Multiphase Flow Characteristics of Unconsolidated Hydrate Slurry in Gas–Water–Hydrate–Sand Systems

In this study, the gas–water–hydrate–sand four-phase flow characteristics were simulated using the Eulerian model and a user-defined function for hydrate particle diameter in fluent. The results indicate that (1) in the straight pipe section, hydrates concentrated in the middle and lower regions, while sand particles settled at the bottom. In the bend section, hydrates were mainly on the outer side, and sand particles were on the inner side. (2) In the straight pipe section, increasing flow velocity reduced hydrate concentration at the pipe center, promoting a more homogeneous cross-sectional distribution due to enhanced turbulent mixing and reduced particle clustering. However, in the bend section, centrifugal forces induced hydrate particle migration toward the outer wall, altering the distribution pattern and leading to localized accumulation. Along the entire pipeline, the maximum hydrate concentration increased with the gas volume fraction and was increasingly concentrated in the lower region of the pipe. (3) The concentration of sand particles at the pipe bottom decreased with increasing flow velocity and hydrate/gas volume fractions. This reduction was attributed to the displacement effects induced by hydrate and gas phases, which mitigated sedimentation and promoted a more uniform cross-sectional distribution. (4) The pressure drop per unit pipe length increased with rising flow velocity and hydrate volume fraction but decreased with gas volume fraction, with the most pronounced increase observed at 3 m/s, where the pressure drop was 6.24 times higher than at 1 m/s. This research provides a theoretical foundation for flow assurance technology in deep-sea hydrate extraction.