An assessment of k-ε turbulence models for gas distribution analysis

This paper presents the gas distribution analysis by injecting air fountain into the containment and simulations with the HYDRAGON code. Turbulence models of standard k-ε(SKE), re-normalization group k-ε(RNG) and a realizable k-ε(RLZ) are used to assess the effects on the gas distribution analysis during a severe accident in a nuclear power plant. By comparing with experimental data,the simulation results of the RNG and SKE turbulence models agree well with the experimental data on the prediction of dimensionless density distributions. The results illustrate that the turbulence model choice had a small effect on the simulation results, particularly the region near to the air fountain source.     

Simulation of the Effects of Several Factors on Arc Plasma Behavior in Low Voltage Circuit Breaker

Taking into account the properties of the arc plasma and the electromagnetic, heat and radiative phenomena, commercial computational fluid dynamics software PHOENICS has been adapted and modified to develop the three-dimensional magneto-hydrodynamic （MHD） model of arc in a low voltage circuit breaker. The effects of the arc ignition location, venting size and gassing material on arc behavior have been investigated. The analysis of the results show that the arc velocity accelerates with the increase in the distance between arc ignition location and of the venting size, and the existence of the gassing material is beneficial to improving the arc voltage and reducing the arc temperature.     

The Influence of Contact Space on Arc Commutation Process in Air Circuit Breaker

In this paper,a 3D magneto-hydrodynamic (MHD) arc simulation model is applied to analyze the arc motion during current interruption in a certain air circuit breaker (ACB).The distributions of pressure,temperature,gas flow and current density of the arc plasma in the arc region are calculated,and the factors influencing the commutation process are analyzed according to the calculated results.Based on the airflow in the arc chamber,the causes of arc commutation asynchrony and the back commutation are investigated.It indicates that a reasonable contact space design is crucial to a successful arc commutation process.To verify the simulation results,the influence of contact space on arc voltage and arc commutation is tested.This research can provide methods and references to the optimization of ACB design.     

Characteristic analysis of CO2 switching arcs under a DC current

The current interruption capability of a gas,when used in high voltage gas-blast circuit breakers,depends not only on its material properties but also the flow field since turbulence plays a dominant role in arc cooling during the interruption process.Based on available experimental results,a study of CO2 switching arcs under a DC (direct current) current in the model circuit breaker has been conducted to calibrate CO2 arc model and to analyse its electric and thermal property.Through detailed analysis of the results mechanisms responsible for the temperature distribution are identified and the domain energy transportation process of different region discussed.The present work provides significant coefficients for CO2 switching arc simulation and gives a better understanding of CO2 arc burning mechanisms.     

Numerical simulation of constricted and diffusive arc–anode attachments in wall-stabilized transferred argon arcs

A numerical simulation is conducted to investigate arc–anode attachment behavior, especially the formation mechanism of the constricted arc attachment mode for the water-cooled anode of wall-stabilized transferred argon arcs. Argon molecular ions and the corresponding kinetic processes are included to the finite-rate chemistry model in order to capture the chemical nonequilibrium characteristics of the arc near the anode region. Modeling results show that constricted and diffusive arc–anode attachments can be self-consistently obtained at different arc currents while keeping other parameters unchanged. The dominant kinetic processes contributing to ionization and recombination in the arc center and fringes are presented. The results show that in arc fringes and the arc attachment region, molecular ion recombination plays an important role which leads to the rapid loss of electrons. The radial evolution of the production, loss and transport processes of electrons is further analyzed. It is found that for the constricted arc attachment mode, both the recombination and convection transport caused by the anode jet result in the loss of electrons at the arc fringes, which leads to the shrinkage of the arc column at the anode. The formation of the anode jet is due to the combined action of radial and axial Lorentz forces in the anode region.     

3D Numerical Analysis of the Arc Plasma Behavior in a Submerged DC Electric Arc Furnace for the Production of Fused MgO

A three dimensional steady-state magnetohydrodynamic model is developed for the arc plasma in a DC submerged electric arc furnace for the production of fused MgO. The arc is generated in a small semi-enclosed space formed by the graphite electrode, the molten bath and unmelted raw materials. The model is first used to solve a similar problem in a steel making furnace, and the calculated results are found to be in good agreement with the published measurements. The behavior of arcs with different arc lengths is also studied in the furnace for MgO production. From the distribution of the arc pressure on the bath surface it is shown that the arc plasma impingement is large enough to cause a crater-like depression on the surface of the MgO bath. The circulation of the high temperature air under the electrode may enhance the arc efficiency, especially for a shorter arc.     

Three-Dimensional Simulation of Plasma Deformation During Contact Opening in a Circuit Breaker,Including the Analysis of Kink Instability and Sausage Instability

A three-dimensional(3-D) transient model has been developed to investigate plasma deformation driven by a magnetic field and its influence on arc stability in a circuit breaker.The 3-D distribution of electric current density is obtained from a current continuity equation along with the generalized Ohm’s law;while the magnetic field induced by the current flowing through the arc column is calculated by the magnetic vector potential equation.When gas interacts with an arc column,fundamental factors,such as Ampere’s law,Ohm’s law,the turbulence model,transport equations of mass,momentum and energy of plasma flow,have to be coupled for analyzing the phenomenon.The coupled interactions between arc and plasma flow are described in the framework of time-dependent magnetohydrodynamic(MHD) equations in conjunction with a K-ε turbulence model.Simulations have been focused on sausage and kink instabilities in plasma(these phenomena are related to pinch effects and electromagnetic fields).The 3-D simulation reveals the relation between plasma deformation and instability phenomena,which affect arc stability during circuit breaker operation.Plasma deformation is the consequence of coupled interactions between the electromagnetic force and plasma flow described in simulations.     

Numerical analysis of turbulent mixed convection air flow in inclined plane channel with k-ε type turbulence model

Numerical study on turbulent mixed convection in inclined plane channels, from 15° to 90° (vertical), was carried out to examine the effect of inclination on fluid flow and heat transfer distributions. The turbulent air flows upward or downward into the duct with one wall heated from bottom. Calculation results with several kinds of k-εtype turbulence models were used to compare the experimental data with those in literatures to determine suitable model. The dependents of Nusselt number on the inclination angle of both the buoyancy-aided and buoyancy-opposed flow are discussed.     

Numerical Simulation of High-Current Vacuum Arc with Consideration of Anode Vapor

A high-current vacuum arc (HCVA) with the consideration of anode vapor is mod-eled and simulated. First, from the HCVA column model, the heat flux density to the anode is obtained, which is put into the anode activity model, and the parameter distributions (such as the vapor temperature and velocity) of anode vapor are obtained from the simulation results of the anode activity model. Then, by iterating and calculating the HCVA column model and anode activity model, the interaction between the HCVA column and the anode vapor is simulated and analyzed. In the simulation, the distribution of the axial magnetic field (AMF) generated by the electrode system is calculated by software ANSYS. The simulation results show that the influence of anode vapor on the parameter distributions in the arc column is significant. The simulation results are also compared with the vacuum arc photograph.     

Numerical Simulation of the Eddy Current Effects on the Arc Splitting Process

This paper focuses on a numerical simulation of the arc plasma behavior in the arc splitting process,considering the eddy currents in the electrodes and the splitter plate.Based on three-dimensional(3D) magneto-hydrodynamic(MHD) theory,a thin layer of nonlinear electrical resistance elements is used in the model to represent the voltage drop of plasma sheath and the formation of new arc root in order to include the arc splitting process in the simulation.In the arcing process,eddy currents in metal parts are generated by a time-varying magnetic field.The arc model is calculated with the time-varying magnetic field term,so that the eddy current effects can be considered.The effect of nonlinear permeability of a ferromagnetic material is also involved in the calculation.Using the simulation results for the temperature,velocity and current density distribution,the arc splitting process is analyzed in detail.The calculated results are compared with the simulation neglecting eddy currents.     

Benchmark Simulation of Turbulent Flow through a Staggered Tube Bundle to Support CFD as a Reactor Design Tool. Part I: SRANS CFD Simulation

Time-invariant and time-variant numerical simulations of flow through a staggered tube bundle array, idealizing the lower plenum (LP) subsystem configuration of a very high temperature reactor (VHTR), were performed. In Part I, the CFD prediction of fully periodic isothermal tube-bundle flow using steady Reynolds-averaged Navier-Stokes (SRANS) equations with common turbulence models was investigated at a Reynolds number (Re) of 1.8 × 10⁴, based on the tube diameter and inlet velocity. Three first-order turbulence models, standard k-ε turbulence, renormalized group (RNG) k-ε, and shear stress transport (SST) k-ω models, and a second-order turbulence model, Reynolds stress model (RSM), were considered. A comparison of CFD simulations and experiment results was made at five locations along (x, y) coordinates. The SRANS simulation showed that no universal model predicted the turbulent Reynolds stresses, and generally, the results were marginal to poor. This is because these models cannot accurately model the periodic, spatiotemporal nature of the complex wake flow structure.     

Numerical Simulation of Heat Transfer of Supercritical Fluids in Circular Tubes Using Different Turbulence Models

In this study, the heat transfer of supercritical fluids in vertical and horizontal circular tubes has been investigated numerically to understand the thermal-hydraulic behavior of supercritical fluids. The simulations are carried out using different turbulence models and the numerical results are compared with the experimental data to evaluate the accuracy and applicability of those turbulence models. Six turbulence models are used in this study, the LB low-Re k-? model, the LS low-Re k-? model, the RNG k-? model, the realizable k-? model, the standard k-? model, and the Reynolds stress model. The comparison shows that the Reynolds stress model gives better agreement with the experimental data than other turbulence models studied in this work.     

基于水工质的有窗散裂靶流场分析及实验测量

散裂靶是加速器驱动的次临界系统(ADS)的重要组成部分,有窗散裂靶是唯一经实验验证、测量的液态金属高功率散裂靶,研究有窗靶内工质的流动对散裂靶的设计优化有重要意义。本文以水为工质对有窗靶件进行了可视化实验及数值模拟研究,实验采用粒子图像测速法对靶件可视化部分进行速度场测量,同时利用计算流体力学软件FLUENT对靶件流场进行数值模拟。通过5种湍流模型（标准k-ε模型、RNG k-ε模型、Realk-ε模型、SST k-ω模型、RSM模型）在不同流速下的模拟结果与实验结果的对比分析,表明采用RNG k-ε模型并结合相应的壁面函数能较准确模拟有窗靶内的流动。     

Plasma-facilitated modification of pumpkin vine-based biochar and its application for efficient elimination of uranyl from aqueous solution

An acrylic modified pumpkin vine-based biochar (p-PVB-PAA) is synthesized by non-thermal plasma-grafting modification of pumpkin vine-based biochar (PVB) for the removal of uranyl from an aqueous solution. Microscopic characterization reveals that compared to PVB the surface of p-PVBPAA has more oxygen-containing functional groups by strong chemical bonding and the specific surface area is increased to 275.3 m² g⁻¹ from 3.8 m² g⁻¹. It is found that p-PVB-PAA showed a much higher maximum adsorption capacity for uranyl from aqueous solutions than PVB, which were 207.02 mg g⁻¹ and 67.58 mg g⁻¹ at pH=5 and 298 K, respectively. Moreover, the adsorption behavior follows a pseudo-second-order kinetics model and the Langmuir adsorption model. Additionally, macroscopic experiments and spectroscopic studies verified that the significantly improved adsorption performance of the p-PVB-PAA is due to surface complexation and electrostatic interactions. Furthermore, the very high removal efficiency and excellent regeneration ability (the percentage of the removal still remained at nearly 90% after five cycles) makes this low-cost, easily obtained, and environmentally friendly material attractive for commercial application.     

超临界CO2在螺旋管中的流动换热特性研究

超临界蒸发器应用到核电中,可大幅提高机组的热效率。超临界压力流体的热物性在准临界温度附近变化非常剧烈,会对其流动和换热产生很大的影响。研究超临界压力流体在螺旋管内的流动和换热规律,有利于对超临界螺旋管蒸发器的设计。本文采用RNG k-ε和SST k-ω模型对超临界CO₂在螺旋管中的流动换热情况进行了数值模拟,发现SST k-ω模型模拟结果与实验结果符合得更好。基于此模型,分析了不同进口质量流速及不同热流密度对管壁温和换热系数的影响,发现随着质量流速的减小、热流密度的增加,峰值向远离h_pc的一侧偏移。最后讨论并分析了周向壁温和换热系数的分布情况,发现壁温在φ＝315°处最高,需在实验操作或实际运行中加以监控,以保障螺旋管蒸发器的安全运行。     

Efficient direction-independent fog harvesting using a corona discharge device with a multi-electrode structure

Efficient collection of water from fog can effectively alleviate the problem of water shortages in foggy but water-scarce areas, such as deserts, islands and so on. Unlike inefficient fog meshes, corona discharge can charge water droplets and further enhance the water-collecting effect. This study proposes a novel multi-electrode collecting structure that can achieve efficient and direction-independent water collection from fog. The multi-electrode structure consists of three parts: a charging electrode, an intercepting electrode and a ground electrode. Four types of water-collecting structures are compared experimentally, and the collection rates from a traditional fog mesh, a wire-mesh electrode with fog coming from a high-voltage electrode, a wire-mesh electrode with fog coming from a ground electrode and a multi-electrode structure are 2–3 g h⁻¹, 100–120 g h⁻¹, 60–80 g h⁻¹ and 200–220 g h⁻¹, respectively. The collection rate of the multi-electrode structure is 100‒150 times that of a traditional fog mesh and 2–4 times that of a wire-mesh electrode. These results demonstrate the superiority of the multi-electrode structure in fog collection. In addition, the motion equation of charged droplets in an electric field is also derived, and the optimization strategy of electrode spacing is also discussed. This structure can be applied not only to fog collection, but also to air purification, factory waste gas treatment and other fields.     

基于核主泵性能预测的数值模拟精度研究

为提高核主泵在全工况点的数值模拟精度,研究了数值模拟过程中近壁面网格尺度、湍流模型、流动状态3种因素对计算精度的影响。结果表明,在定常状态下,重整化群（RNG） k-ε湍流模型和标准壁面函数法在近壁面网格尺度（y+）为50左右时具有较高的计算精度,并且其计算精度高于RNG k-ε增强壁面函数法、低雷诺数k-ε和剪切应力传输（SST）k-ω这3种湍流模型的计算精度,但上述不同网格尺度和湍流模型的计算结果均存在较大的计算误差;采用非定常计算时的计算精度明显高于定常计算,能够反映出扬程曲线在关死点附近的驼峰现象,效率的计算精度也有一定改善,更适合于对核主泵进行性能预测。      

二维孔隙结构内单相流动阻力特性数值模拟

利用计算流体力学(CFD)商业软件CFX 10.0,采用标准k-ε、RNG k-ε以及SST模型3种不同的湍流模型,对矩形管内球形颗粒作2维有序排列所形成的孔隙流道的等温单相流动进行数值模拟,并与Ergun关系式预计值进行对比;探讨球形颗粒的排列方式、直径等对单相流动阻力的影响;研究矩形管内单位长度压降及阻力系数随孔隙雷诺数Re的变化规律(1.5≤Re≤1497)。     

竖直圆管内超临界压力氟利昂传热试验研究

深入研究超临界压力下流体特殊的对流传热特性,对超临界水冷反应堆的堆芯设计至关重要。在上海交通大学SMOTH氟利昂回路上开展了压力4.3~4.7 MPa、质量流速600~2 500kg/(m2·s)、热流密度20~180kW/m2参数下的圆管内超临界上升流传热试验。远离拟临界温度区间内换热系数和Dittus-Boelter公式计算值很接近,热流密度越大,近拟临界区换热系数越小,小质量流速大热流密度下,发生显著传热恶化。加速效应无量纲数和浮升力无量纲数对传热特性显示了强烈的相关性。提出了氟利昂工质传热试验的传热恶化起始点关系式。Bishop关系式计算换热系数和试验值之间标准差很小,但整体略偏大;Jackson关系式计算值和试验值之间平均偏差很小,但标准差偏大。     

Microchannel cooling technique for dissipating high heat flux on W/Cu flat-type mock-up for EAST divertor

As an important component of tokamaks, the divertor is mainly responsible for extracting heat and helium ash, and the targets of the divertor need to withstand high heat flux of 10 MW m⁻² for steady-state operation. In this study, we proposed a new strategy, using microchannel cooling technology to remove high heat load on the targets of the divertor. The results demonstrated that the microchannel-based W/Cu flat-type mock-up successfully withstood the thermal fatigue test of 1000 cycles at 10 MW m⁻² with cooling water of 26 l min⁻¹, 30 °C (inlet), 0.8 MPa (inlet), 15 s power on and 15 s dwell time; the maximum temperature on the heat-loaded surface (W surface) of the mock-up was 493 °C, which is much lower than the recrystallization temperature of W (1200 °C). Moreover, no occurrence of macrocrack and 'hot spot' at the W surface, as well as no detachment of W/Cu tiles were observed during the thermal fatigue testing. These results indicate that microchannel cooling technology is an efficient method for removing the heat load of the divertor at a low flow rate. The present study offers a promising solution to replace the monoblock design for the EAST divertor