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垂直管道中含天然气水合物相变的气液固多相流动
Alternative TitleGas–liquid–solid Flow with Gas Hydrate Dissociation in a Vertical Pipe
李鹏
Thesis Advisor鲁晓兵 ; 张旭辉
2019-05-27
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline工程力学
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
Abstract

    天然气水合物由于储量大、污染低等优点,已成为我国非常重要的战略能源,世界各国也加快了天然气水合物的勘探和开发工作。天然气水合物的试验性开采表明降压、注热等方法难以满足商业化开采的需求,经济高效的开采方案已成为天然气水合物资源开发利用的瓶颈问题。天然气水合物机械-热联合开采法是一种新的概念模式,具有开采可控、高效且能有效降低地层安全性风险的优点,而垂直管道中含水合物相变的多相流输运过程是机械-热联合开采方法中的关键环节之一,有必要对其进行深入的研究。

       本文的研究工作主要是针对机械-热联合开采法,重点研究含天然气水合物的沉积物颗粒和水的混合物在垂直管道中的流动问题。研究手段以数值模拟为主,同时结合物理实验和理论分析。首先提出了一个考虑多相流动对分解速率影响的沉积物中水合物的分解模型;考察了垂直管道中流动从初始固液两相流到气液固三相流的转变,得到了流动过程中浓度、速度、温度和分解速率的分布规律;发现了管道内存在水合物分解的平衡高度,并给出了定量表达式;在上述分析的基础上提出了含水合物分解时流型变化的判定依据和管道输送最优工况的设计原则。本文得到的主要结论如下:

 (1)  提出了含水合物沉积物颗粒在水流条件下的分解模型

        首先基于水合物的分解速率与剩余水合物的量成正比的假设,提出了水合物分解速率模型,其中考虑了变化的温度、压力和颗粒浓度对分解速率的影响;然后研制了一套相应的水合物沉积物合成与分解实验装置,制备球形的含水合物沉积物颗粒(包括水合物、水和土);再者观测土球在水中对流传热条件下的分解过程并记录土球完全分解完毕所需要的时间;最后基于观测数据得到了水合物分解的本征速率模型。

(2) 气液固三相流动与水合物分解的相互作用研究

        利用FLUENT软件,采用其中的欧拉多相流模型来模拟气液固三相流动,考虑相间作用以及传热效应,并考虑颗粒间的碰撞;采用(1)中提出的水合物的分解模型来模拟其分解,实现了管道中流动从固液两相流到含相变的气液固三相流的全过程分析;固体颗粒由水合物、水和砂土组成,分析了水合物分解导致的固体颗粒内部组分的变化规律。研究表明,多相流动所造成的管道中速度场和浓度场的变化会对水合物的分解产生明显的影响,反之亦然。通过本文的研究对两者之间的相互作用机理有了较清晰的认识。

 (3) 提出了连续进料条件下天然气水合物管道输送最优工况设计原则

       首先通过量纲分析的方法给出了控制整个问题的无量纲参数,然后定量分析了管内水合物颗粒的分解平衡高度及总产气量变化,探索了管道内多相流动的流型变化及相应的流动稳定性条件。从分解平衡高度、产气量和流型变化三个角度综合考虑,提出了最优工况中各个参数的设计依据。

Other Abstract

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.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79088
Collection流固耦合系统力学重点实验室
Recommended Citation
GB/T 7714
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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