CFD-PBM Numerical Simulation Of Gas–Liquid–Solid Flow Characteristics In Natural Gas Hydrate Transportation Pipelines

Natural gas hydrate is considered as one of the new clean energy resources with the greatest potential in the 21 st century due to its high energy density,large resource and good cleanliness.Therefore,it has important environmental,resource and strategic significance for the exploration,exploitation and utilization of natural gas hydrate.Since 2011,China has carried out five trial mining operations in Muli area of Qilian Mountains and Shenhu area of South China Sea,and achieved a major breakthrough.However,there are still a series of technical,economic and environmental problems to be solved in order to realize the safe exploitation and efficient commercial utilization of natural gas hydrate.Among them,pipelines transportation of the extracted natural gas hydrate is one of the key links.Based on the CFD-PBM coupling model and particle dynamics theory,this paper uses the open source CFD software Open FOAM to study the gas-liquid-solid flow characteristics in gas hydrate pipelines.The main research results are as follows.(1)Based on the CFD-PBM coupled model and particle dynamics theory,and considering the coalescence and breakage behavior of bubbles and hydrate particles,the interphase force model,turbulence model,and the coalescence and breakage rate model of bubbles and hydrate particles are selected.A numerical model of gas-liquid-hydrate pipeline transportation is established.The selected model is added to the Open FOAM solver,and the model is verified with the relevant experimental data.The results show that the established model and the adopted solution process are reasonable and effective,and meet the accuracy requirements.(2)The established model is used to simulate the three-phase flow characteristics of gas-liquid-hydrate in the vertical pipeline,the effects of gas phase volume fraction,hydrate volume fraction,inlet velocity and hydrate particle density on the flow characteristics were analyzed.The results show that with the increase of the gas phase volume fraction,air masses of different sizes will be formed in the tube,which makes the velocity distribution and concentration distribution of each phase differ greatly along the radial direction,the particle size of hydrate particles decreases,and the pressure loss gradient decreases.With the increase of hydrate volume fraction,the particle size of hydrate particles increases significantly,and the pressure loss gradient first increases and then decreases.The radial distribution curve of the concentration of each phase is flatter at high flow rate.The bubble size increases with the increase of the flow rate,and the hydrate particle size decreases with the increase of the flow rate.As the density of hydrate particles increases,the flow velocity distribution of each phase becomes more uniform,and the particle size of hydrate particles increases.(3)The three-phase flow characteristics of gas-liquid-hydrate in pipelines with different diameters and inclination angles were simulated and studied,and the influence of the inner diameter and inclination angle of pipelines on the flow characteristics was analyzed.The results show that in the range of small pipe diameter,the pressure loss gradient is larger,and the pressure loss gradient fluctuates more violently along the axial direction.With the increase of the pipe diameter,the pressure loss gradient decreases,the flow velocity of each phase in the pipe fluctuates greatly along the radial distribution,and the gas phase concentration at different positions on the radial section of the pipe will fluctuate greatly.When the pipe diameter reaches 400 mm,the particle size of bubbles and hydrate particles increases significantly,and the gas phase concentration distribution is more uniform.The flow rates of liquid phase and hydrate in the inclined pipeline are no longer exactly the same,and the distribution of the three-phase concentration of gas-liquid-hydrate in the radial direction is quite different.With the increase of the inclination angle,the particle size of the hydrate particles first decreased and then increased,and the pressure loss gradient decreased,and the magnitude of the decrease increased with the increase of the inclination angle.