三峡右岸机组水力性能数值模拟

目前,对大型水轮机组稳定性问题的研究成为热点和前沿问题。三峡电站的运行水头变幅大,负荷变幅也大, 机组稳定性问题更为突出,成为人们关注的焦点。应用CFD技术对混流式水轮机的内部流场进行模拟分析,对可能影响 水轮机运行性能的若干因素进行研究,可以为改善水轮机的水力稳定性能提供依据。     

低温气液两相流研究中的CFD技术

介绍了 CFD技术的优势及发展现状 :论证了将 CFD技术引入到低温气液两相流系统研究及开发中的可行性 ;通过分析 CFD通用软件中理论模型的不足 ,找到了限制 CFD技术在低温气液两相流研究中应用的关键因素 ,并拟定了解决方案     

吸力面带凹窝轴流风扇内流分析与实验研究

本文采用Navier-Stokes方程和κ-ε两方程模型分别对空调用轴流风轮叶片吸力面设置凹窝和不设置凹窝的的内流特性进行了三维数值模拟,同时对两者的风量噪音特性进行了实验研究.结果表明,在叶片吸力面设置凹窝没有改善风轮的整体流量─压力特性,不能提高风扇的静压升,凹窝只能提高凹窝局部位置的静压升,且凹窝中心压力大,凹窝边缘压力低.小流量时,风扇吸力面设置凹窝反而增加风机噪音,大流量时,凹窝对风机的噪音没有影响.     

FFP在黏土中贯入过程的CFD模拟

圆锥静力触探法(Cone penetration test, CPT)广泛应用于土的抗剪强度测试中。在此基础上发展的自由落体式贯入仪(Free fall penetrometer, FFP),依靠自由下落获得的动能和自身重力势能贯入土中,不需要借助外部加载装置,提高其使用的便捷性。但在动力贯入过程中FFP与土的相互作用更为复杂,涉及到土体的率效应和拖曳阻力等的影响。因此,对FFP各项受力和相关参数的准确分析有助于提高其实用性以及测量的准确性。作者采用基于计算流体动力学(computational fluid dynamics, CFD)的分析软件ANSYS CFX 17.0模拟FFP在均质黏土中的贯入过程,借助动网格的大变形分析方法来模拟FFP贯入过程中的运动边界问题。提出了薄层单元法模拟FFP与土体的界面摩擦接触行为。在CFD模拟中,土体材料采用非牛顿流体来模拟,其剪切强度受土体切应变率的影响(即土体的率效应)。通过模拟贯入仪在黏土中以不同的速度贯入的过程,研究FFP的端部阻力和侧壁阻力与贯入速率、土体强度和密度、界面摩擦系数以及率效应参数之间的关系,建立了端部承载力系数、端部和侧壁率效应参数及拖曳系数的表达式,并提出了土体不排水抗剪强度的预测方法,为FFP测试数据的解析提供依据。      

5×5棒束CFD模拟计算域和网格标准

采用不可压N-S方程、标准k-ε湍流模型、SIMPLE算法,对某压水堆的两跨距5×5燃料组件的水循环区域流动进行数值分析.对比流道横截面上的压降、速度及其横向分量的相对误差,得到计算域的进出口段长度、网格形式、网格尺寸的选取标准.结果显示：计算域的进口段、出口段长度不宜过短或过长,分别为2.0倍、2.3~2.8倍格架中心距长度;格架区宜采用非结构蜂窝网格,光棒区宜采用结构网格;格架区网格尺寸可取0.3 mm,最大不宜超过0.35 mm,而结构网格横向尺寸可取0.3 mm~0.5 mm,纵向稀疏比应小于1.5为宜.满足上述标准,数值分析能保证较高的可信度,同时保持可接受的计算量.     

Ultrasound coupled with supercritical carbon dioxide for exfoliation of graphene: Simulation and experiment

Ultrasound coupled with supercritical CO₂ has become an important method for exfoliation of graphene, but behind which a peeling mechanism is unclear. In this work, CFD simulation and experiment were both investigated to elucidate the mechanism and the effects of the process parameters on the exfoliation yield. The experiments and the CFD simulation were conducted under pressure ranging from 8 MPa to 16 MPa, the ultrasonic power ranging from 12 W to 240 W and the frequency of 20 kHz. The numerical analysis of fluid flow patterns and pressure distributions revealed that the fluid shear stress and the periodical pressure fluctuation generated by ultrasound were primary factors in exfoliating graphene. The distribution of the fluid shear stress decided the effective exfoliation area, which, in turn, affected the yield. The effective area increased from 5.339 cm³ to 8.074 cm³ with increasing ultrasonic power from 12 W to 240 W, corresponding to the yield increasing from 5.2% to 21.5%. The pressure fluctuation would cause the expansion of the interlayers of graphite. The degree of the expansion increased with the increase of the operating pressure but decreased beyond 12 MPa. Thus, the maximum yield was obtained at 12 MPa. The cavitation might be generated by ultrasound in supercritical CO₂. But it is too weak to exfoliate graphite into graphene. These results provide a strategy in optimizing and scaling up the ultrasound-assisted supercritical CO₂ technique for producing graphene.     

Experimental and numerical investigation on performance of a swirling jet reactor

In this work, a three-dimensional Computational Fluid Dynamic (CFD) analysis of a swirling jet reactor was implemented to gain a better understanding of fluid dynamics into the reactor. The effect of different geometries of the reactor, by considering different diameters of the injection slots of the reactor, on flow velocity and flow pressure distributions was investigated. Firstly, a one-phase model was implemented by considering only water into the reactor. Then, a two-phase model was defined including dissolved air into the water. The inlet flow pressure was set to 0.25 bar to consider non-cavitating conditions and, then, to get more accurate results on fluid dynamics into the reactor due to the absence of cavitating conditions. Data collected from experimental tests were used to calibrate and validate the model. Results of numerical simulations were in good agreement with experimental data, showing for all the geometries a rotating flow around the central axis of the reactor and at the exit of the double cone. The highest flow velocities and flow pressure drops were observed for the reactor geometry with the smallest injection slots diameters. Finally, noise measurements were performed during another set of experimental tests by considering different inlet flow pressures.     

CFD–DEM simulation of particle deposition characteristics of pleated air filter media based on porous media model

In this study, the three-dimensional physical model of pleated air filtration media was simplified to porous media model, and the calculation parameters of porous media were obtained based on experimental data. The model of V-shaped pleated air filter media is constructed, the height of the media pleat is 50 mm and the pleat thickness is 4 mm, the pleat angle is 3.7°. The Hertz-Mindlin contact model was modified by Johnson Kendall Roberts (JKR) adhesion contact model. The deposition process of particles in media was simulated based on computational fluid dynamics (CFD) theory and discrete element method (DEM). Results show that the CFD–DEM coupling method can be effectively applied to the macro research of pleated air filter media. The particles will form dust layer and dendrite structure on the fiber surface, and the dust layer will affect the subsequent air flow organization, and the dendrite structure will eventually form a “particle wall”. The formation of the “particle wall” will prevent the particles from moving further in the fluid domain, which makes area of pleated angle become the “low efficiency” part about the particle deposition. Compared with area of pleated angle, the particles are concentrated in the opening area and the middle area of the pleated to agglomerate and deposit.     

Numerical simulation of viscous flow: Buckling of planar jets

The phenomenon of viscous fluid buckling has a long and distinguished history, dating back to Taylor (1968). This paper is concerned with demonstrating that a numerical method, GENSMAC, is capable of simulating this physical instability. A table of the parameter values (e.g. the Reynolds number, the Froude number, inlet width, inlet velocity and aspect ratio) is provided giving details of when buckling occurs and when it does not. This allows the deduction of a possible buckling condition in terms of the Reynolds number and the ratio of height of the jet to the inlet width, modifying a previous hypothesis. Visualization of jet buckling is provided. This work has been motivated by the need of industry to understand jet filling of containers; jet buckling can lead to air entrapment and this is undesirable. Copyright © 1999 John Wiley & Sons, Ltd.     

Simulation of vortex shedding past a square cylinder with different turbulence models

This paper presents the results of numerical simulations of vortex shedding past a free-standing square cylinder at Re_D=22 000, obtained with different turbulence models. Using wall functions, the standard k–ε model is compared with a modification suggested by Kato and Launder (Proc. 9th Symp. Turbulent Shear Flows, Kyoto, 10-4-1 (1993)). In addition, both versions are used in a two-layer approach, in which the flow close to the cylinder is computed with a locally more suitable one-equation turbulence model and only outside the viscous near-wall layer with the two mentioned high-Re model versions. To allow a comparison, the simulations are performed first using the same computational domain and boundary conditions as in previous investigations. Then results are presented that were obtained on a computational domain and with boundary conditions more suitable for a comparison with the experiments. © 1998 John Wiley & Sons, Ltd.