双转子对旋风力机多工况数值模拟

以实验室双转子对旋风力机为研究对象,应用数值模拟方法获取了该风力机56个工况的性能数据,分析了其多工况性能以及典型工况S1面流谱,结果表明:相同转速条件下,风力机产生的功率随着来流风速的增加而增加;在功率图谱上,不同转速的功率等转速线斜率差异较大,高等转速线大于低等转速线的斜率;下游转子的进口气流角小于上游转子,导致其产生的功率大于上游转子;对旋风力机工作趋于区域里的点是速度三角形与翼型匹配,保证翼型工作的合理攻角范围内的工况.     

Numerical study on the internal flow characteristics of an axial-flow pump under stall conditions

When an axial-flow pump works in low flow rate conditions, rotating stall phenomena will probably occur, and the pump will enter hydraulic unsteady conditions. The rotating stall can lead to violent vibration, noise, turbulent flow, and a sharp drop in efficiency. This affects the safety and stability of the pump unit. To study the rotating stall flow characteristics of an axial-flow pump, the steady and unsteady internal flow field in a large vertical axial-flow pump was investigated using 3D computational fluid dynamic (CFD) technology. Numerical calculations were carried out using the Reynolds-averaged Navier–Stokes (RANS) solver and Menter's shear stress transport (SST) k-ω turbulence model. Steady flow characteristics including streamline, velocity vector, pressure and turbulent kinetic energy are presented and analyzed. Unsteady flow characteristics are described using post-processing signals for pressure monitoring points in the time and frequency domains. Using Q-criterion, the locations and evolution rules of the core region of the vortex structure in guide vanes under deep stall conditions were investigated. The reliability of the numerical simulation results was verified using the experimental prototype pressure fluctuation test. In this way, typical flow structure and pressure fluctuation characteristics in an axial-flow pump were analyzed, with contrastive analysis in design condition and stall conditions. Finally, the mechanism of low-frequency pressure fluctuation in a pump unit under the stall condition was revealed.     

阀控双相交流液压振动系统特性研究

基于自激振动原理的阀控双相液压振动系统,能在较小能量输入下产生高频振动。此类系统在以冲击振动形式进行工作的工程机具中有广泛的应用。论文在描述阀控双相交流液压振动系统的工作原理的基础上,分析了系统自激起振条件、功率流分布形式及各相流体参数耦合关系等问题。通过仿真计算,获得了系统主要结构参数及功能要素对于系统振动特性的影响规律。为系统工作性能的匹配和结构参数的优化提供了依据。     

Numerical investigation of an Organic Rankine cycle radial inflow two-stage turbine

The Organic Rankine cycle power plants have recently been the subject of intensive and remarkable growth in research worldwide. The system operates using very impressive technology to convert low to moderate heat sources into useful electrical power by means of a turbine. It offers the advantages of better operating performance, less crowded components compared to the classical steam and gas power plants for low power ranges, and more importantly, no greenhouse gas effects on the atmosphere. Numerical investigations of the turbine play a major role in improving the performance of the system, and an accurate CFD simulation interfaced with accurate thermodynamic models is considered a significant step in the design and prediction of the turbine performance. This paper draws both numerical and experimental performance maps of an ORC radial inflow two-stage turbine, starting with experimental data collection and refinement, passing by the performance parameters calculation. A commercial CFD tool was used to study the grid-independency of the numerical performance maps; a medium and fine mesh were generated and checked against the experiments. The look-up-tables interpolation method was implemented in this study to evaluate the working fluid thermodynamic properties. The main objective of this work is the validation and improvement of the numerical model prediction with respect to the experimental results.     

Experimental study of the pressure fluctuations in a pump turbine at large partial flow conditions

Frequent shifts of output and operating mode require a pump turbine with excellent stability. Current researches show that large partial flow conditions in pump mode experience positive-slope phenomena with a large head drop. The pressure fluctuation at the positive slope is crucial to the pump turbine unit safety. The operating instabilities at large partial flow conditions for a pump turbine are analyzed. The hydraulic performance of a model pump turbine is tested with the pressure fluctuations measured at unstable operating points near a positive slope in the performance curve. The hydraulic performance tests show that there are two separated positive-slope regions for the pump turbine, with the flow discharge for the first positive slope from 0.85 to 0.91 times that at the maximum efficiency point. The amplitudes of the pressure fluctuations at these unstable large partial flow conditions near the first positive slope are much larger than those at stable operating condtions. A dominant frequency is measured at 0.2 times the impeller rotational frequency in the flow passage near the impeller exit, which is believed to be induced by the rotating stall in the flow passage of the wicket gates. The test results also show hysteresis with pressure fluctuations when the pump turbine is operated near the first positive slope. The hysteresis creates different pressure fluctuations for those operation points even though their flow rates and heads are similar respectively. The pressure fluctuation characteristics at large partial flow conditions obtained by the present study will be helpful for the safe operation of pumped storage units.     

某重型燃气轮机燃烧室燃烧流动的数值模拟

对某重型燃气轮机燃烧室燃烧天然气进行数值模拟,在模拟过程中采用了雷诺应力模型、EBU-Arrheniue湍流燃烧模型和六通量辐射模型来描述其燃烧流动过程,运用FLUENT软件求解了三维流场和温场分布.计算结果能够很好地反映该重型燃气轮机燃烧室燃烧流动特点,对预测燃烧室内的燃烧流动有一定参考价值.     

Numerical study of droplet vaporization under acoustic pulsing conditions

Journal of Mechanical Science and Technology - Bubble growth and collapse in acoustic droplet vaporization (ADV) are numerically studied by employing the level-set interface tracking method, which...     

Numerical analysis of pulsating heat pipe based on separated flow model

The examination on the operating mechanism of a pulsating heat pipe (PHP) using visualization revealed that the working fluid in the PHP oscillated to the axial direction by the contraction and expansion of vapor plugs. This contraction and expansion is due to the formation and extinction of bubbles in the evaporating and condensing section, respectively. In this paper, a theoretical model of PHP was presented. The theoretical model was based on the separated flow model with two liquid slugs and three vapor plugs. The results show that the diameter, surface tension and charge ratio of working fluid have significant effects on the performance of the PHP. The following conclusions were obtained. The periodic oscillations of liquid slugs and vapor plugs were obtained under specified parameters. When the hydraulic diameter of the PHP was increased to d=3mm, the frequency of oscillation decreased. By increasing the charging ratio from 40 to 60 by volume ratio, the pressure difference between the evaporating section and condensing section increased, the amplitude of oscillation reduced, and the oscillation frequency decreased. The working fluid with higher surface tension resulted in an increase in the amplitude and frequency of oscillation. Also the average temperature of vapor plugs decreased.     

极大流量工况下离心泵内部流场数值分析

针对离心泵极大流量工况下内部流动特性的问题,应用流体动力学软件Fluent,采用RNGκ-ε湍流模型与SIMPLEC算法,对某一高比转速离心泵内部流场进行了数值模拟,并与实验结果进行了比较。对比分析了4种不同流量工况下离心泵内部流体速度和压力分布以及离心泵的外特性。研究结果表明,在设计流量工况下,离心泵内部压力分布均匀,速度迹线平滑;较大流量工况下,蜗壳压力不断减小,速度分布不均匀;极大流量(1.7Qopt)工况下,蜗壳出口处出现局部负压现象,速度流线产生的漩涡增大,在扩散管局部位置流体受到冲击,容易出现回流现象。针对离心泵在不同工况下以及达到极大流量工况下内部流动随流量变化规律的研究,可为高比转速离心泵多工况优化设计、延长使用寿命提供参考。     

跨声速涡轮级三维流场的数值模拟

采用FLUENT软件对某变循环发动机的高压跨声速涡轮级的三维粘性流场进行了数值模拟计算,使用混合平面法实现涡轮级动、静三维流场联算,计算中分别采用两个典型的双方程模型RNGk-ε模型和realizable k-ε模型,并将计算结果进行了对比,结果在一定程度上表明在分离流计算和带二次流的复杂流动计算中,Realizable k-ε模型的计算结果更精细一些.同时分析了其内部流场以及通道内总压损失的分布情况,为进一步改进叶型设计提供了理论依据.     

SK型静态混合器内的流动特性数值研究

基于微尺度流动特征的角度,研究SK型静态混合器内部湍流时的流动规律.利用计算流体力学专业软件并采用雷诺时均方程和标准的k-ε湍流模型对SK静态混合器内的湍流状态下的三维不可压缩流场进行数值模拟.数值计算和研究表明在含有多个SK型螺旋元件的静态混合器内,受元件衔接处的切割作用的流体将在其后的混合元件的3L/8处完成重新汇合;研究了SK型静态混合器的主要参数对流动阻力的影响,从而为静态混合器的优化设计提供了依据.     

燃气涡轮机导片与转子叶片交互作用的非定常流场数值模拟

采用ICEM-CFD软件建构O-H网格系统,分别模拟二种不同的涡轮导片及转子叶片间隙在转动效应下流场结构.模拟的数据与实验值做比较后,发现在涡轮导片及转子叶片流场预测上,已能有效预测流场的边界层流、分离流的发展以及导角叶片与转子叶片间的多震波的流场现象,另外亦能针对涡轮叶片压力总损失及轮机效率加以分析,为涡轮机的改进设计和优化提供了理论依据.     

有机工质在透平静叶栅内流动的数值研究

针对有机工质在透平中流动产生气动损失的问题,对有机工质R245fa在SC11静叶栅中的流动进行了数值研究。提出了考虑粘性的有机工质物性定义方法,分别采用理想气体状态方程和SW气体状态方程对透平静叶栅中的流场进行了研究,对比了采用不同气体状态方程得到的压缩因子及密度沿叶型表面的分布规律,评价了有机工质在透平静叶栅内流动的非理想程度。分析了有机工质在透平静叶栅内流动的参数分布,并得到了有机工质在静叶栅内不同区域的膨胀规律。研究结果表明,应用理想气体状态方程与SW气体状态方程得到的计算结果偏差很大,理想气体状态方程不适用于有机工质在透平内流动的计算;在透平静叶栅中,叶片压力面压降幅度较吸力面更平缓,吸力面下游马赫数达到最大值,叶片尾迹中存在损失。     

高密实度H型风力机气动性能的数值分析

针对高密实度H型风力机在不同叶片数下的气动特性及风场布置等问题,将采用k-ω SST湍流模型进行数值模拟的方法应用到对高密实度H型风力机的研究中.开展了对风力机做功特性的分析,建立了流场与风力机功率之间的关系,提出了单个风力机设计选择叶片数时应综合考虑风力机效率和轴承安全这两个因素;在尾流场分析的基础上对不同叶片数的风力机在风场前后串列布置进行了评价.研究结果表明:叶片在上游(θ=90°附近)的气动性能决定整个风力机的性能;由正常工况点下的流场图显示,叶片数的增加导致流场复杂,以及叶片的内外压差逐渐减小,从而使得功率下降;尾流场流向速度恢复至来流速度的距离随叶片数的增加而减小了28.1％,这对风力机的前后串列布置提供了依据.     

工业汽轮机调节级部分进汽瞬态数值研究

针对采用喷嘴调节的汽轮机调节级效率和安全性问题,基于时均的N-S方程,采用六面体结构化网格和有限容积法,对某机组调节级内部流动进行了数值计算。给出了不同工况下调节级内部压力和速度分布图,分析了4阀开和3阀开工况下调节级内部流动。给出了动叶旋转一周扭矩的变化,并将非定常扭矩进行了傅里叶变换,得到了频谱图。研究结果表明,部分进汽使调节级内部流动不均匀,4阀开,动叶所承受最大瞬时扭矩是进汽弧段平均扭矩的2.2倍,3阀开,动叶所承受最大瞬时扭矩是进汽弧段平均扭矩的1.8倍,这可以为调节级动叶安全性校核提供参考;频谱图显示,动叶受到与转速相关的低频激振力;3阀开工况下调节级的效率比4阀开工况下降低7%。     

Numerical study on flow characteristics at blade passage and tip clearance in a linear cascade of high performance turbine blade

A numerical analysis has been conducted in order to simulate the characteristics of complex flow through linear cascades of high performance turbine blade with/without tip clearance by using a pressure-correction based, generalized 3D incompressible Navier-Stokes CFD code. The development and generation of horseshoe vortex, passage vortex, leakage vortex, tip vortex within tip clearance, etc. are clearly identified through the present simulation which uses the RNG k-ε turbulent model with wall function method and a second-order linear upwind scheme for convective terms. The present simulation results are consistent with the generally known tendency that occurs in the blade passage and tip clearance. A 3D model for secondary and leakage flows through turbine cascades with/without tip clearance is also suggested from the present simulation results, including the effects of tip clearance height.     

A study of the flow characteristics of developing turbulent pulsating flows in a curved duct

Flow characteristics of turbulent pulsating flows in a square-sectional curved duct were experimentally investigated. Experimental studies for air flow were conducted to measure axial velocity profiles, secondary flow and pressure distributions in a square-sectional 180° curved duct by using an LDV system with a data acquisition and processing system which includes a Rotating Machinery Resolve (RMR) and PHASE software. Measurements were made at the seven cross-sections from the inlet (ø=0°) to the outlet (ø=180°) of the duct with 30° intervals. Pressure was measured by using a magnetic differential pressure gage. The experiment was conducted in nineteen sections from the inlet to the outlet of the duct at 10° intervals.Velocity profiles for turbulent pulsating flows were large at the outer wall for a bend angle of ø=30° because of the centrifugal force. The velocity profiles were similar to those of turbulent steady flows. The secondary flow of the turbulent pulsating flow had a positive value at a bend angle of 150° without regarding the phase. The dimensionless value of the secondary flow became gradually weak and approached to zero in the region of a bend angle of 180° regardless of the ratio of velocity amplitude. The pressure difference of turbulent pulsating flows was the largest near the region of a bend of angle of 90° in the case of the middle region and became small beyond 90.     

Numerical simulation of fully developed flow and heat transfer characteristics in a curved tube with pulsating pressure gradient

Characteristics of fluid flow and convective heat transfer of a pulsating flow in a curved tube have been investigated numerically. The tube wall is assumed to be maintained at a uniform temperature peripherally in a fully developed pulsating flow region. The temperature and flow distributions over a cross-section of a curved tube with the associated velocity field need to be studied in detail. This problem is of particular interest in the design of Stirling engine heat exchangers and in understanding the blood flow in the aorta. The time-dependent, elliptic governing equations are solved, employing finite volume technique. The periodic steady state results are obtained for various governing dimensionless parameters, such as Womersley number, pulsation amplitude ration, curvature ratio and Reynolds number. The numerical results indicate that the phase difference between the pressure gradient and averaged axial velocity increases gradually up to π/2 as Womersley number increases. However, this phase difference is almost independent of the amplitude ratio of pulsation. It is also found that the secondary flow patterns are strongly affected by the curvature ratio and Reynolds number. These, in turn, give a strong influence on the convective heat transfer from the pipe wall to the pulsating flow. The results obtained lead to a better understanding of the underlying physical process and also provide input that may be used to design the relevant system. The numerical approach is discussed in detail, and the aspects that must be included for an accurate simulation are discussed.     

Effect of wave conditions on the performance and internal flow of a direct drive turbine

The purpose of the present study is to investigate the effect of wave conditions on the performance and internal flow of a newly developed direct drive turbine (DDT) model for wave energy conversion experimentally. All the experiments using the test turbine models are conducted in a 2-D wave channel. Monochromatic waves of various conditions of wave height and wave period are applied to the turbine performance test. The influences of turbine configuration on turbine performance are also investigated. Test results show that rotational speed, differential pressure, incident flow rate, maximum output power, and best efficiency of the turbine model vary considerably depending on the wave conditions. Installation of a front guide nozzle and a rear water reservoir to the test turbine improves the turbine performance. Large passage vortex occurs both at the front and rear turbine nozzles in turn through a reciprocating flow in the turbine passage. This paper was recommended for publication in revised form by Associate Editor Jun Sang Park Young-Do Choi received his B.E. and M.E. degrees in Mechanical Engineering from Korea Maritime University, Korea in 1996 and 1998, respectively. He received his Ph.D. in Engineering from the Yokohama National University, Japan in 2003. Dr. Choi is currently a research professor at the School of Mechatronics, Chanwon National University in Changwon, Korea. His research interests include ocean energy, wind power, small hydro power, fluid machinery, PIV and CFD. Chang-Goo Kim received his B.E. and M.E. degrees in Mechanical Engineering from Korea Maritime University, Korea in 2007 and 2009, respectively. Mr. Kim is currently a doctorate student in the Department of Mechanical Engineering, Graduate School, Korea Maritime University. His research interest includes ocean energy. Young-Ho Lee received his B.E. and M.E. degrees from Korea Maritime University, Korea. He received his Ph.D. in Engineering from the University of Tokyo, Japan. Dr. Lee is currently a Professor at the Division of Mechanical and Information Engineering, Korea Maritime University. His research interests include ocean energy, wind energy, small hydro power, fluid machinery, PIV, and CFD.     

Numerical analysis of flow in a Francis turbine on an equal critical cavitation coefficient line

Numerical simulation and model test were applied to study the cavitation flow in Francis turbines. The SST k-ω turbulence model and the mixture model were used to simulate the cavitating flow in the Francis turbine. An equal critical cavitation coefficient line was calculated and the flow in the Francis turbine was analyzed. Simulation results show reasonable agreement with the experimental data. It is confirmed that these cavitation model and numerical method is a useful way to study the two-phase cavitation flow in Francis turbines. On the equal critical cavitation coefficient line, the energy loss in the turbine may be caused by the rotating of vortex rope in the draft tube or flow separation in the runner. The study of equal critical cavitation coefficient line can provide a basic guidance for industry practice.