大颗粒与壁面碰撞的离散单元法模拟与分析

颗粒与壁面碰撞普遍存在于散体物料输送过程,研究颗粒与壁面碰撞有助于优化输送系统、减小物料磨损或提高输送经济性。本文基于离散单元法(DEM),采用Hertz-Mindlin无滑移接触模型,对单个6mm直径大颗粒与壁面碰撞进行了数值模拟和分析,研究了碰撞速度、碰撞角度和剪切模量对碰撞过程和法向最大接触力的影响。研究结果表明,Hertz-Mindlin无滑移接触理论描述的法向接触过程具有自相似特性,法向卸载时长与法向加载时长比值为定值。模拟的接触时长与Thornton等的关系式预测值相符。碰撞速度和碰撞角度对碰撞过程中的法向最大接触力均有明显影响,法向最大接触力随法向碰撞速度的增加近似线性增加;碰撞速度不变时,法向最大接触力随碰撞角度的增大而减小。剪切模量对法向接触力具有重要影响,在考虑颗粒磨损和破碎的DEM模拟时,不宜采用降低剪切模量加快计算速度。本研究对颗粒磨损和破碎研究以及高温气冷堆吸收球气力输送过程优化均具有重要意义。     

不同棒束结构稠密栅元通道内的湍流CFD研究

采用URANS（UnsteadyReynoldsAveragedNavierStokes）方法对不同棒束结构稠密栅元通道（P/D=1.001～1.2）内的湍流流动进行CFD模拟。研究分析了不同Re（Re=5000~215000）的湍流流动的主流速度、壁面剪应力、湍动能等参数。研究表明：在较稠密的棒束（P/D<1.1）通道内,P/D的变化对子通道内主流速度和剪应力分布均有较大影响。本文的模拟结果也验证了在达到临界P/D前（即使δ/D<0.011）,交混因子Y和δ/D成反比关系。对于固定的棒束结构（P/D=1.062）,当Re达到一定值（Re=9600）时,子通道内主流速度和剪应力分布对Re的变化不敏感。     

Plasma Confinement Modification in IR-T1 Tokamak by Velocity Shear Stabilization

E × B velocity shear effects on the plasma confinement were investigated in the IR-T1 tokamak. The investigations have been done at the presence of external applied electric and Resonant Helical magnetic Fields (RHF). In this work, experimental data have been measured by using two arrays of the Langmuir probes in both the radial and poloidal directions. A velocity shear stabilization mechanism has also been proposed to be responsible for an improvement in plasma confinement. The results show that E_r × B drift velocity (V_E×B) reduces about 90 % due to applied biasing and RHF at edge plasma. We have also observed that positive biasing and RHF lead to a significant decrease (>80 %) for radial turbulent transport (Γ_E×B) at edge plasma. In this paper, the electrostatic Reynolds stress (Rs) and the shearing rate γ_E×B have been calculated. We have also compared the Rs and γ_E×B at presence of the biasing and RHF and without biasing and RHF. A good correlation between confinement modifications and E_r × B velocity shear has been found suggesting that confinement enhancement originates at the edge plasma as a consequence of the formation of a particle transport barrier just inside the limiter.     

Numerical analysis of bubble motion with the VOF method

Numerical analyses of a two-dimensional single bubble in a stagnant liquid and in a linear shear flow were conducted in the present study using the volume of fluid method, which is based on the local-instantaneous field equations. It was clarified that this method gives qualitatively appropriate predictions for the effects of the Morton number and the Eotvos number on fluctuating bubble motion in a stagnant liquid. Calculated velocity and pressure distributions indicated that the Karman vortex causes a sinuous movement of the bubble. As for the bubble motion in a linear shear flow, calculated bubbles migrated in a lateral direction. The direction of the lateral migration agreed to available experimental data. It was also confirmed that (i) the direction or the magnitude of the lateral migration is affected by the Eotvos and the Morton numbers, and (ii) the interaction among the internal flow of the bubble, the wake of the bubble and the external shear flow plays an essential role for the lateral migration.     

流动锂液帘对堆芯等离子体影响的评估(英文)

运用零维模型评估了流动液态锂幕帘作为聚变实验增殖堆工程概要设计 (FEB-E) 第一壁对聚变等离子体的影响。得到了锂液帘工作温度对堆芯有效平均等离子体电荷?Zeff?,燃料稀释以及聚变功率之间的关系。表明在正常工作情况下,液态锂的蒸发对?Zeff?的影响不是很严重,但对燃料稀释和聚变功率的影响却较为敏感。作为一个例子,对较高功率密度的反剪切位形聚变实验增殖堆FEB-E设计方案 II,计算了液帘的流速与它表面最大温升的关系,结果表明:即便0.5m/s的低速流动液帘第一壁, 蒸发对聚变等离子体的影响也甚微。     

Investigation of turbulence rotation in the SOL and plasma edge of W7-X for different magnetic configurations

The W7-X stellarator is optimized with respect to neoclassical transport. Therefore turbulent transport plays an important role. It is equipped with an inertial cooled graphite divertor which intersects the island chain at the plasma edge depending on the magnetic configuration. Additional control coils and the plasma current modify the iota profile at the plasma edge and shift the position of the island chain. To monitor the effects on the poloidal propagation velocity in the scrape-off layer(SOL) and the plasma edge, an O-mode Poloidal Correlation Reflectometer(PCR) is used which simultaneously monitors the propagation of low-k turbulence. Operating in the density range of 0.6?×?10~(19) m~(-3) to2?×?10~(19) m~(-3) it covers a large part of the SOL and the plasma edge and allows for the experimental determination in the last closed flux surface(LCFS) and the associated shear layer in low to middensity discharges. In this paper it is shown that the propagation in the shear layer and its vicinity is determined best, when based on an elliptical model. Different magnetic configurations with magnetic edge topology of five independent islands for ι?=?1 and six linked islands for ι?=?0.81 are investigated. Also the effects of the plasma current and additional control coils on the edge magnetic topology are studied. The coherence spectra of antenna pairs for different poloidal separations is investigated. Using a decomposition method for the measured coherence spectra the characterization of turbulence spectra is possible with respect to e.g. broad band turbulence and quasi coherent modes.A strong reduction of the broad band turbulence is observed in the vicinity of the LCFS which is evidence for the suppression of low-k turbulence at the shear layer.     

Simple heat transfer model for laminar film condensation in a vertical tube

A new physical model for estimating the liquid film thickness and condensation heat transfer coefficient in a vertical tube, considering the effects of gravity, liquid viscosity, and vapor flow in the core region, is proposed. In particular, for calculating the velocity profile in the liquid film, the liquid is assumed to be in Couette flow forced by the interfacial velocity at the liquid-vapor interface. The interfacial velocity is calculated using an empirical power-law velocity profile. The film thickness and heat transfer coefficient from the new model are compared with existing experimental data and the original Nusslet condensation theory. The new model describes the liquid film thinning effect due to the vapor shear flow and predicts the condensation heat transfer coefficient from experiments reasonably well.     

A review of CCFL phenomenon

Counter current flow limitation CCFL is an important phenomenon for numerous engineering applications and safety of light water reactors. In particular, the possible occurrence of CCFL in the hot-leg of a PWR during SBLOCA or LOCA accidents is of special interest for nuclear safety research. A review of the related literature has made in order to present the most important studies about the phenomenon and to reach common general understanding of the different factors that govern CCFL. Eventually this will allow explaining contradictions among different explanations provided by different authors. Most important factors were geometrical characteristics, liquid superficial velocity, and physical properties. The review shows that despite numerous experimental works, many scaling and geometrical effects are still not fully understood. For Instance there exist no consistent explanation of the channel diameter and inclined riser length effect upon results. The same can be stated-though to a minimum extent – for the inclination angle while channel length (or channel to diameter ratio) effect was clear and consistent. Since most experimental work was done in down-scaled hot-leg simulators, it becomes interesting to build a coherent knowledge about these effects and to explain arising contradictions in order to safely extrapolate results to full-scale hot-leg. The review has shown that many differences were simply due to geometrical effects, this leads to the need to “standardize” experimental data according to geometrical parameters. This should results in a better understanding of the phenomenon and corresponding scaling effects. Additionally, important variables such as: pressure drop, void fraction and shear stress were also investigated and discussed. A compilation of CCFL data was built and analyzed. Since the new simulation trends tend to use CFD codes where geometrical and spatial deviations are excluded by using 3D modeling, emphasis was placed upon introducing correlations for onset of CCFL out of collected data. Existing correlations for interfacial shear stress friction factor and the void fraction as a function of gas superficial velocity were also gathered and briefly discussed. The effect of condensation, physical parameters, and hysteresis upon CCFL was also introduced.     

Fully developed rod bundle flow over a large range of Reynolds number

In an investigation of the fluid mechanics of single phase reactor cores, extensive measurements of mean axial velocity, wall shear stress and all six Reynolds stresses have been made in fully developed flow through a square pitched rod bundle array with pitch to diameter ratio of 1.107. The range of Reynolds numbers, based on bulk velocity and hydraulic diameter was 22600 to 207600. The mean secondary flow velocities could not be measured at any Reynolds number, implying that they were always less than about 1% of the bulk velocity. The axial momentum integral equation is used to show that the wall shear stress distribution is determined primarily by the pressure gradient and the transverse shear stress |ovbar|uw, a result that confirms the negligible size of the mean secondary flow. The implications of the results for current engineering calculation methods are discussed.     

Annealing Study on Neutron Irradiation Effects in Resonance Frequencies of Zircaloy Plates by EMAR Method

To develop the nondestructive hydrogen concentration measurement method for the irradiated zirconium alloy, the effect of neutron irradiation damage on acoustic properties obtained by the electromagnetic acoustic resonance (EMAR) method was investigated through annealing tests. It is confirmed that the recovery of irradiation damage begins at a lower annealing temperature than that in unirradiated coldworked materials. Unirradiated recrystallization-annealed materials did not show any change in acoustic properties or hardness during the additional RX annealing at 580°C, whereas the acoustic anisotropy (Δf) of the as-cold-worked unirradiated specimen was significantly increased. Four-cycle irradiation clearly decreased the shear wave velocity of the specimen by 1% compared to the RX-annealed specimen. In comparison with the wave velocity change, the acoustic parameters defined in this study are found to be less sensitive to irradiation damage. From the annealing study of the as-irradiated specimens to the RX condition, it is concluded that the absolute value of (Δf) increases by 0.1% and the frequency ratio (fl/fr) by about 0.006 as a result of the damage induced by the four-cycle irradiation in BWRs. These values are applicable to the relationships between the acoustic parameters and hydrogen concentrations of unirradiated materials as the correction factors to compensate the effects of irradiation damage.     

A turbulence model study for simulating flow inside tight lattice rod bundles

Performances of various turbulence models are evaluated for calculation of detailed coolant velocity distribution in a tight lattice fuel bundle. The individual models are briefly outlined and compared with respect to the prediction of wall shear stress and velocity field, for a fully developed flow inside a triangular lattice bundle. Comparisons clearly show the importance of proper modeling of the turbulence-driven secondary flows in subchannels. A quadratic k– model, which showed promising capability in this respect, is adjusted in its coefficients, and the adjusted model is applied to fully developed flow in an infinite triangular array, with various Reynolds numbers. The results show that the inclusion of adequate anisotropy modeling enables to accurately reproduce the wall shear stress distribution and velocity field in tight lattice fuel bundles.     

On normal impact of an infinite elastic-plastic thin-walled plate by a finite elastic rod

The problem of the transverse collision of a finite circular cylinder (flat-ended metal projectile) of length L and radius R₀, moving with the initial velocity V₀, with an immobile thin infinite plate, is considered. It is assumed that the plate is in a pure shear state. The one-dimensional waves both in the plate (cylindrical elastic-plastic waves) and in cylinder (plane longitudinal elastic waves) are investigated. Graphs of the peak dimensionless shear stress at the point of impact versus four non-dimensional quantities representing the material and geometrical properties of the plate and the projectile, are presented. These graphs allow the projectile speed just sufficient to initiate ejection of the slug to be estimated.     

Identification of damping mechanism of TRR-II reactor control rod during free fall insertion

In light water reactors, control rods are in general inserted into reactors by gravity. In order to achieve a rapid shutdown, it is required to insert control rods as fast as possible. On the other hand, a control rod with a fast falling velocity would impose a substantial impact to reactor structure as well as to the rod itself. Therefore, a damping force must come into effect, especially during the final stage of the free fall of the control rod. The purpose of this study is to develop a mathematical model and a numerical simulation to describe and identify the damping mechanism; and apply this model to the design of the control rod used in TRR-II reactor of the Institute of Nuclear Energy Research (INER) of Taiwan.The damping effect of a falling control rod comes from two factors: the viscous shear stress occurred in a narrow gap between the rod and an outer tube which confines the lateral movement of the rod, and the pressure force exerted on the rod by the compressed water under the rod. The viscous shear stress can be analyzed by assuming a couette flow between the rod and the outer tube similar to the viscous force occurred in rheology. In doing this, the flow rate in each flow path is closely related to the pressure gradient in the flow path and can be evaluated using an electrical circuit analogy. The results of the code prediction were compared to the experimental results as carried out by the INER. Finally, a parametric study was applied to estimate the effects of the various factors including gap thickness, size of the flow holes, and other geometric considerations on the rod falling velocity. The results of this study can serve some technical support during the stage of rod design and manufacture.     

Effect of compensation error in drift-flux parameters on predictions of thermal-hydraulic parameters in nuclear safety system analysis codes

Void fraction in a nuclear reactor core is one of the most important parameters in a safety analysis using nuclear reactor thermal-hydraulics system analysis codes such as TRAC-BF1, RELAP5 and TRACE. Interfacial shear term governing void fraction in the two-fluid code is often estimated by Andersen approach which uses drift-flux type correlation to compute the interfacial shear term. The accuracy of two drift-flux parameters such as distribution parameter and drift velocity is anticipated to affect the accuracy of predicted void fraction significantly. In principle, the distribution parameter and drift velocity are independent parameters which should be determined by local gas and liquid velocities and void fraction. However, due to very limited local data, the distribution parameter and drift velocity are commonly determined by area-averaged void fraction and superficial gas and liquid velocities. This “approximate method” is acceptable when the distribution parameter and drift velocity are used together. However, in the Anderson's approach, the distribution parameter and drift velocity determined by the approximate method are used separately which may cause some compensation error in code calculations. In view of the great importance in accurately computing the interfacial shear term, the effect of the compensation error on the predicted void fraction is investigated. Intensive sensitivity analysis suggests the compensation error propagating to void fraction only up to 1% for steady state computations, whereas the effect of the compensation error on the predicted void fraction for transient computations becomes larger because temporal reduction of drag force may cause the increase in void fraction. A prototypic nuclear power plant analysis for ATWS scenario suggests that the overestimation of the void fraction may affect the neutron flux calculation.     

Longitudinal laminar flow in asymmetrical finite bundles of rods

Theoretical analysis is presented on the asymmetrical effects, i.e. peripheral displacement of rods or unequal rod diameters, on longitudinal laminar flow in a finite bundle of rods. Solution is by the method of superposition. Numerical results are obtained for shear stress and velocity distributions when only one rod is displaced in a seven rod cluster of equal-diameter rods.     

Enhanced Plasma Performance by ICRF Boronization

Boronization with carborane(C2B10H12)by ICRF been applied routinely to the walls of HT-7 super-conducting tokamak for the reduction of impurity influx.especially carbon and oxygen.Significant suppression of metallic impurties and radiating power fraction are achieved.The imporved confinement for both particle and energy is observed in full range of operation parameters.Energy balance analysis shows that electron heat diffusion coefficient is strongly reduced.Measurements by Langmuir probes at the edge plasma show that poloidal velocity shear after boronization is changed to a profile to profile favoring to good confinement.The main emphasis of this paper is to describe effects of boronization on aspects of the enhanced plasma performance.     

Void fraction estimation within rod bundles based on three-fluid model and comparison with x-ray CT void data

An interfacial shear stress equation in the dispersed-annular two-phase flow regime has been developed, which is based on a three-fluid model consisting of a liquid film on a rod, vapor and entrained liquid associated with a vapor flow. It is an extension of J.G.M. Andersen's procedure that provides a two-fluid interfacial shear stress equation using the drift flux parameters C₀ and V_gj. This interfacial shear stress equation can take into account a phase and velocity distribution through an equivalence between the drift flux parameters and the interfacial shear stress.

Using the three-fluid subchannel analysis code TEMPO with the three-fluid interfacial shear stress model, the capability of a three-fluid calculation using the drift flux parameters C₀ and V_gj that reproduce a measured void fraction is demonstrated. A comparison was made with advanced X-ray computed tomography (CT) void fraction data within a 4×4 rod bundle in diabatic 1 MPa pressure conditions. The three-fluid velocity field was estimated to be in good agreement with the experimental result of a void fraction.     

Mode structure symmetry breaking of reversed shear Alfvén eigenmodes and its impact on the generation of parallel velocity asymmetries in energetic particle distribution

In this work, the gyrokinetic eigenvalue code LIGKA, the drift-kinetic/MHD hybrid code HMGC and the gyrokinetic full-f code TRIMEG-GKX are employed to study the mode structure details of reversed shear Alfvén eigenmodes (RSAEs). Using the parameters from an ASDEX-Upgrade plasma, a benchmark with the three different physical models for RSAE without and with energetic particles (EPs) is carried out. Reasonable agreement has been found for the mode frequency and the growth rate. Mode structure symmetry breaking (MSSB) is observed when EPs are included, due to the EPs' non-perturbative effects. It is found that the MSSB properties are featured by a finite radial wave phase velocity, and the linear mode structure can be well described by an analytical complex Gaussian expression ${\rm{\Phi }}(s)={{\rm{e}}}^{-\sigma {\left(s-{s}_{0}\right)}^{2}}$ with complex parameters σ and s0, where s is the normalized radial coordinate. The mode structure is distorted in opposite manners when the EP drive shifted from one side of ${q}_{\min }$ to the other side, and specifically, a non-zero average radial wave number 〈ks〉 with opposite signs is generated. The initial EP density profiles and the corresponding mode structures have been used as the input of HAGIS code to study the EP transport. The parallel velocity of EPs is generated in opposite directions, due to different values of the average radial wave number 〈ks〉, corresponding to different initial EP density profiles with EP drive shifted away from the ${q}_{\min }$.     

Optimal design of the positions of the hoops for a hydraulic pipelines system

This paper focuses on decreasing the vibration and improving the dynamics performances for a hydraulic pipelines system. The parametric model of the hydraulic pipelines system under the random excitations is constructed and the dynamics characteristics are obtained by the finite element analysis, then an optimization model is presented to reduce the vibration by rationally designing the positions of the hoops in the pipelines system. The dimensions determining the locations of the hoops are defined as design variables, and the dynamics performances, such as the maximum displacement, the maximum axial stress, the maximum shear stress, the maximum axial strain, the maximum hoop strain, the maximum shear strain and the failure probability of the first passage are regarded as nonlinear constraints whereas the failure probability of cumulative fatigue damage is viewed as an optimization objective. The results show that the dynamics performances of the hydraulic pipelines system are distinctly improved by the optimization procedure, such as, the maximum displacement and velocity are reduced by 67.5% and 58.6%, respectively, and the maximum axial stress and strain are both decreased by 61.5% while the maximum shear stress and strain are reduced by 66.1%, and the failure probability of the first passage and cumulative fatigue damage are allayed by more than 99%, etc.     

Internal-Shear Mode Instabilities on High-Speed Liquid Jet, (II) Experimental Analysis of Curved Target Flow

The wave generation on high-speed liquid jets, induced by the instabilities of a laminar shear layer underneath the free surface (the internal-shear mode instabilities), are investigated theoretically and experimentally. Part II analyzes the jet curvature effect on the velocity profile and the surface configuration near the nozzle exit. The initial characteristics of the free-surface shear layer and the relaxation of the shear layer with the distance from the nozzle exit are also analyzed. Based on these results, the stability of the shear layer is analyzed for curved jet geometries simulating high-energy beam targets. The analytical results show that the gravity and the jet curvature have little effect on the stability of the shear layer, and that the shear layer stabilizes quickly with its relaxation. The predicted most-unstable wave numbers are in fair agreement with available experimental data for curved jets of both water and Li. This indicates that the internal-shear mode instabilities are responsible for the experimentally observed waves.