Experimental study on holdup and pseudo-slug flow pattern in horizontal and slightly inclined dense-gas/liquid flow

Abstract The formation of dense gases is driven by high pressures, critical temperatures, and high content of CO₂, conditions characteristic of ultra-deepwater production environments such as the Brazilian pre-salt. This study presents an experimental investigation of horizontal and slightly upward inclined dense-gas and liquid two-phase flow under hydrodynamic conditions representative of these scenarios. Due to the lack of experimental data in such regimes, the accuracy of existing predictive models remains limited, potentially leading to economic losses and environmental or safety risks. Experiments were conducted using a high-pressure inclinable loop equipped with a 30 m pipeline, employing pressurized sulfur hexafluoride (SF₆) as the gas phase and mineral oil as the liquid phase. The test section was configured at 0°, 5°, and 10°, and 125 experimental points were collected, including pressure, temperature, flow, holdup, and flow pattern data. Two-phase flow visualization was carried out using a high-speed camera, while liquid holdup and phase fraction distribution were determined with a collimated gamma-ray densitometer. The observed flow patterns included stratified smooth, stratified wavy, stratified wavy with mixing at the interface, slug, pseudo-slug, dispersed, and the rarely reported dual-continuous pattern. Notably, the dual-continuous flow pattern was identified for the first time under upward inclined conditions (5° and 10°). The experimental results demonstrate that pipe inclination and dense gas velocity are key factors in the transition between slug and pseudo-slug flow patterns. Additionally, interfacial instabilities and liquid splashing were observed at high dense-gas velocities. These findings address important knowledge gaps and support the development of more accurate predictive models for multiphase flow in complex production scenarios.