Flow pattern and holdup phenomena of low velocity oil-water flows in a vertical upward small diameter pipe

This paper is dedicated to the investigation of flow pattern and holdup phenomena in low velocity oil-water flows through a vertical upward pipe with 20 mm inner diameter (ID). Four typical flow patterns, namely dispersed oil-in-water slug flow (D OS/W), dispersed oil-in-water flow (D O/W), very fine dispersed oil-in-water flow (VFD O/W) and transition flow (TF) were detected using a radial dual-sensors conductance probe array. We compared our experimental flow pattern boundaries with those from published physical models. To further explore the local flow structures of different flow patterns, we extracted probability density function (PDF) of oil droplet size across pipe section. In addition, slippage effect was evaluated by water holdup measurement using quick-closing valve (QCV). It was found that slippage effect generally weakens with increasing water-cut and mixture velocity. Finally, some typical drift-flux models were used to calculate water holdup, which was further compared with the measured data from published literatures. The results show that flow pattern-based modified drift-flux models proposed in the present study perform well on water holdup prediction for small ID pipes, while drift-flux models proposed by Flores et al. present superiorities on predicting water holdup in large ID pipes.