不同流道宽度下流体掠过方柱的旋涡脱落特性数值模拟

方柱结构的旋涡脱落特性一直是学术研究的热点。流体掠过方柱体时在方柱后产生的旋涡脱落有可能引发方柱的流致振动。文章使用BELIEF程序,通过模拟在不同流道宽度条件下流体掠过方柱的旋涡脱落情况,研究流道宽度对流体掠过方柱的旋涡脱落特性的影响。结果表明:流体旋涡脱落斯特鲁哈数(St)随着流道变窄而增大;当流道宽度(H)远大于方柱迎流面特征尺寸(方柱横截面边长w)时,St随流道宽度变窄变化缓慢,流道宽度的变化对旋涡脱落特性的影响很小;随着流道宽度继续变窄,流道宽度对流体旋涡脱落特性的影响逐渐增大。当流道宽度与方柱特征尺寸之比B(H/w)减小至一定值时,旋涡脱落频率随流道变窄而急剧增大。     

Vibration of nuclear fuel bundles

Several mathematical models have been proposed for calculating fuel rod responses in axial flows based on a single rod consideration. The spacing between fuel rods in liquid metal fast breeder reactors (LMFBRs) is small; hence fuel rods will interact with one another due to fluid coupling. The objective of this paper is to study the coupled vibration of fuel bundles. To account for the fluid coupling, a computer code (AMASS) is developed to calculate added mass coefficients for a group of circular cylinders based on the potential flow theory. The equations of motion for rod bundles are then derived including hydrodynamic forces, drag forces, fluid pressure, gravity effect, axial tension and damping. Based on the equations, a method of analysis is presented to study the free and forced vibrations of rod bundles. Finally, the method is applied to a typical LMFBR fuel bundle consisting of seven rods.     

Two-dimensional simulation of drop deformation and breakup at around the critical Weber number

The breakup of two-dimensional liquid drops is numerically investigated at around the critical Weber number. The Moving Particle Semi-implicit (MPS) method is used to solve the unsteady Navier–Stokes equations for both the drops and the ambient fluid. The two dominant forces affecting drop breakup, pressure drag and surface tension force, are verified using the two benchmarks: pressure distribution on the surface of a cylinder in a uniform flow and oscillation of a square drop under surface tension force. The results show that the breakup process occurs in two stages. During the first stage, the drops become stretched and thinned normal to the flow direction of the ambient fluid. During the second stage, detached points appear on the surface of the drops, which are ascribed to the unstable growth of surface waves. The post-breakup topology of the drops is dependent on the value of the Weber number: the larger the Weber number is, the more detached points on the surface of the drops emerge. Another feature commonly shared in the two stages is that some fine fragmentations are stripped from the equatorial edges of the drops. The critical Weber number is predicted to be 13 for uranium dioxide drops in water, at which breakup regime is consistent with the so-called vibration regime.     

非线性振动下水平通道内气液两相流动研究

在地震等行为产生的非线性振动下,两相流体会影响回路传热并对装置结构进行冲击,因此对气液界面行为的把握对核安全具有十分重要的意义。本文通过将振动装置与两相流实验回路相结合的方法,对非线性振动下水平通道内气液两相流问题进行了实验研究。基于FLUENT平台,结合动网格模型及UDF编程手段建立了数学模型,并对数学模型进行验证。研究结果表明：模拟结果与实验结果具有很好的一致性;振动工况下气液两相流动形式不同于稳态工况,会出现更复杂的气液界面,主要流型有泡状流、弹状流、搅拌流、波状流及环状流;瞬时摩擦压降的波动幅度随振动幅度和频率的增大而增大,且与振动幅度相比,振动频率对其影响更大。     

ALE finite element method for gas–liquid two-phase flow including moving boundary based on an incompressible two-fluid model

This paper proposes an ALE (Arbitrary Lagrangian–Eulerian) finite element method for gas–liquid two-phase flow, based on an incompressible two-fluid model, to analyze the two-phase flow including moving boundaries. The basic equations are derived by describing the two-fluid model in the ALE form. The solution algorithm is parallel to a fractional step method, and the Galerkin method is employed for the formulation. A quadrilateral element with four nodes is used for the discretization of the computational domain. The present method is also applied to calculate the flow around a circular cylinder, which is forced to oscillate in a quiescent air–water two-phase mixture. The drag coefficients of the cylinder exhibit periodical change in accordance with the variation of the flow around the cylinder. The time variations of the flow field and drag coefficients are discussed in relation to the oscillation of the cylinder.     

The effect of axial extension on the fluidelastic vibration of an array of cylinders in cross-flow

In this paper an improved model is developed aimed at analyzing the fluidelastic vibration of a single flexible cylinder surrounded by rigid cylinders and subject to cross-flow. Compared with other dynamical models reported previously, the nonlinearity associated with the mean axial extension of the cylinder has been accounted for in the current work. Calculations are performed to explore the effect on the nonlinear dynamics of the nonlinearity presented in the equation of motion. Numerical results show that, with increasing flow velocity just beyond a critical value, a post-Hopf limit-cycle motion occurs. For a system without loose supports, the vibration amplitude of the limit-cycle motion can be predicted by using the presented model. To understand the nonlinear dynamics of the cylinder better, a modified system with loose supports is also investigated. The impacting force induced by the loose supports was modelled by a cubic spring or by a trilinear spring. It is found, by using the theoretical model developed here, that the post-Hopf dynamics predicted with the cubic-spring representation is quite different from those predicted with the trilinear-spring representation. However, it is shown that the effect of the nonlinearity associated with the mean axial extension on the nonlinear dynamics of cylinder with loose supports is not pronounced.     

Void fraction distributions of inert gas jets across a single cylinder with non-wetting surface in liquid sodium

Little work on the void fraction behaviors along structural materials with poor-wettability for liquid metals has been performed. In the present study, void fraction behaviors around a single cylinder with non-wetting surface condition were quantitatively discussed by using a gas jet–cylinder system where the impinging jet flow, the boundary layer flow, the separation flow, and the wake flow appear. One cylinder with a non-wetting surface and two cylinders with a wetting surface were used to vary the wettability for liquid sodium, and void fraction distributions were measured around the cylinders. In the case of wetting condition, void fraction distributions around the cylinder decrease clearly in the backward region of the cylinder, and liquid-rich region is formed due to bubble separation from the cylinder surface. On the other hand, under non-wetting condition, because of two-phase flow without bubble separation on the cylinder surface, void fraction distributions show almost steady values around the cylinder compared to those with wetting surface. The void behaviors on a non-wetting surface were also confirmed by a visualization experiment conducted in water. The observed differences can be basically attributed to the work of adhesion required for liquid–solid interfacial separation.     

Experimental investigation of a flow-induced oscillating cylinder with two degrees-of-freedom

The phenomenon of flow-induced vibration of bluff bodies has been studied extensively. The vast majority of these studies have concentrated solely on one degree-of-freedom oscillation in the inline or cross-flow directions. Herein, experiments were carried out with a cylinder in a water channel with two degrees-of-freedom. The cylinder was cantilever mounted with a low natural frequency (typically 65 Hz) in the inline and cross-flow directions. The Reynolds number fell in the range 1.17 × 10³ < Re < 2.6 × 10⁴. The oscillating frequency of the cylinder and the surrounding flow were measured simultaneously using high temporal resolution particle image velocimetry (PIV), which is non-intrusive with respect to the flow and has high spatial and temporal resolutions. The vibration of the cylinder was found to be anisotropic. There was a discrepancy between the vibration frequencies in the inline and cross-flow directions, the difference being a function of reduced velocity.     

Numerical prediction of added mass and damping for a cylinder oscillating in confined incompressible gas–liquid two-phase mixture

This paper is concerned with the numerical analysis of the added mass and damping of a circular cylinder, which oscillates in an air–water bubbly mixture enclosed by a concentric shell. The mixture is assumed to be incompressible. This is because the oscillation frequency of the cylinder is low in this study, and accordingly the pressure change around the cylinder is not so large. An incompressible two-fluid model is solved by the finite element method, proposed by the author in a prior paper, to calculate the bubbly flow around the oscillating cylinder. The analysis reveals the effects of the diameter ratio of the cylinder to the shell, the air volumetric fraction and the bubble diameter. It also clarifies that the increase of damping ratio in the bubbly mixture is attributable to the phase lag of the drag force acting on the cylinder behind the cylinder displacement.     

Some models for the prediction of fluid elastic instability

Single and two-phase cross flow induced fluid-elastic instabilities are addressed. The evolution of models are shown to be the result of changing experimental configurations. One-dimensional control volume models are adequate for a single flexible cylinder in an array of rigid cylinders. A row of flexible cylinders in an array of rigid cylinders requires the basic equations be solved at least in normal mode form. More general flexibility of the cylinders leads to solving the vorticity equation. Two-phase flow is addressed using modern two-fluid model equations. The results are shown to compare well with experimental data in spite of the many assumptions required.     

Simplified method to evaluate upper limit stress intensity factor range of an inner-surface circumferential crack under steady state thermal striping

Simplified method to evaluate the upper limit stress intensity factor (SIF) range of an inner-surface circumferential crack in a thin- to thick-walled cylinder under steady state thermal striping was considered in this paper. The edges of the cylinder were rotation-restrained and the outer surface was adiabatically insulated. The inner surface of the cylinder was heated by a fluid with constant heat transfer coefficient whose temperature fluctuated sinusoidally at constant amplitude ΔT. By combining our analytical temperature solution for the problem and our semi-analytical-numerical SIF evaluation method for the crack, we showed that the desired maximum steady state SIF range can be evaluated with an engineering accuracy after ΔT, the mean radius to wall thickness ratio r_m/W of the cylinder, the thermal expansion coefficient and Poisson's ratio are specified. By applying our method, no transient SIF analysis nor sensitivity analysis of the striping frequency on the SIF range is necessary. Numerical results showed that our method is valid for cylinders in a range of r_m/W = 10–1.     

Fluid damping for circular cylindrical structures

Fluid damping plays an important role for structures submerged in fluid, subjected to flow, or conveying fluid. This paper presents a summary of fluid damping for circular cylinders vibrating in stationary fluid, cross flow, and parallel flow.     

窄流道中柔性单板流固耦合数值模拟

板状燃料组件在先进核反应堆中得到了广泛应用。流体以一定流速轴向掠过平行板组件可能导致板的流致振动（FIV）,而板的振动又会影响流场的重新分布,两者之间构成强烈的流固耦合（FSI）关系。针对板状燃料组件的FIV现象开发了计算程序。程序基于物理组成贴体坐标系（PCBFC）,结合任意拉格朗日欧拉坐标法（ALE）实现网格的移动。本工作详细模拟了在窄通道中移动边界条件下流场的分布;数值求解板在流体压力下的梁式振动方程,从而实现窄流道中柔性单板流固耦合的数值模拟。     

Interface Waves Excited by Vertical Vibration of Stratified Fluids in a Circular Cylinder

The excitation of interface waves by vertical vibration of stratified immiscible fluids in a circular cylinder is studied experimentally and analytically. The excitation of linear capillary-gravity waves by the time-dependent fluid acceleration combined with the fluid-wall relative motion is considered in the analysis. The analytical results show that the interface responses are dominated by the modes predicted by the Mathieu equation when the excitation amplitude and frequency lie in an unstable region of the Mathieu equation governing the acceleration-driven interface oscillation. Otherwise, symmetrical harmonic modes are excited by the capillary force associated with the fluid-wall relative motion. These changes in the instability modes agree to those observed in the experiments conducted with oil (kerosene) and water in an acrylic pipe. The amplitudes of the symmetrical harmonic waves in the experiments are well predicted by using the contact line model of Miles. The interface motion in the experiments, however, is found to include cyclic formation of a thin film of oil on which the oil-water interface behaves as if it “slips” without being much affected by the capillary force, and thus to be far more complicated than assumed by the model.     

Experimental study on two-phase flow instability of natural circulation under rolling motion condition

Two-phase flow instability of natural circulation under a rolling motion condition is experimentally studied. The experimental results show the rolling motion induces a fluid flow fluctuation. At the trough point of the flow fluctuation, rolling motion can cause the early occurrence of natural circulation two-phase flow instability, and this case is defined as trough-type flow oscillation. The system stability decreases with increasing rolling amplitude and effect of rolling frequency is nonlinear. The complex overlap effect of trough-type flow oscillation and density wave oscillation can enhance the system coolant fluctuation; this case is defined as complex flow oscillation. Complex flow oscillation may be divided into two types: regular and irregular complex flow oscillations. Irregular complex flow oscillation is a transition type from trough-type flow oscillation to regular complex flow oscillation. Under the same thermal hydraulic conditions, the marginal stability boundary (MSB) of regular complex flow oscillation is similar to that of density wave oscillation without rolling motion, and the influences of rolling parameters on the MSB are slight.     

换热器管束流体激振研究的新思路

提出了一种快速简便地求解非定常粘性流方程的方法、以得到换热器管束复杂流道内作用于振动管子上的流体激振力。简述了如何将管束流动的流体弹性稳定性分析与工程实际的振动疲劳破坏相联系，以及怎样在求解流动基本方程的基础上分析管束复杂流道内的振荡压力传播。将求解复杂流动的快速方法—参数多项式方法与流体激振的全功能分析及振荡压力传播理论结合起来，为管束流体激振研究开拓了一个新的途径。     

用涡格子CFD方法研究反应堆堆芯元件流致振动的流体力学机理

将反应堆堆芯元件简化为圆柱排列,用涡格子CFD(计算流体力学)方法对其中基本并列双圆柱元素进行数值研究.利用这种方法直接计算出涡的对流和扩散,通过对流场中涡运动的物理机理的计算与描述,得到流体运动绕流双圆柱作用力的变化,揭示出堆芯元件的流致振动流体力学机理.     

A review of heat exchanger tube bundle vibrations in two-phase cross-flow

Flow-induced vibration is an important concern to the designers of heat exchangers subjected to high flows of gases or liquids. Two-phase cross-flow occurs in industrial heat exchangers, such as nuclear steam generators, condensers, and boilers, etc. Under certain flow regimes and fluid velocities, the fluid forces result in tube vibration and damage due to fretting and fatigue. Prediction of these forces requires an understanding of the flow regimes found in heat exchanger tube bundles. Excessive vibrations under normal operating conditions can lead to tube failure.

Relatively little information exists on two-phase vibration. This is not surprising as single-phase flow induced vibration; a simpler topic is not yet fully understood. Vibration in two-phase is much more complex because it depends upon two-phase flow regime, i.e. characteristics of two-phase mixture and involves an important consideration, which is the void fraction. The effect of characteristics of two-phase mixture on flow-induced vibration is still largely unknown. Two-phase flow experiments are much more expensive and difficult to carry out as they usually require pressurized loops with the ability to produce two-phase mixtures. Although convenient from an experimental point of view, air–water mixture if used as a simulation fluid, is quite different from high-pressure steam–water. A reasonable compromise between experimental convenience and simulation of steam–water two-phase flow is desired.

This paper reviews known models and experimental research on two-phase cross-flow induced vibration in tube bundles. Despite the considerable differences in the models, there is some agreement in the general conclusions. The effect of tube bundle geometry, random turbulence excitations, hydrodynamic mass and damping ratio on tube response has also been reviewed. Fluid–structure interaction, void fraction modeling/measurements and finally Tubular Exchanger Manufacturers Association (TEMA) considerations have also been highlighted.     

Seismic design method of clamped-free thin cylindrical shells immersed in fluid

We carried out excitation experiments on clamped-free thin cylindrical shells immersed in fluid that represented thermal baffles of a fast breeder reactor. At a certain excitation level, occurred external pressure buckling. We also observed that parametric vibration, which involved high-order circumferential vibration modes, occurred at a certain combination of excitation frequency and excitation level. Concerning seismic design of the thermal baffles, we, therefore, take not only the buckling but also the effect of the parametric vibration into consideration. We adopt buckling eigenvalue analyses to estimate buckling pressure and propose a formula to prevent the buckling. Further, we discuss important factors such as buckling strength reduction caused by initial shape imperfections and interaction between horizontal and vertical seismic response. Concerning the parametric vibration, a significant deformation of cylinders should be prevented. A practical method applying the dynamic stability theory is proposed to obtain the condition, under which the parametric vibration takes place.     

核承压热交换器两相流流致振动现象研究

为保障核承压热交换器的安全运行,采用数值模拟以及软件计算相结合的方法,对核承压热交换器两相流流致振动现象及减振措施进行了探究。研究结果表明:基于流致振动发生机理,热交换器横流速度、固有频率、卡门旋涡脱落频率以及紊流抖振频率为重点分析因素;由公式得出流量、换热管直径、换热管壁厚、管束排列等对流致振动有直接影响,无支撑跨距是影响管束流致振动较大因素;最易发生流致振动的部位包括入口区域、出口区域、折流板缺口区域以及无支撑跨距大管束;设计中,应在流量、换热管直径、壁厚、无支撑跨距、管束排列及入口防冲挡板设置等方面优化,以减小流致振动危害。