Study on three-phase flow characteristics of natural gas hydrate pipeline transmission

Based on the basic principles of thermodynamics and fluid mechanics, the temperature and pressure model of natural gas hydrate hydraulic lifting pipeline, the hydrate decomposition mass transfer model and the mathematical model of pipeline multiphase flow are established. The relationship between the back pressure of the pipeline outlet, the decomposition surface and the depth of the seawater was analyzed. On this basis, the numerical simulation and experimental verification of natural gas hydrate pipeline transmission were carried out. The effects of pipe diameter, inlet velocity and solid phase parameters on the three-phase flow of natural gas hydrate pipelines were analyzed. The result shows that with the outlet pressure increasing, the position at which the hydrate begins to decompose moves up and the decomposition rate slows down. As the depth of mining increases, the rate of hydrate decomposition slows and the position at which decomposition begins to move up. In deep sea mining below 1500 m, the increase of mining depth has little effect on the decomposition rate of hydrate, and the decomposition starting position is always near 510 m in sea depth. The pipe pressure loss gradient decreases as the pipe diameter increases. After the pipe diameter is larger than 300 mm, the pressure loss gradient decreases slowly, and the pressure loss gradient drops sharply at 450 mm, and eventually stabilizes after 500 mm. When the inlet velocity of the pipe is low, the fluid flow in the pipe is unstable and reflow occurs. The pipe pressure loss gradient first decreases and then increases as the fluid flow rate increases, and there is an optimal flow rate value that minimizes the pressure loss gradient.