|Alternative Title||Gas–liquid–solid Flow with Gas Hydrate Dissociation in a Vertical Pipe|
|Thesis Advisor||鲁晓兵 ; 张旭辉|
|Place of Conferral||北京|
|Keyword||天然气水合物 机械-热联合开采 水合物分解 气液固多相流 最优工况|
Gas hydrates (GH) is a kind of important strategic energy resource in China due to its tremendous reserve and small contamination compared to traditional fossil fuels. Many countries have accelerated the exploitation and research of GH. The trial exploitation of GH revealed that the methods such as depressurization and thermal injection are difficult to meet the demand for the exploitation efficiency in commercial exploitation. The economical and efficient exploitation is the bottleneck in the GH exploitation. The combined mechanical-thermal method for GH exploitation is a new concept, which has the advantages of high efficiency and controllability and meanwhile the safety of formation can be reduced effectively. The pipe transportation of gas hydrate-bearing sediments (GHBS) is one of the key problems in the mechanical-thermal method and so important to study.
The study in this dissertation mainly focuses on the flow of mixture of sediment particles containing GH and water in a vertical pipe which is a vital problem of the combined mechanical-thermal method for GH exploitation. The main research method was numerical simulation, combined with physical experiments and theoretical analysis. A kinetic model for GH dissociation in the GHBS particles was first presented, considering the influence of multiphase flow on the dissociation rate. The transition of flow from the initial liquid-solid two-phase flow to gas-liquid-solid three-phase flow and the distribution of the phase volume fraction, velocity, temperature and dissociation rate were analyzed. The equilibrium height of GH dissociation in the pipe was discovered, and the quantitative expression for dissociation equilibrium height of the pipe was obtained. On this basis, the criterion of flow pattern evolution and the design principle of optimal condition were proposed. The main achievements are as follows:
(1) The dissociation rate model of GHBS particles considering water flow
An intrinsic dissociation model was presented based on the assumption that the dissociation rate of the GHBS particle is exponential with the concentration of remaining hydrate. The model considered the influences of changing temperature, pressure and the particle concentration on the dissociation rate. Then an apparatus for measuring the model parameters was developed and the spherical GHBS particles (containing water, GH and sand) were made. The dissociation process of soil particles under convective heat transfer conditions in water was observed and the time required for the particle to dissociate completely was recorded. Finally the dissociation model was obtained by fitting the experimental data.
(2) Interaction of gas-liquid-solid three-phase flow and hydrate dissociation
The Eulerian multiphase flow model was used to simulate the gas-liquid-solid three-phase flow within the CFD software FLUENT, taking into account the phase interaction, the heat transfer, and the collision among particles. The dissociation model of GH presented in (1) was used to model GH dissociation. During the transportation of GHBS particles in the pipe, the initial solid-liquid two-phase flow gradually changes to the gas-liquid-solid three-phase flow due to GH dissociation. The GHBS particles consist of GH, water and sand. The evolution of internal components of solid particles during the dissociation was analyzed. The simulated results show that the hydrodynamics behaviors in the pipe cause the convective heat transfer among the liquid and the solid particles, accelerating the GH dissociation process, which in turn leads to a significant change in the gas-liquid-solid three-phase flow.
(3) Design priciples for optimal condition of the pipe transportation of GH under continuous import of GHBS particles
The essential dimensionless numbers controlling the problem were deduced. The dissociation equilibrium height (the height of particles in the pipe after GH totally dissociation) and total gas production of GHBS particles in the pipe were quantitatively analyzed. The flow pattern evolution of multiphase flow and the corresponding flow stability conditions in the pipe were explored. Considering the dissociation equilibrium height, gas production and flow pattern evolution, the design principle of each parameter in the optimal condition was proposed.
|李鹏. 垂直管道中含天然气水合物相变的气液固多相流动[D]. 北京. 中国科学院大学,2019.|
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