基于PIV技术真空开关电弧流场实验研究

为了对真空开关电弧燃烧过程及热形态变化规律研究,本文采用粒子成像测速(PIV)技术对短间隙真空开关电弧进行了实验研究,观察分析了电弧流场的信息,有明显的漩涡区。实验结果表明,运用PIV技术能较好地获取真空开关电弧燃烧二维速度场分布;随着电弧电流不断增大,真空间隙中金属蒸气压力不断增大,电弧加速向四周扩散运动,当电弧电流增大到一定值时,在电弧四周产生明显的涡流区域;电弧电流峰值过后,涡流区域不断减小,当电弧电流减小到一定值时,电弧不再向外扩散,而是向弧柱中心做集聚运动。     

高压变电柜自动控制装置在电弧加热器中的应用

高压变电柜自动控制装置的体积比较小，其中涉及了高压变电柜技术，它可以自动控制高压变电柜，并可以对受电设备的工作环境和工作状态进行有效的实时监测。本文主要阐述高压变电柜自动控制装置在电弧加热器中的应用，以及其中使用的背景技术、设计的装置内容和技术特点。     

射流增压通风系统流场特性分析

根据实验数据,绘制射流增压通风系统的流场分布图,并分析速度分布和压力分布特性,从而全面了解射流发展的整个过程,有利于调控隧道气流的流动和控制隧道防灾.射流增压通风系统射流风机的纵向布置间距相应地增加,以上分析结果可供同行参考.     

小麦气力输送流场颗粒流化特性数值模拟

目的 探索仓泵式气力输送小麦颗粒时不同输送压力下罐体及引出管内颗粒的流化特性,从而得出最佳操作压力。方法 利用Solidworks建立简易的等比例发送装置三维模型,采用模拟仿真软件Fluent对0.25、0.3、0.35 MPa等3种不同输送压力进行数值模拟,并利用CFD–Post进行数据后处理。结果 当进气口压力为0.35 MPa时物料最先输送完毕,用时为10 s。整体发料过程从引出管入口至出口处三者压力分别降低了97.1%、96.8%、98.1%,其中当进气口压力为0.3 MPa时,压力降低最小,能量利用率最高。结论 输送压力越大输送速度越快,其压降也最大。考虑经济性与高效性可得,最佳进气口压力为0.3 MPa。     

非淹没双空化射流流场及冲击特性数值模拟

空化射流具有比同等条件下普通水射流更强的冲击力,被广泛应用于各种领域。为进一步提高空化射流的工作效率,提出了新型的非淹没双空化射流。基于多相流Mixture模型和RNG k-ε输运方程,研究了双空化喷嘴中内外喷嘴径向间距D₀与出口轴向间距L,以及外喷嘴来流压力P₁对射流轴线速度、压力、含气率与比能的影响,并分析了最优D₀,L,P₁组合下射流冲击平板的壁面压力分布特征。研究结果表明,当D₀=1.5 mm,且L=2 mm时射流的流场特性最优,在此结构下的最优靶距为20～25 mm。对该非淹没双空化射流冲击特性分析发现,相比于传统中心体空化喷嘴,其可产生体积更大的空化云和更好的冲击效果。研究结果可为空化射流的高效应用提供理论基础。     

圆形静压油腔流场特性的PIV实验与数值模拟研究

利用粒子图像测速技术(Particle Image Velocimetry, PIV)和计算流体动力学(CFD)方法,研究了高档数控机床液体静压支承系统中油腔内部的流场特性,包括入口雷诺数(Re)、油腔深度(H)和封油边间隙(h)对油腔内部流场的涡胞结构、壁面压强及剪应力分布等的影响。研究结果表明：实验结果与数值模拟结果相一致;油腔内部存在复杂的涡胞结构;随Re(70~1400)的增大,涡胞数量由1个增加为3个,涡胞尺寸和位置也不断变化;随着油腔深度H从6mm增大到18mm,涡胞尺寸逐渐增大,但涡胞数量减小;随封油边间隙h的增大,涡胞尺寸明显增大;相同条件下,增大入口雷诺数Re或减小封油边高度h都会增大承载面压强和剪应力。该结论为提高数控机床静压油腔的稳定性和承载力提供理论依据。     

双弧脉冲MIG焊耦合电弧特性的数值模拟研究

针对双弧脉冲MIG焊热源稳定性差的问题,本文对双弧脉冲MIG焊耦合电弧进行瞬态数值模拟,分析了不同脉冲电流参数下耦合电弧形态、温度和压力的分布及变化规律。研究表明：耦合电弧呈驼峰状,脉冲电流发生跳变时,耦合电弧伸展或收缩,并逐渐稳定,峰值电流越小,越快达到稳定;增大脉冲电流,耦合电弧温度和电弧压力随之升高;保持总电流不变,减小主弧电流,增大旁弧电流,主弧温度和电弧压力减小,旁弧温度和电弧压力增大,当旁弧电流足够大时,耦合电弧温度和电弧压力呈双峰分布。数值模拟结果与双弧脉冲MIG焊工艺实验结果吻合良好,模拟结果对调控双弧脉冲MIG焊脉冲电流参数,改善其耦合电弧稳定性及工艺性能具有重要意义。     

消声器内部流场的数值模拟

通过建立消声器的内部流场的物理模型和数学模型,利用Fluent软件对其流场进行了数值模拟,并就消声器内部流场对消声性能的影响进行了分析,为消声器的结构设计提供了一种可靠的指导方法。     

凝汽器管道壁面泄漏流场数值模拟研究

凝汽器作为核电厂热力循环中的冷却设备,乏汽通过与其内的冷却水进行热交换形成冷凝水以进行再次热力循环。凝汽器所用的海水冷却水一旦泄漏就会对二回路的蒸汽发生器造成污染,进而影响核电站的安全运行。本文针对凝汽器液相泄漏过程的流动特性进行了数值模拟研究,采用Euler-Euler两相流模型和k-ε湍流模型分析了不同流动工况下两相泄漏过程的流场,在常温常压情况下通过粒子图像测速(PIV)对凝汽器冷凝管的泄漏过程进行了可视化测量分析,并与数值模拟作了对比。结果表明：气液两相同时存在于泄漏过程中时,泄漏量与管道内液相流动工况无关,只与管道内外侧压差相关;泄漏的检测时间可设定在发生泄漏后0.003s内,此时泄漏射流附近存在的两个回流涡会导致示踪气体的回流,可能会对示踪气体的传质和对流扩散产生影响;管道内外侧压差为100000Pa情况下,冷却剂泄漏的质量流量约为3.925×10^-3kg/s至9.813×10^-3kg/s。     

W-Cu复合材料在不同电压下的电弧烧蚀性能研究

使用化学镀的方法制得包覆完整的W-Cu复合粉末,随后采用热压烧结法制得分布均匀、界面结合良好的W-Cu复合材料.在2、4、6和8 kV不同电压下对W-Cu复合材料进行电弧烧蚀实验,发现随着电压的升高,击穿电流增大,电极之间放电距离增大,击穿强度降低,燃弧时间增长.利用扫描电子显微镜及3D激光共聚焦显微镜观察材料表面在不...     

计算流体力学在纳滤膜分离技术研究中应用

介绍了计算流体力学(CFD)在膜分离过程模拟中的基本原理,探讨了CFD在膜隔网优化设计、纳滤膜污染机理研究、纳滤膜对无机离子截留等领域中的应用,并对CFD在纳滤膜分离技术研究中的应用前景进行了展望.     

计算流体力学对膜生物反应器水力学特征的模拟研究

通过计算流体力学(CFD)中的FLUENT软件,对膜生物反应器内流场和流态进行模拟,定量给出了其流速与剪切力的分布.研究了膜组件距离反应器底部不同距离时的水力学情况和水力学条件.对比了膜生物反应器中,玻璃纤维编织管式膜组件与平板膜组件,由于构型不同所导致的水力学条件差异.结果表明,从雷诺数和质量流速率参数分析,玻璃纤维编织管式膜组件的水力学条件更好,更有利于膜污染的防治.探讨了单个与多个曝气器情况下的流速分布,结果表明,后者流速分布更为均匀.由本文研究结果可知,计算流体力学可以作为一个有力的工具应用于MBR及其膜污染控制研究中.     

Shape optimization of bi-directional flow passage components based on a genetic algorithm and computational fluid dynamics

The inlet/outlet is an important part of a water conveyance system in a pumped storage power station (PSPS). Its hydraulic characteristics are directly related to the operation and economic benefit of the PSPS. Frequent changes between inflow and outflow operations pose significant challenges in the design of the inlet/outlet diffusion segment shape. In this study, an effective optimization method, including three-dimensional parametric modelling, computational fluid dynamics and a genetic algorithm, is introduced and coupled to the design of the diffusion segment shape. The hydraulic characteristics of bi-directional flow, including the head loss, velocity uneven distribution and uneven discharge distribution, are selected as the objective function in the optimization method. Using this method, the recommended shape of the inlet/outlet is studied and its hydraulic characteristics are discussed. The results indicate that the optimized inlet/outlet has better performance.     

Rapid pivot feeding in pipefish: flow effects on prey and evaluation of simple dynamic modelling via computational fluid dynamics

Most theoretical models of unsteady aquatic movement in organisms assume that including steady-state drag force and added mass approximates the hydrodynamic force exerted on an organism''s body. However, animals often perform explosively quick movements where high accelerations are realized in a few milliseconds and are followed closely by rapid decelerations. For such highly unsteady movements, the accuracy of this modelling approach may be limited. This type of movement can be found during pivot feeding in pipefish that abruptly rotate their head and snout towards prey. We used computational fluid dynamics (CFD) to validate a simple analytical model of cranial rotation in pipefish. CFD simulations also allowed us to assess prey displacement by head rotation. CFD showed that the analytical model accurately calculates the forces exerted on the pipefish. Although the initial phase of acceleration changes the flow patterns during the subsequent deceleration phase, the accuracy of the analytical model was not reduced during this deceleration phase. Our analysis also showed that prey are left approximately stationary despite the quickly approaching pipefish snout. This suggests that pivot-feeding fish need little or no suction to compensate for the effects of the flow induced by cranial rotation.     

Numerical simulation of fluid flow and temperature field in keyhole double‐sided arc welding process on stainless steel

Double‐sided arc welding process powered by a single supply is a type of novel high‐production process. In comparison with the conventional single‐sided arc welding, this process has remarkable advantages in enhancing penetration, minimizing distortion and improving welding production. In this paper, a three‐dimensional steady numerical model is developed for the heat transfer and fluid flow in plasma arc (PA)–gas tungsten arc (GTA) double‐sided keyhole welding process. The model considers the surface tension gradient, electromagnetic force and buoyancy force. A CCD camera is used to observe the size and shape of the keyhole and weld pool. The acquired images are analysed through image processing to obtain the surface diameters of the keyhole on the two sides. A double‐V‐shaped keyhole geometry is then proposed and its characteristic parameters are derived from the images and cross‐section of weld bead. In the numerical model, the keyhole cavum within the weld pool is treated as a whole quality, whose temperature is fixed at the boiling point of the workpiece material. The heat exchange between the keyhole and weld pool is treated as an interior boundary of the workpiece. Based on the numerical model, the distributions of the fluid flow and temperature field are calculated. A comparison of cross‐section of the weld bead with the experimental result shows that the numerical model's accuracy is reasonable. Copyright © 2005 John Wiley & Sons, Ltd.     

Structural optimization of downhole float valve via computational fluid dynamics

The internal parts of currently used float valve have serious erosion and unstable performance situations. If the drilling float valve fails, it will take a long time to deal with accidents and increase the risk of drilling operations. To solve this problem, we use the computational fluid dynamics method (CFD) to analyze the flow field structure of common downhole float valve, including the velocity field, turbulence intensity field, and wall shear stress field. The study found that: the flow field is quite disordered near the sealing surface of valve core, as well as near the spring and the opening parts of lower seat. Meanwhile, strong vortexes have been generated. This is a good corresponding relationship between the CFD simulation and the erosion (location and extent) in real float valve. This indicates that the CFD method can be used for optimization of existing float valve. On this basis, we present the optimal design, and use CFD method to analyze the new design. The analysis results show that the optimized design has not high velocity region, simultaneously, disturbance and vortex phenomenon of the flow field are significantly reduced and even disappeared. Its flow field distribution is more reasonable, which can reduce the factors causing erosion and performance failure. Therefore, we can infer that the new float valve has better anti-erosion performance and working stability.     

A modified drag model for power-law fluid-particle flow used in computational fluid dynamics simulation

A modified drag model for the power-law fluid-particle flow considering effects of rheological properties was proposed. At high particle concentrations (ε_s ≥ 0.2), based on the Ergun equation, the cross-sectional shape and the tortuosity of the pore channel are considered, and the apparent flow behavior index and consistency coefficient of the power-law fluid at the surface of the particles are corrected. At low particle concentrations (ε_s < 0.2), based on the Wen-Yu drag model, the modified Reynolds number for power-law fluid and the relational expression between drag coefficient for single particle and Reynolds number that considers the effect of the flow behavior index are adopted. Numerical simulations for the power-law fluid-particle flow in the fluidized bed were carried out using the non-Newtonian drag model. The effects of rheological parameters on the drag coefficient were analyzed. The comparisons of simulation and experiment show that the modified drag model predicts reasonable void fraction under different rheological parameters, particle diameters, and liquid velocities in both low particle concentrations and high particle concentrations. The increase in flow behavior index and consistency coefficient increases the drag coefficient between the two phases and decreases the average particle concentration within the bed.     

The flow field near a venous needle in hemodialysis: A computational study

The vascular access used in hemodialysis can suffer from numerous complications, which may lead to failure of the access, patient morbidity, and significant costs. The flow field in the region of the venous needle may be a source of damaging hemodynamics and hence adverse effects on the fistula. In this study, the venous needle flow has been considered, using three‐dimensional computational methods. Four scenarios where the venous needle flow could potentially influence dialysis treatment outcome were identified and examined: Variation of the needle placement angle (10°, 20°, 30°), variation of the blood flow rate settings (200, 300, 400 mL/min), variation of the needle depth (top, middle, bottom), and the inclusion of a back eye in the needle design. The presence of the needle has significant effect on the flow field, with different scenarios having varying influence. In general, wall shear stresses were elevated above normal physiological values, and increased presence of areas of low velocity and recirculation—indicating increased likelihood of intimal hyperplasia development—were found. Computational results showed that the presence of the venous needle in a hemodialysis fistula leads to abnormal and potentially damaging flow conditions and that optimization of needle parameters could aid in the reduction of vascular access complications. Results indicate shallow needle angles and lower blood flow rates may minimize vessel damage.     

Use of computational fluid dynamics to study forces exerted on prey by aquatic suction feeders

Suction feeding is the most commonly used mechanism of prey capture among aquatic vertebrates. Most previous models of the fluid flow caused by suction feeders involve making several untested assumptions. In this paper, a Chimera overset grids approach is used to solve the governing equations of fluid dynamics in order to investigate the assumptions that prey do not interact with the flow and that the flow can be modelled as a one-dimensional flow. Results show that, for small prey, both neglecting the prey and considering prey interaction give similar calculated forces exerted on the prey. However, as the prey item increases in size toward the size of the gape, its effect on the flow becomes more pronounced. This in turn affects both the magnitude of the hydrodynamic forces imparted to the prey and the time when maximum force is delivered. Maximum force is delivered most quickly to intermediate sized prey, about one-third of mouth diameter, and most slowly to prey less than 7 per cent or greater than 67 per cent of mouth diameter. This suggests that the effect of prey size on the timing of suction forces may have substantial consequences for the feeding ecology of suction feeders that are known to prefer prey between 25 and 50 per cent of mouth diameter. Moreover, for a 15 cm fish with a 15 mm gape, assuming a radial one-dimensional flow field can result in underestimating the maximum force exerted on a 5 mm diameter spherical prey 1 gape distance from the mouth by up to 28.7 per cent.     

Using design of experiments methods for applied computational fluid dynamics: A case study

This article presents an application of design of experiments (DOE) in a computational fluid dynamics (CFD) environment to study forces and moments acting on a missile through various speeds and angles of attack. Researchers employed a four-factor Latin hypercube space-filling design and the Gaussian Process to build a surrogate model of the CFD environment. The surrogate model was used to characterize missile aerodynamic coefficients across the transonic flight regime. The DOE process completed the task with less computational resources than a traditional one-factor-at-a-time (OFAT) approach. To validate the surrogate model, specific OFAT angle of attack sweeps were performed. This provided a direct comparison between the Gaussian Process model and OFAT analysis. In most cases, the surrogate computer model was able to accurately capture the nonlinear response variables. Moreover, the surrogate model enabled a dynamic prediction tool that could investigate untested scenarios, a capability not available with OFAT. The DOE process consequently received support from engineers who do not typically use DOE.