Proposing a novel combined cycle for optimal exergy recovery of liquefied natural gas

The effective utilization of the cryogenic exergy associated with liquefied natural gas (LNG) vaporization is important. In this paper, a novel combined power cycle is proposed which utilizes LNG in different ways to enhance the power generation of a power plant. In addition to the direct expansion in the appropriate expander, LNG is used as a low-temperature heat sink for a middle-pressure gas cycle which uses nitrogen as working fluid. Also, LNG is used to cool the inlet air of an open Brayton gas turbine cycle. These measures are accomplished to improve the exergy recovery of LNG. In order to analyze the performance of the system, the influence of several key parameters such as pressure ratio of LNG turbine, ratio of the mass flow rate of LNG to the mass flow rate of air, pressure ratio of different compressors, LNG pressure and inlet pressure of nitrogen compressor, on the thermal efficiency and exergy efficiency of the offered cycle is investigated. Finally, the proposed combined cycle is optimized on the basis of first and second laws of thermodynamics.     

液化天然气储罐混凝土穹顶的冲击破坏模式与机理

为获得大型全容式LNG储罐混凝土穹顶结构在圆柱形刚体冲击作用下的动力响应、失效模式以及失效机理,基于160 000 m³的LNG储罐,应用ANSYS/LS-DYNA建立LNG储罐精细化有限元数值模型,并通过对弹丸冲击混凝土靶板实验的数值模拟,验证了有限元模拟方法及材料模型的适用性。通过分析圆柱体冲击物撞击下LNG储罐的动力响应,提出储罐穹顶结构的3种失效模式即局部凹陷、混凝土剥落、击穿破坏,并根据冲击过程中能量的传递特征揭示了每类失效模式对应的失效机理。最后通过大量参数分析获得不同冲击物直径、冲击位置、冲击角度对LNG储罐结构的最大冲击响应及失效模式的影响规律。结果表明,冲击角度、冲击物直径对LNG储罐混凝土外罐穹顶的失效模式影响较大,冲击位置对储罐穹顶失效模式的影响较小,可以忽略。

     

LNG耐超低温柔性管道研究进展综述

LNG (液化天然气)耐超低温柔性管道是开采、运输、存储LNG过程中的关键装备之一, 被誉为是LNG外输系统的“血管”. 近年来, 随着LNG的开发逐渐由近海走向深远海, 耐超低温柔性管道作为LNG外输系统中的核心输运装备迎来了更加广阔的发展前景, 同时也面临着由更加严苛的海洋环境带来的结构失效的挑战. 本文针对LNG耐超低温柔性管道的工程应用背景、结构设计、内流分析等方面进行了调研与综述, 总结了LNG耐超低温柔性管道上述各项技术的研究进展. 分析了LNG耐超低温柔性管道的波纹管状结构、螺旋缠绕结构和高分子材料的柔顺性结构特征的力学机理, 总结了实现柔顺性结构的方法, 梳理了LNG耐超低温柔性管道管内流体计算分析的规律, 并对LNG耐超低温柔性管道相关技术的未来研究热点提出了展望. 我国在LNG耐超低温柔性管道相关技术的研究工作中起步相对较晚, 突破LNG耐超低温柔性管道的结构设计分析与工业应用中的关键力学问题, 实现LNG耐超低温柔性管道的国产化研制, 对于实现我国深远海天然气资源开发的“卡脖子”技术的自主可控, 助力“碳达峰”国家战略目标的实现具有重要意义.      

Numerical simulation of liquefied fuel spills: II. Instantaneous and continuous LNG spills on an unconfined water surface

A simple numerical model based on the shallow water equations in radial symmetry is used to simulate both instantaneous and continuous spills of liquefied natural gas (LNG) onto a water surface. Using the computed results, a study is made of the similarities and differences in the pool structure resulting from the two types of spills. For instantaneous spills a relation linear on a logarithmic plot is suggested between the maximum pool size and the spill volume. The effects of shear forces and surface cohesivity on the evolution of the spill are also examined.     

开敞空间液化天然气泄漏低温扩散及爆炸传播规律

低温可导致人员冻伤及物品脆裂,气体爆炸传播规律是爆炸演化过程和事故分析的基础。采用数值模拟方法,研究液化天然气大面积泄漏汽化过程、甲烷与空气混合过程及爆炸传播过程。结果表明:随着扩散距离的增大,低温区域的温度谷值升高,且升高趋势变缓;在距泄漏源中心110 m范围内,温度低于273 K;随着风速的增加,温度谷值呈线性下降;随着泄漏时间的延长,温度谷值降低,且下降趋势变缓;随着距泄漏中心距离的增加,爆炸后超压峰值先升高后降低;在距泄漏源中心200 m范围内,爆炸产生的高温会对人员造成伤害。     

Sloshing

本文列举了诸多工程领域中的液体共振运动现象,详细探讨了船舱中伴有剧烈流动的晃荡问题.描述了基于理论分析的非线性多模态方法,该方法便于波动稳定性分区、多分支解和物理稳定性的研究.强调了方形舱、垂向圆柱舱以及球形舱内伴有旋转和混沌（不规则波动）的三维流动的重要性.晃荡引起的砰击涉及到各种各样的内流条件,这些条件随液体深度与舱体长度之比而变化.针对棱柱状LNG舱,讨论了许多与流体力学和热力学参数、影响砰击载荷效应的水弹性以及模型实验缩尺比的物理现象.     

Numerical simulation of liquefied fuel spills: I. Instantaneous release into a confined area

The shallow-water equations in radial symmetry are solved numerically to simulate the collapse of a cylindrical liquid column into an area surrounded by a concentric dike. The following three subcases of this problem are considered: a liquid column collapsing onto a layer of the same liquid, a liquid column collapsing onto a solid surface, and a column of lighter liquid collapsing onto a heavier liquid (i.e. liquefied natural gas (LNG) spilled onto water). The results for the three categories are compared and the differences and similarities between them are analysed.     

Structural failures precipitated by weld discontinuities

A literature review was conducted to assess the effects of weld defects upon the failure of welded structures. This effort was focussed on storage tank failures with emphasis on those fabricated for cryogenic liquid containment in order to assess the significance of past storage tank failures relative to the 9% nickel steel storage tanks currently in service. Consideration of past failures can be instructive for insuring the integrity of these vessels.Along with other documented failures, three cryogenic storage tank failures are documented herein. One of these was a liquefied natural gas (LNG) tank, the other two were designed to store liquid ethylene. Although details of design differed slightly, the overall design was the ‘tank within a tank’ concept. In each of the three failures detailed in this review, a breach of the outer tank integrity resulted when cryogenic liquid came in contact with the outer tank wall. This can be attributed to the fact that the outer steel wall was brittle at service temperatures.In addition to the above failures, this review has revealed a pattern of failures in pressure vessels resulting from faulty welds (usually fillet welds) associated with nozzle attachments and branch connections. These are undoubtedly areas of concern in cryogenic storage tanks along with the ones delineated more fully in this review.     

晃荡

本文列举了诸多工程领域中的液体共振运动现象,详细探讨了船舱中伴有剧烈流动的晃荡问题.描述了基于理论分析的非线性多模态方法,该方法便于波动稳定性分区、多分支解和物理稳定性的研究.强调了方形舱、垂向圆柱舱以及球形舱内伴有旋转和混沌(不规则波动)的三维流动的重要性.晃荡引起的砰击涉及到各种各样的内流条件,这些条件随液体深度与舱体长度之比而变化.针对棱柱状LNG舱,讨论了许多与流体力学和热力学参数、影响砰击载荷效应的水弹性以及模型实验缩尺比的物理现象.     

A meshless numerical wave tank for simulation of nonlinear irregular waves in shallow water

Time domain simulation of the interaction between offshore structures and irregular waves in shallow water becomes a focus due to significant increase of liquefied natural gas (LNG) terminals. To obtain the time series of irregular waves in shallow water, a numerical wave tank is developed by using the meshless method for simulation of 2D nonlinear irregular waves propagating from deep water to shallow water. Using the fundamental solution of Laplace equation as the radial basis function (RBF) and locating the source points outside the computational domain, the problem of water wave propagation is solved by collocation of boundary points. In order to improve the computation stability, both the incident wave elevation and velocity potential are applied to the wave generation. A sponge damping layer combined with the Sommerfeld radiation condition is used on the radiation boundary. The present model is applied to simulate the propagation of regular and irregular waves. The numerical results are validated by analytical solutions and experimental data and good agreements are observed. Copyright © 2008 John Wiley & Sons, Ltd.     

MPS与GPU结合数值模拟LNG液舱晃荡

液舱晃荡是一种在外部激励作用下部分装载的液舱内液体的波动现象,它会对液舱结构强度和运输船舶稳性产生危害.移动粒子半隐式法(moving particle semi-implicit,MPS)是一种典型的无网格粒子类方法,可以有效地模拟剧烈的液舱晃荡问题.但MPS方法存在计算效率低的缺点,难以模拟大规模三维问题,而GPU并行加速技术已广泛应用于科学计算领域.因此,本文将MPS方法与GPU并行加速技术相结合,采用CUDA程序语言编写,自主开发了MPSGPU-SJTU求解器,对三维液化天然气(liquefiednatural gas, LNG)型液舱晃荡进行了数值模拟.通过三种不同粒子间距的数值模拟,验证了求解器的收敛性,其中最大计算粒子数达到了200多万.与其他研究结果相比,MPSGPU-SJTU求解器能够准确地预测壁面砰击压力,并且捕捉晃荡过程中自由面的大幅度变形和强非线性破碎现象.相比CPU求解器的计算时间,GPU并行加速技术可以大幅度地减小计算时长,提高MPS方法的计算效率.本文将LNG型液舱与方型液舱的晃荡进行对比,结果表明在高充液率下LNG型液舱可以有效地减小晃荡幅值和壁面砰击压力.但在中低充液率下,LNG型液舱则会加剧晃荡,自由面呈现明显的三维特征.本文还进一步研究了水和LNG两种不同介质的液舱晃荡现象,数值模拟结果表明二者的流场基本相似,砰击压力则正比于液体密度.      

Direct numerical simulation of turbulent flow with evaporating droplets at high temperature

Direct numerical simulation (DNS) is used to investigate turbulent flows with evaporating fuel droplets. For the solution of the carrier gas fluid, the Eulerian method is employed, while for fuel droplets, the Lagrangian method is used. The two-way coupling interactions between the carrier fluid and the fuel droplets are described by the mass, momentum and energy transfers. Direct numerical simulation is performed by a compressible code, named S3D. In this paper, the effects of evaporating and non-evaporating droplets on isotropic turbulent flows are investigated. From the simulations it is found that for the case without evaporation, the inclusion of small droplets suppresses the turbulence, while evaporation usually enhances turbulence at later times for higher mass-loading ratios.     

Numerical simulation of terrain-induced severe slugging coupled by hydrodynamic slugs in a pipeline–riser system

A numerical study based on a one-dimensional two-fluid model is carried out to describe the transient hydrodynamic slugging and terrain-induced severe slugging in a pipeline–riser system. The system of equations is rendered well-posed by interfacial pressure model for the riser. The selected flow conditions are restricted in the well-posed region for the horizontal and the downward inclined pipes to ensure the hydrodynamic slug characteristics are predicted correctly. The validity of the model is examined by water faucet problem and horizontal slug flow experiments. Simulations with and without slug capturing are conducted to address the effect of hydrodynamic slugs on severe slugging. It has been found that more accurate predictions are obtained by taking hydrodynamic slugs into account. At low superficial gas velocity, the simulation without slug capturing tends to overestimate the severe slugging period. When hydrodynamic slugs are captured, the upstream gas expansion is suppressed by the hydrodynamic slugs. At relatively high superficial gas velocity, the simulation without slug capturing tends to underestimate the severe slugging period. When hydrodynamic slugs are captured, the upstream compressible volume is greatly enlarged by the blowout of the hydrodynamic slugs. In both situations, the influences of the hydrodynamic slugs can reduce the errors of the predicted severe slugging characteristics.     

Vortex-induced vibrations of two mechanically coupled circular cylinders with asymmetrical stiffness in side-by-side arrangements

Vortex-induced vibrations of two mechanically coupled circular cylinders with asymmetrical stiffness in side-by-side arrangements are numerically investigated in a uniform flow at a low Reynolds number of 100. The oscillation system is restricted to the cross-flow direction, giving rise to a coupled two-degree-of-freedom response. Attention is placed on the two cylinders with a center-to-center gap ratio of 4 and a mass ratio of 10. The flow dynamics are described by the two-dimensional incompressible Navier–Stokes equations and resolved by the Characteristic-Based-Split finite element method. The stiffness of the first spring that connects the lower cylinder to the wall is chosen such that the vortex-induced vibration of the associated single cylinder with the same stiffness undergoes a pre-synchronization (state A), synchronization (state B) and post-synchronization (state C), respectively. In each state, the stiffness of the second spring connecting the lower and upper cylinders is varied to cover both synchronization and de-synchronization regimes. Numerical results show that the mechanically coupled system locks on the first-mode natural frequency in state A, while on the second-mode natural frequency in states B and C. In such a lock-in regime, the amplitude ratios of the two oscillating and coupled cylinders collapse well onto the corresponding first or second free-vibration mode. The overall coupling mechanism is further explained in terms of the hydrodynamic coefficients, frequency characteristics, wake patterns and effective added mass, quantifying the associated fluid-structure interactions against those governing a single-degree-of-freedom, single-cylinder system.     

Hydroelastic Effects of the Camber Ratio on A Ducted Marine Propeller in A Wake Flow

The effects of the camber ratio on the hydrodynamic and structural behaviors of a NACA-based ducted marine propeller in the wake flow behind an underwater axisymmetric body are numerically studied by computational fluid dynamics methods, in particular, the finite element method. The results are presented in terms of the efficiency, deflection, pressure coefficient, and natural frequencies. It is shown that the wake flow strongly affects the performance of the selected propulsion system. It is shown that the distributions of the camber ratio over the blades of the propeller nonlinearly changes its resistance against cavitation occurrence and deflection, and also changes its hydrodynamic performance and vibrational behavior.     

Resonant frequencies in an elevated spherical container partially filled with water: FEM and measurement

In this paper, a numerical-experimental study of the overall dynamical response of elevated spherical tanks subjected to horizontal base motion is presented. The main objective is to gain insight in the physical response of this particular structural typology widely used in the petrochemical industry as liquefied petroleum gas (LPG) containers. In order to identify the natural frequencies of the modes that mainly contribute to the response, experimental free vibration tests on an elevated spherical tank model for different liquid levels were carried out. Next, a numerical model that takes into account the coupling between fluid and structure was developed and validated against the experimental results. A very good agreement between experimental and numerical results was obtained. The results obtained show the influence of liquid levels on natural frequencies and indicate that the sloshing has a significant effect on the dynamical characteristics of the analyzed system. In order to obtain a good representation of the overall dynamical behaviour of the system by means of a simplified lumped mass model, a minimum of three masses is suggested. Finally, appropriate names of these three masses are proposed in the present paper.     

Modeling the electrostatic effect on the hydrodynamic behavior in FCC risers: From understanding to application

A CFD simulation was proposed to investigate the electrostatic effect on the hydrodynamic behavior of turbulent gas–solid flow in FCC risers. The simulation was first verified using the open experimental data with expected electrostatic effects observed in FCC risers. The influences of several operating parameters on the degree of electrification in FCC risers were analyzed, such as surface charge densities, pressure, gas velocity. It was noted that the gas velocity played a highly significant role compared with solid flux, while the effect of pressure was relatively weak. Further analysis showed that a much stronger electrostatic effect was found in small-scale FCC risers than their large-scale counterparts, and in addition, the major regions affected by the electrostatic charge depend on the scale of the riser. Finally, an external electric field was applied to optimize the flow field distribution in the FCC riser. The results of the electrostatic effects on the hydrodynamic behaviors in FCC risers are of great use in providing a reference for the optimization of FCC risers and their scaling.     

充液管道系统的模态分析

考虑管道与液体之间的泊松耦合与连接耦合,推导了低频情况下的充液直管轴向、横向振动传递矩阵与弯管单元的传递矩阵,引入了弯管弯曲因子,对－“L”形充液管系进行了研究,使用ANSYS有限元分析软件与传递矩阵法计算了两种边界条件下充液、充所管系的模态频率与模态振型,结果表明,气体与管道之间的耦合作用对管系模态的影响可忽略;而对充液管系而言,因液耦合作用不能忽略。     

Numerical simulation of a supersonic gas–solid flow over a blunt body: The role of inter-particle collisions and two-way coupling effects

A supersonic dusty gas flow over a blunt body is considered. The mathematical model of the two-phase gas–particle flow takes into account the inter-particle collisions and the two-way coupling effects. The carrier gas is treated as a continuum, the averaged flow field of which is described by the complete Navier–Stokes equations with additional source terms modeling the reverse action of the dispersed phase. The dispersed phase is treated as a discrete set of solid particles, and its behavior is described by a kinetic Boltzmann-type equation. Particles impinging on the body surface are assumed to bounce from it. Numerical analysis is carried out for the cross-wise flow over a cylinder. The method of computational simulation represents a combination of a CFD-method for the carrier gas and a Monte Carlo method for the “gas” of particles. The dependence of the fine flow structure of the continuous and dispersed phases upon the free stream particle volume fraction α_p∞ and the particle radius r_p is investigated, particularly in the shock layer and in the boundary layer at the body surface. The particle volume fraction α_p∞ is varied from a negligibly low value to the value α_p∞ = 3 × 10⁵ at which inter-particle collisions and two-way coupling effects are simultaneously essential. Particular attention has been given to the particles of radii close to the critical value r_p1, because in this range of particle size the behavior of the particles and their effect on the carrier gas flow are not yet completely understood. An estimate of the turbulent kinetic energy produced by the particles in the shock layer is obtained.     

饱和砂土液化研究进展

首先对20世纪90年代以前有关饱和砂土液化的研究工作进行了简要回顾,然后重点对近期液化的离心机实验研究、液化引起的地面大位移及对结构物的破坏等进行了综述.同时也对液化的判别方法、粉砂液化、静态液化等进行了评述.认为今后的研究重点是液化引起的地面大位移及对结构物的破坏等,其中涉及到结构与土的耦合问题、流变体(液化区域内的砂土)和固体(未液化区域的砂土)耦合及演变的问题,需要多学科(岩土力学、数值分析、流体力学、结构力学等)的研究人员共同研究.其中,液化前后的土体本构关系是需要研究的核心内容.今后需利用离心机实验、现场原位实验开展工作,并进行相关实验技术研究工作.图0参118