Dynamics of a small bubble in a compressible fluid

An efficient (simplified) method for solving problems of spherically symmetric dynamics of a small gas bubble in a compressible fluid is proposed. The method is based on the joint use of the full problem statement (the gas dynamics equations for the gas and the fluid) and its relevant simplifications. Some approximate statements are discussed. In the proposed method, the rarefaction and compression of the gas during the slow motion of the bubble surface is assumed to be uniform over the bubble volume. At the same time the fluid in the thin zone adjacent to the bubble is considered to be slightly compressible. Otherwise the gas dynamics equations are used for the gas and the fluid. The dynamics of the fluid in the thick external zone are described by the linear acoustics only. The proposed simplified method and two others used in literature are estimated by comparison of their numerical results with those obtained in full statement. Copyright © 2000 John Wiley & Sons, Ltd.     

Direct 2D simulation of small gas bubble clusters: from the expansion step to the equilibrium state

A numerical strategy, based on an adaptive finite element method, is proposed for the direct two‐dimensional simulation of the expansion of small clusters of gas bubbles within a Newtonian liquid matrix. The velocity and pressure fields in the liquid are first defined through the Stokes equations and are subsequently extended to the gas bubbles. The liquid–gas coupling is imposed through the stress exerted on the liquid by gas pressure (ruled by an ideal gas law) and by surface tension. A level set method, combined with a mesh adaptation technique, is used to track liquid–gas interfaces. Many numerical simulations are presented. The single bubble case allows to compare the simulations to an analytical model. Simulations of the expansion of small clusters are then presented showing the interaction and evolution of the gas bubbles to an equilibrium state, involving topological rearrangements induced by Plateau's rule. Copyright © 2006 John Wiley & Sons, Ltd.     

The simulation of gas production from oceanic gas hydrate reservoir by the combination of ocean surface warm water flooding with depressurization

A new method is proposed to produce gas from oceanic gas hydrate reservoir by combining the ocean surface warm water flooding with depressurization which can efficiently utilize the synthetic effects of thermal, salt and depressurization on gas hydrate dissociation. The method has the advantage of high efficiency, low cost and enhanced safety. Based on the proposed conceptual method, the physical and mathematical models are established, in which the effects of the flow of multiphase fluid, the kinetic process of hydrate dissociation, the endothermic process of hydrate dissociation, ice-water phase equilibrium, salt inhibition, dispersion, convection and conduction on the hydrate dissociation and gas and water production are considered. The gas and water rates, formation pressure for the combination method are compared with that of the single depressurization, which is referred to the method in which only depressurization is used. The results show that the combination method can remedy the deficiency of individual producing methods. It has the advantage of longer stable period of high gas rate than the single depressurization. It can also reduce the geologic hazard caused by the formation deformation due to the maintaining of the formation pressure by injected ocean warm water.     

基于Boltzmann模型方程的气体运动论统一算法研究

模型方程出发,研究确立含流态控制参数可描述不同流域气体流动特征的气体分子速度分布函数方程; 研究发展气体运动论离散速度坐标法, 借助非定常时间分裂数值计算方法和NND差分格式, 结合DSMC方法关于分子运动与碰撞去耦技术, 发展直接求解速度分布函数的气体运动论耦合迭代数值格式; 研制可用于物理空间各点宏观流动取矩的离散速度数值积分方法, 由此提出一套能有效模拟稀薄流到连续流不同流域气体流动问题统一算法. 通过对不同Knudsen数下一维激波内流动、二维圆柱、三维球体绕流数值计算表明, 计算结果与有关实验数据及其它途径研究结果(如DSMC模拟值、N-S数值解)吻合较好, 证实气体运动论统一算法求解各流域气体流动问题的可行性. 尝试将统一算法进行HPF并行化程序设计, 基于对球体绕流及类``神舟'返回舱外形绕流问题进行HPF初步并行试算, 显示出统一算法具有很好的并行可扩展性, 可望建立起新型的能有效模拟各流域飞行器绕流HPF并行算法研究方向. 通过将气体运动论统一算法推广应用于微槽道流动计算研究, 已初步发展起可靠模拟二维短微槽道流动数值算法; 通过对Couette流、Poiseuille流、压力驱动的二维短槽道流数值模拟, 证实该算法对微槽道气体流动问题具有较强的模拟能力, 可望发展起基于Boltzmann模型方程能可靠模拟MEMS微流动问题气体运动论数值计算方法研究途径.      

Wave dispersion of a gas in a liquid

A novel method of dispersing a gas in a liquid by pressure pulses generated by waves propagating from a hydrodynamic oscillation generator is proposed. Devices that realize this method, wave dispersers, are created and investigated experimentally. The amplitude-frequency characteristics of the wave processes in the dispersers and the size distribution density functions of the gas bubbles are obtained. It is shown that there are optimal pressure values at the disperser inlet at which a minimum bubble size is achieved. The average diameter of the gas bubbles produced by water wave dispersers in the optimal operation regimes varied on the range from 0.3 to 0.6 mm, depending on the gas flow-rate.     

压裂页岩气藏多尺度耦合流动数值模拟研究

在碳达峰的国策背景之下,页岩气成为传统能源向绿色清洁低碳能源转型的重要过渡和能源支点.压后页岩气藏流体流动力学成为高效开发页岩气的关键力学问题.文章将小尺度低导流天然裂缝等效升级为连续介质,建立有机质-无机质-天然裂缝三重连续介质模型,同时对大尺度高导流裂缝采用离散裂缝模型刻画,嵌入天然裂缝连续介质中,构建多重连续/离散裂缝模型.综合考虑吸附气的非平衡非线性解吸附和表面扩散,自由气的黏性流和克努森扩散,给出页岩气在多尺度复杂介质中的非线性耦合流动数学模型.提出多尺度扩展有限单元法对离散裂缝进行显式求解,创新性构建三类加强形函数捕捉离散裂缝的局部流场特征,解决了压后页岩海量裂缝及多尺度流动通道的流动模拟难题.文章提出的模型和方法既能准确刻画高导流裂缝对渗流的影响,又克服了海量多尺度离散裂缝导致计算量增大的问题.通过算例展示了压后页岩各连续介质的压力衰减规律,发现裂缝中自由气、有机质中自由气、无机质中吸附气依次滞后的压力(浓度)扩散现象,重点分析了吸附气表面扩散系数、自由气克努森扩散系数、天然裂缝连续介质渗透率和吸附气解吸附速率对页岩气产量的影响.文章重点解决压后页岩多尺度流动通道的表征和...     

考虑气体压缩性的多孔材料渗透率和惯性系数的测定

多孔介质材料的渗透率和惯性系数是决定多孔介质中流体流动特性的重要参数，一般需要通过实验进行测定，在测定渗透律和惯性系数量，选用气体作为工作介质可以为实验带来极大的方便，然而通常的实验都将气体看作不可压缩流体，直接根据Darcy-Forchheimer定律得到这两个参数，这种近似对实验条件如样品厚度、工作压力等提出了很多要求，本文提出了在考虑气体压缩性的情况下测定渗透率和惯性系数的方法，该方法可以大大降低实验时对样品厚度、工作压力等条件的要求。本文还根据该方法对多孔材料PVF进行了渗透率和惯性系数的测定，并对测量结果进行了验证。     

Characteristics of Seepage and Diffusion in Gas Drainage and Its Application for Enhancing the gas utilization rate

Although gas drainage technology has greatly developed, gas concentration and utilization rates are still very low, resulting in substantial quantities of low concentration gas emissions in coal mine. To study the roles of suction pressure on gas drainage, a mathematical model is developed in this study for coupled gas migration and coal deformation based on a dual-porosity medium. The simulation results reveal the initial gas production is mainly contributed by the seepage in the fracture, and then the dominating factor rapidly transitions to diffusion which provides relatively stable gas production in most drainage time. In addition, the increase in gas production is tiny while the gas concentration clearly decreases because of air leakage as the suction pressure increases. Therefore, a concentration-based suction pressure regulating method is proposed to extend the time period of effective gas drainage and increase the gas utilization rate through adjusting the suction pressure of the bedding borehole. Field tests were performed to constrain the gas drainage process under different suction pressures, and the results gained verify the effectiveness and applicability of this method. This study proves that the concentration-based suction pressure regulating method may be a promising technology to realize safe, economical and efficient underground gas drainage in coal mines in the future.

     

Analytical Solutions to Gas Flow Problems in Low Permeability Porous Media

In most of conventional porous media the flow of gas is basically controlled by the permeability and the contribution of gas flow due to gas diffusion is ignored. The diffusion effect may have significant impact on gas flow behavior, especially in low permeability porous media. In this study, a dual mechanism based on Darcy flow as well as diffusion is presented for the gas flow in homogeneous porous media. Then, a novel form of pseudo pressure function was defined. This study presents a set of novel analytical solutions developed for analyzing steady-state and transient gas flow through porous media including effective diffusion. The analytical solutions are obtained using the real gas pseudo pressure function that incorporates the effective diffusion. Furthermore, the conventional assumption was used for linearizing the gas flow equation. As application examples, the new analytical solutions have been used to design new laboratory and field testing method to determine the porous media parameters. The proposed laboratory analysis method is also used to analyze data from steady-state flow tests of three core plugs. Then, permeability (k) and effective diffusion coefficient (D _e) was determined; however, the new method allows one to analyze data from both transient and steady-state tests in various flow geometries.     

Characterizing the Gas Permeability of Natural and Synthetic Materials

The objective of this article is to propose an experimental method to compare the gas permeability of all the different materials used as gas barrier, such as compacted clay liners or geomembranes. This method is based on the falling pressure experiment, allowing the determination of a single coefficient whatever the material tested. This coefficient is the time constant τ, which is obtained by analytical solutions of the simplified equations describing the transport of gas through the material. The domain of validity is specified for porous media, thanks to a numerical solution of the complete equations system. Two applications are presented, showing the applicability of the method on compacted clay liners and on high density polyethylene geomembranes.     

Simulation of Gas Dipole Tests in Fractures at the Intermediate Scale Using a New Upscaling Method

A high-level radioactive waste disposal site may lead to gas generation by different physical mechanisms. As these sites are to be located in areas with low water flow, any small amount of gas can lead to relative high gas pressures, so that multiphase flow analysis becomes relevant. The movement of gas and water through the system has two important implications. Firstly, water flow takes place in unsaturated conditions, and thus travel times of the radioactive particles transported are affected; and secondly, gas can also carry radioactive particles. Therefore, one of the key points in such studies is the time when gas would break through the biosphere under a number of different flow conditions. In fractured zones, gas would flow preferentially through the most conductive features. We consider a two-dimensional system representing an isolated fracture. In each point we assign a local porosity and permeability and a local pressure-saturation relationship. A dipole (injector-producer) gas flow system is generated and the variation in water saturation is studied. A simple method is proposed for obtaining upscaled values for several parameters involved in two-phase flow. It is based on numerical simulation on a block scale assuming steady-state conditions and absence of capillary pressure gradients. The proposed method of upscaling is applied to simulate a dipole test using a coarser grid than that of the reference field. The comparison between the results in both scales shows an encouraging agreement.     

数量化理论Ⅲ在瓦斯突出预测敏感地质指标筛选中的应用

论述了数量化理论及数量化理论Ⅲ的基本原理,提出了反应标度的概念和计算方法,扩展了原来反应取值的适用范围和科学性。以中马村矿区为研究实例,研究了影响煤与瓦斯突出的地质因素,并采用统计单元划分结合数量化理论Ⅲ的方法,从11个地质因素中筛选出8个瓦斯突出预测敏感地质指标,并据此对中马村矿已采区进行了瓦斯地质区划。实践表明:将数量化理论Ⅲ应用于瓦斯地质学是可行的,为瓦斯突出预测敏感地质指标的筛选提供了新的思路。     

深海天然气水合物机械?热联合开采方法研究综述

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. 经济高效的开采方法以及相关的灾害控制和环境保护是对天然气水合物进行商业化开采必须要解决好的两个关键问题. 目前, 注热法和降压法的联合使用被认为是最为有效的天然气水合物开采方法. 在降压法和注热法中, 天然气水合物开采涉及传热、相变、渗流和变形等物理过程和效应, 而传热最慢且相变会消耗大量的热量, 无法直接采用常规的单纯依靠渗流原理的油气开采方案来开采天然气水合物. 我国南海的天然气水合物主要赋存于粉砂质黏土和粉细砂等类型的沉积物中, 胶结性差且埋深较浅. 常规的开采方法还不适合我国南海的水合物开采, 需要考虑新型的开采方式, 这其中提高沉积层中的热传导效率是天然气水合物开采的关键. 郑哲敏提出了机械?热联合开采的新概念方法, 利用无穷无尽表层海水的热量, 基于对流传热的原理和管道输送技术, 并兼顾类似采煤挖掘可能导致的深海浅软地层安全问题. 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. 本文针对该新方法的能量、装备、经济可行性进行综合评估, 阐述了针对核心问题管道含相变气液固多相流动、地层安全方面的研究进展, 展望了未来推广应用的空间.      

A high‐precision unstructured adaptive mesh technique for gas–liquid two‐phase flows

Adaptive mesh techniques are used widely in the numerical simulations of fluid flows, and the simulation results with high accuracies are obtained by appropriate mesh adaptations. However, gas–liquid two‐phase flows are still difficult to be simulated on adaptive meshes, especially on unstructured adaptive meshes, because the physical phenomena near gas–liquid interfaces are highly complicated and in general, not modeled appropriately on adaptive meshes. In this paper, a high‐precision unstructured adaptive mesh technique for gas–liquid two‐phase flows is developed and verified/validated. In the unstructured adaptive mesh technique, the PLIC algorithm is employed to simulate interfacial dynamic behaviors and, therefore, the reconstruction method for the interfaces in refined cells is developed, which satisfies the gas and liquid volume conservations and geometrical conservations of interfaces. In addition, the physics‐based consideration is performed on the momentum calculations near interfaces, and the calculation method with gas and liquid momentum conservations is developed. For verification, the slotted‐disk revolution problem is solved. As a result, the unstructured adaptive mesh technique succeeds in reproducing the slotted‐disk shape accurately and well maintaining the shape after one full‐revolution. The dam‐break problem is also simulated and the momentum conservative calculation method succeeds in providing physically appropriate results, which show good agreements with experimental data. Therefore, it is confirmed that the developed unstructured adaptive mesh technique is very efficient to simulate gas–liquid two‐phase flows accurately. Copyright © 2010 John Wiley & Sons, Ltd.     

前进式U形通风采空区风流与自燃的数值模拟

基于渗流方程、多组分气体渗流-扩散方程和多孔介质渗流传热方程,建立前进式开采U型 通风采空区自然发火模型. 通过迭代方法计算沿空巷道的侧漏风量,以此边界条件数值模拟 计算出前进式开采U型通风采空区的漏风规律;根据瓦斯-Ohttp://d.wanfangdata.com.cn/Periodical_lxysj200804005.aspx前进式开采U型通风;采空区;风流规律;自燃温度场;有限元; 基于渗流方程、多组分气体渗流-扩散方程和多孔介质渗流传热方程,建立前进式开采U型 通风采空区自然发火模型. 通过迭代方法计算沿空巷道的侧漏风量,以此边界条件数值模拟 计算出前进式开采U型通风采空区的漏风规律;根据瓦斯-Ohttp://d.wanfangdata.com.cn/Periodical_lxysj200804005.aspx前进式开采U型通风;采空区;风流规律;自燃温度场;有限元; 基于渗流方程、多组分气体渗流-扩散方程和多孔介质渗流传热方程,建立前进式开采U型 通风采空区自然发火模型. 通过迭代方法计算沿空巷道的侧漏风量,以此边界条件数值模拟 计算出前进式开采U型通风采空区的漏风规律;根据瓦斯-Ohttp://d.wanfangdata.com.cn/Periodical_lxysj200804005.aspx前进式开采U型通风;采空区;风流规律;自燃温度场;有限元;基于渗流方程、多组分气体渗流-扩散方程和多孔介质渗流传热方程,建立前进式开采U型通风采空区自然发火模型.通过迭代方法计算沿空巷道的侧漏风量,以此边界条件数值模拟计算出前进式开采U型通风采空区的漏风规律;根据瓦斯-O2-CO浓度和温度分布状态及其变化来描述自燃过程.在计算中,采空区非均质性按自然冒落考虑,兼顾瓦斯涌出对自燃的耦合作用;模拟得到,在前进式开采特有的3面漏风边界条件下,采空区自燃位置(最高温点)在开切眼冒落区进风一侧,自燃产生的有害气体和瓦斯随漏风流从回风边界渗出.在前进开采U型通风形式下,通风量在到达工作面时衰减很大,因此,前进开采布置一次推进距离不易过大;解决好沿空留巷的封堵材料和正确的堵漏措施,是保证前进式开采顺利进行的技术关键.     

An implicit characteristic-flux-averaging method for the euler equations for real gases

A formulation of an implicit characteristic-flux-averaging method for the unsteady Euler equations with real gas effects is presented. Incorporation of a real gas into a general equation of state is achieved by considering the pressure as a function of density and specific internal energy. The Ricmann solver as well as the flux-split algorithm are modified by introducing the pressure derivatives with respect to density and internal energy. Expressions for calculating the values of the flow variables for a real gas at the cell faces are derived. The Jacobian matrices and the eigenvectors are defined for a general equation of state. The solution of the system of equations is obtained by using a mesh-sequencing method for acceleration of the convergence. Finally, a test case for a simple form of equation of state displays the differences from the corresponding solution for an ideal gas.     

Slip and Macroparameter Jumps of a Polyatomic Gas with Rotational Degrees of Freedom on a Weakly Curved Spherical Surface

The problem of the slip of a temperature-inhomogeneous polyatomic gas along a spherical surface of small curvature is solved. The solution is obtained using the half-space moment method on the basis of a previously proposed model kinetic equation which takes into account the rotational degrees of freedom of the polyatomic gas. Both the first- and second-order (in the Knudsen number) slip coefficients and the polyatomic gas macroparameter jump coefficients on the phase interface are obtained. These coefficients are given as functions of the tangential momentum accommodation coefficients, the translational and rotational energy accommodation coefficients, and the Prandtl number. The kinetic coefficients are calculated for certain polyatomic gases.     

Calculation of thermal conductivity of polar-nonpolar gas mixtures

Summary An approximate method, an empirical one, a semi theoretical one, and the procedure due to Lindsay and Bromley are examined for prediction of thermal conductivity of polar-nonpolar gas mixtures. With a modification of the approximate method we find, by analyzing experimental and calculated results of twelve different systems, that prediction of thermal conductivity is possible with an uncertainty of about 2%. This is important because experimental data on such gas systems are rare. The methods also permit computation of thermal conductivity at moderately high temperatures and for multicomponent mixtures.     

Numerical simulations of gas-dynamic conductivity of microchannels with allowance for their surface structure

The effect of the microchannel surface structure on the free-molecular gas flow is studied by the test-particle method. Simulations are performed for channels whose surface either is obtained by means of statistical modeling or is reconstructed from the data of atomic-force microscopy of real surfaces. Dependences of monochromatic molecular beam scattering on the angle of incidence and the average height of microscopic roughness elements on the surface are considered. It is demonstrated that the method developed allows one to obtain the distribution function for particles reflected from a rough surface and to use it in the boundary conditions in problems of rarefied gas dynamics.