液控调节阀空化与冲蚀仿真模拟与分析

基于液控调节阀的结构特性和流体物性参数,以开度40%为例建立数学模型,并采用Mixture模型和DPM(Discrete Phase Model)模型,对液控调节阀的空化和冲蚀现象进行数值模拟并分析。研究结果表明:当入口压力由4.5 MPa提高到5.5MPa时,阀芯与阀座之间的窄通道内流速变化剧烈,阀芯顶部抛物线段压力下降的区域增大,形成对称小椭圆形空化区域并脱离整个空化区域;当入口颗粒质量浓度一定时,阀芯顶部抛物线段磨损率分布趋势为:随颗粒直径的增大,磨损率先减小、后增大,然后再减小;当入口颗粒直径不变时,磨损率随入口颗粒质量浓度的增大而增大。     

离心泵内部空化特性的数值模拟与试验研究

以一台典型的清水式离心泵为研究对象,采用数值模拟和试验研究的方法,对3种不同流量工况下的离心泵内部空化特性进行了对比分析。结果表明:数值模拟与试验结果存在一定差异,具体表现为小流量区比较吻合,大流量区差异较大;临界汽蚀余量时,离心泵空化已经很严重,通过数值模拟发现,初生空化发生在临界汽蚀余量之前很长一段时间。因此后续应针对该泵展开初生空化研究。     

Numerical investigation of cavitating flow behind the cone of a poppet valve in water hydraulic system

Computational Fluid Dynamics (CFD) simulations of cavitating flow through water hydraulic poppet valves were performed using advanced RNGk-epsilon turbulence model. The flow was turbulent, incompressible and unsteady, for Reynolds numbers greater than 43000. The working fluid was water, and the structure of the valve was simplified as a two dimensional axisymmetric geometrical model. Flow field visualization was numerically achieved. The effects of inlet velocity, outlet pressure, opening size as well as poppet angle on cavitation intensity in the poppet valve were numerically investigated. Experimental flow visualization was conducted to capture cavitation images near the orifice in the poppet valve with 30° poppet angle using high speed video camera. The binary cavitating flow field distribution obtained from digital processing of the original cavitation image showed a good agreement with the numerical result. Project supported by the National Natural Science Foundation of China (No. 59835160) and Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry of China (No. 50175097)     

Numerical analysis of buried pipe characteristics

Voluminous statistical data on the breakage and leakage of pipelines occurring frequently in water supply and drainage engineering, were collected and analyzed to find the causes and solutions for the breakage and leakage. Then the original parameter method of foundation beam on elastic semi-infinite plane subgrade was applied to verify the statistical results. Numerical computational results showed that the performance of the original parameter method of foundation beam was satisfactory. Project supported by Zhejiang Province Science & Technology Foundation for Construction.     

Numerical analysis on car-following traffic flow models with delay time

INTRODUCTION For the past few decades, many researchers have made studies of modelling driver behavior to repro- duce the observed features of traffic flow. Recently Kerner and Rehborn (1997) observed that there are three distinct dynamic phases on highways: free traf- fic flow, coexisting traffic flow and traffic jams. The occurrence of traffic jams without obvious reasons had been explained in terms of the conventional phase transition (Nagatani, 1998). Various traffic flow models, suc…     

Experimental and numerical investigation of flow forces in a seat valve using a damping sleeve with orifices

The power of hydraulic piston engines is much affected by the on-off valves which control the fuel injection of the piston assembly. Therefore, the opening time of the seat valve used as the on-off valve is optimized by minimizing the axial flow forces on the spool. A damping sleeve with orifices is proposed to change the valve internal geometry. Experimental and numerical investigations of the flow forces acting on the spool with and without the proposed damping sleeve are carried out to identify the differences in the flow field and to minimize the forces’ effect. The simulated results fit the experimental results well. Both results show that the proposed damping sleeve affects the pressure distribution along the spool cone surface and the jet stream direction significantly. The effects of the orifice’s width, height, and relative sleeve installation positions on the flow field and cavitation are assessed using simulation methods. As a result of the flow field changing, the damping sleeve can reduce the flow forces significantly and even reverse the forces’ direction at the cost of a little flow loss. The opening time of the seat valve can be reduced by 31% to 0.67 ms by using the proposed damping sleeve.     

不同学科的实验特点比较分析

不同学科运用实验方法的时间、范围、程度等之所以不同,主要源于不同学科的实验特点,亦即实验性的不同.实验性既与学科对象以及学科的方法论特征直接相关,又与具体的实验技术密切相关.在不同的实验技术条件下,自然科学、社会科学、人文学科体现出不同的实验特点.     

差分精度对液压滑阀内流体数值解析的影响

：以液压控制系统中常用的滑阀为研究对象，通过对滑阀内流体流动的仿真计算，对本中采用的数值解析手法的差分方法、空间计算格子系等计算参数对计算结果的影响进行了分析和讨论。结果表明，上述计算参数对流场构造以及流量的计算结果影响相对较小。2种不同计算方法对滑阀轴向流体力的分析比较得出：作用在滑阀轴方向上的流体力对差分方法、空间计算格子系等计算参数非常敏感。如要得到较高精度的计算结果，采用压强和壁面切应力来直接计算流体力要比应用控制体内流体的动量守恒原理需要更密的计算格子系。这一结果有助于我们在进行流场数值计算时，根据不同的对象选取合适的解析手法和计算参数，节省计算时间，提高计算效率。     

两相自然循环流量漂移分岔特性数值模拟

为了研究自然循环的静态不稳定特性,通过对一个实验系统建立两相自然循环物理模型和数学模型,编程计算了两相情况下自然循环流量,研究自然循环流量漂移不稳定性的静态分岔特性及其影响因素。其中在实验段内,单相和两相段由净蒸汽产生点划分,净蒸汽产生点由萨哈-朱伯模型计算,两相摩擦压降采用均相流模型计算。最终得到了流量漂移的分岔特性曲线,并给出了相应的两相自然循环的稳定性准则,同时讨论了实验段入口欠热度、系统压力对分岔特性曲线的影响,并比较了分相流与均相流模型的计算结果。结果表明,提高入口温度、提高系统压力有利于系统稳定,分相流的计算结果比均相流稳定。     

矩形结构壁面上液膜流动特性的数值研究

壁面结构是影响液膜传热传质特性的重要因素之一。采用数值模拟技术研究矩形结构壁面上的液膜流动特性,得到了液膜的自由液面分布及涡结构特性。结果表明:矩形结构宽度w=3.0mm时,液膜下自由液面位置最低。调整矩形结构宽度,液膜上、下自由液面位置呈现相反的变化趋势,但总体效果变化不明显。在改变矩形结构宽度的基础上,随着液相入口流速的增大,液膜上自由液面位置均急速上升,而且w=1.0mm与w=1.5mm时,液膜流动稳定性良好,而液膜下自由液面位置的变化无明显规律;此外,不同结构尺寸的矩形结构内均有近似对称的旋涡存在。不同的矩形结构宽度下,旋涡的大小和产生的位置不同,w=1.5mm时,矩形结构前端处出现小旋涡。     

三角形结构对液膜流动特性影响的数值模拟

基于VOF方法,利用CFD软件FLUENT对倾斜壁面上二维气-液两相流进行数值模拟,分析了三角形高度和结构间距均为1mm的6种尺寸壁面上的液膜流动特性,获得了三角形结构的壁面上液膜自由液面的分布图以及不同结构宽度w对液膜流动特性的影响规律。结果表明:与平壁面上液膜流动相比,三角形结构壁面上液膜自由液面位置随三角形结构呈规律性变化;在三角形结构壁面上,气体漩涡存在于液膜和壁面之间,当三角形结构宽度w改变时,漩涡的形状和位置均发生变化。     

Numerical computations of the flow in a finite diverging channel

The flow in a finite diverging channel opening into a large space and resembling the experimental prototype of Putkaradze and Vorobieff （2006） was numerically investigated. The effects of the Reynolds number, initial condition, intersection angle, length of the wedge edges, and the outer boundary condition were examined. The numerical results showed that the flow in the wedge undergoes a change from symmetrical flow to unsymmetrical flow with a weak backflow, then a vortical （circulation） flow and finally an unsteady jet flow as the Reynolds number is increased for an intersection angle of 32° and a wedge edge of length 30 times the width of the inlet slit. For the unsteady flow, the jet attached to one side of the wedge constantly loses stability and rolls up into a mushroom-shaped vortex-pair near the outlet of the wedge. As the intersection angle is increased to 50°, a stable jet flow is observed as a new regime between the vortex and unsteady regimes. Both the intersection angle and the wedge length have negative effects on the stability of the flow, although the effect of the wedge length on the critical Reynolds number for the symmetry-breaking instability is not pronounced. The outer boundary condition was found not to affect the flow patterns inside the wedge significantly. At a certain Re regime above the onset of symmetry-breaking instability, the flows evolve into steady state very slowly except for the initial stage in the case of decreasing flow flux. Two different solutions can be observed within the normal observation time for the experiment, providing a possible explanation for the hysteresis phenomenon in the experiment.     

带有调压室的长距离引水系统水力特性研究

为了解和掌握带有调压室的长距离引水系统的特殊水力特性,依托锦屏二级水电站,采用压力引水系统水力过渡过程的数学模型,计算和分析了引水隧洞长度对引水系统大波动过渡过程的影响,考虑在各种大波动过渡过程中不利工况情况下,分析了阻抗式和差动式两种型式调压室引水系统的水力特性,揭示了长距离引水隧洞沿线压力分布、调压室涌浪特性、蜗壳进口压力随引水隧洞长度变化的规律。     

Numerical simulation and structure improvement of double throttling in a high parameter pressure reducing valve

In this paper,a new pressure reducing valve(PRV) with an orifice plate is proposed.The main objective is to explain the mechanisms of pressure reduction and energy conversion in the new PRV.A numerical simulation method was used to investigate the PRV internal flow field and to analyze the throttling effects of the orifice plate and the transform of thermal parameters as outlet pressure,outlet temperature,velocity,and superheat.A structure improvement method for the valve body and orifice plate is put forward to reduce energy loss.The governing equations for internal flow numerical simulation are composed of the continuity,momentum,energy and k-ε transport equations,based on isotropic eddy viscosity theory.Different valve plug displacement models were built to describe the double throttling process.Our analysis shows that the steam pressure drops twice and the degree of superheat increases.There are also lots of eddies which clog the flow channel and disturb the steam flow in the valve cavity after the valve plug and the outlet cavity.After modifying the structure,the numerical results show a better performance of steam flow.     

Numerical study of heat transfer characteristics of downward supercritical kerosene flow inside circular tubes

The heat transfer characteristics of China RP-3 aviation kerosene flowing in a vertical downward tube with an inner diameter of 4 mm under supercritical pressures are numerically studied. A ten-species surrogate model is used to calculate the thermophysical properties of kerosene and the re-normalization group (RNG) k-ε turbulent model with the enhanced wall treatment is adopted to consider the turbulent effect. The effects of mass flow rate, wall heat flux, inlet temperature, and pressure on heat transfer are investigated. The numerical results show that three types of heat transfer deterioration exist for the aviation kerosene flow. The first type of deterioration occurred at the tube inlet region and is caused by the development of the thermal boundary layer, while the other two types are observed when the inner wall temperature or the bulk fuel temperature approaches the pseudo-critical temperature. The heat transfer coefficient increases with the increasing mass flow rate and the decreasing wall heat flux, while the inlet bulk fluid temperature only influences the starting point of the heat transfer coefficient curve plotted against the bulk fluid temperature. The increase of inlet pressure can effectively eliminate the deterioration due to the small variations of properties near the pseudo-critical point at relatively high pressure. The numerical heat transfer coefficients fit well with the empirical correlations, especially at higher pressures (about 5 MPa).     

机械壅塞孔板流动特性实验分析

采用一种新型机械壅塞孔板装置研究孔板流动特性与流量控制,可在较低压力损失下实现不可压缩流体的壅塞流动。利用可视化实验方法研究机械壅塞孔板中环形孔流量系数、压强分布及其壅塞流特性。实验结果表明:雷诺数、孔板厚度、塞体锥度和偏心距对环形孔流量系数影响明显;流体流经环形孔板时会出现明显的压降;孔板上游压强在流动方向上略微降低,孔板下游压强存在恢复趋势;在设计的参数范围内,机械壅塞孔板的流量控制偏差为3.73%。     

Numerical prediction of vortex flow and thermal separation in a subsonic vortex tube

INTRODUCTION Vortex tube is a device for producing hot and cold air when compressed air flows tangentially into the vortex chamber through the inlet nozzles. This causes the vortex and swirl flow movement inside the vortex tube. The air in the middle region of the tube has lower velocity and lower temperature than the inlet air. So the air near the tube wall has higher ve-locity and higher temperature than the inlet air. The cold air in the core region of the tube flows out through the o…     

试析中国特色社会主义理论体系的五大特征

中国特色社会主义理论体系内在地具有五大特征,即继承性、创新性、科学性、实践性、开放性。它的继承性主要体现在始终坚持无产阶级立场、具有与时俱进的理论品质及其理论成果上的内在一致性：它的创新性主要表现在明确提出了初级阶段理论、市场经济理论和科学发展观：它的科学性主要体现在以马克思主义、科学社会主义为指导并坚持把它的基本原理与中国实际相结合．坚持党的领导并把中国特色社会主义事业引向新胜利;它的实践性来自于它雄厚的科学基础、变革社会的实际需要和人民群众实践的首创精神;它的开放性则源于中国特色社会主义实践的内在要求,体现了马克思主义发展的中国方向和时代立场。     

对新加坡生物学教材中核心概念编写的分析

新加坡的教育制度具有东西方文化的色彩,通过对其生物学教材中核心概念引入、解释、巩固及运用的研究和分析,可以对我国的教材中概念的编排和教师的教学提供借鉴。