Prediction of slug-liquid holdup for high-viscosity oils in upward gas/liquid vertical-pipe flow

Slug-liquid holdup is a critical slug-flow parameter, which affects average liquid holdup and pressure gradient in pipes. Most experimental slug-liquid-holdup studies in the literature were conducted either by use of low-viscosity liquid for all inclination angles or high-viscosity liquid for horizontal and slightly inclined pipes, indicating a lack of experimental data for vertical flow of high-viscosity liquid. Therefore, the objective of this study is to experimentally and theoretically investigate the effect of oil viscosity on slug-liquid holdup in gas/liquid upward vertical flow, and to develop a new closure model to predict slug-liquid holdup in vertical pipes. In this study, experiments were conducted in a 50.8-mm inner-diameter (ID) vertical pipe for six oil viscosities: 586, 401, 287, 213, 162, and 127 mPa·s. A new slug-liquid-holdup closure model derived from Froude and inverse viscosity numbers was developed in this study for high-viscosity-liquid two-phase upward vertical flow. The proposed model was validated against independent experimental data and showed excellent prediction for high-viscosity data. Furthermore, the proposed model was compared with existing models that take into account the viscosity effects showing better performance. The new model was incorporated in the Tulsa University Fluid Flow Projects (TUFFP) unified model (all versions; Zhang et al. 2003b), improving the prediction of pressure gradient and average liquid holdup for high-viscosity upward vertical flow.