characterized as an intermittent flow where the liquid does not occupy the whole pipe cross section as does a traditional slug flow. In the annular flow pattern, the bulk of the liquid was observed to flow at the pipe bottom in a stratified configuration; however, a thin liquid film covered the whole pipe circumference. In both stratified and annular flow patterns, the interface between the gas core and the bottom liquid film presented a flat shape. The superficial gas Froude number, Fr-Sg, was found to be an important dimensionless parameter to scale the pressure effects on the measured parameters. In the pseudo-slug flow pattern, the flow is gravity-dominated. Pressure gradient is a function of Fr-Sg and liquid superficial velocity, v(SL). Liquid holdup is independent of v(SL) and a function of Fr-Sg. In the stratified and annular flow patterns, the flow is friction-dominated. Both pressure gradient and liquid holdup are functions of Fr-Sg and v(SL). Interfacial-roughness measurements showed a small variation in the stratified and annular flow patterns. Model comparison produced mixed results, depending on the specific flow conditions. A relation between the measured interfacial roughness and the interfacial friction factor was proposed, and the results agreed with the existing measurements.