Magneto-hydrodynamic simulation study of direct current multi-contact circuit breaker for equalizing breaking arc

This work is based on a direct current(DC)natural current commutation topology,which uses load-carrying branch contacts carrying rated current and multiple sets of series arcing branch contacts in parallel to achieve circuit breaking.The proposed topology can meet the new requirements of higher voltage DC switches in aviation,aerospace,energy and other fields.First,a magneto-hydrodynamic arc model is built using COMSOL Multiphysics,and the different arc breaking characteristics of the arcing branch contacts in different gas environments are simulated.Then,a voltage uniformity coefficient is used to measure the voltage sharing effect in the process of dynamic interruption.In order to solve the dispersion of arcing contact action,a structural control method is adopted to improve the voltage uniformity coefficient.The uniform voltage distribution can improve the breaking capacity and electrical life of the series connection structure.

垂直上升通道内气泡–液态金属MHD流动与传热的数值研究

在聚变堆的冷却系统中,由于强磁场的影响,液态金属流动受到抑制,通过掺入气泡到液态金属中可减小磁流体力学(Magnetohydrodynamic,MHD)效应,提升传热速率。该文基于VOF多相流模型,在不同的磁场强度(0~1.3 T)和不同的气泡雷诺数(1.3×10^(4)~6.76×10^(4))范围内,对横向磁场作用下垂直上升通道中气泡-液态金属两相流动传热特性进行数值研究。结果表明,无磁场时气体呈柱状流,而在磁场作用下,由于MHD箍缩效应的影响,气柱破裂、分离,形成了气泡流;随着磁场增大,其分离点由入口逐渐向上移动,且气泡运动速度随之增加。与液态金属单相流相比,气泡-液态金属两相流动的平均换热系数随磁场强度和气泡雷诺数增加均呈下降趋势,但因两相流动显著减小由MHD效应引起的压降,从而提升通道内综合换热因子,且在磁场较小时,提升效果更好。

Simulation of Natural Convection Flow with Magneto-Hydrodynamics in a Wavy Top Enclosure with a Semi-Circular Heater

Natural convection flow in enclosure has different applications such as room ventilation, heat exchangers, the cooling system of a building etc. The Finite-Element method based on the Galerkin weighted residual approach is used to solve two-dimensional governing mass, momentum and energy-equations for natural convection flow in the presence of a magnetic field on a roof top with semi-circular heater. In the enclosure the horizontal lower wall was heated, the vertical two walls were adiabatic, inside the semi-circular heater, the wavy top wall cooled. The parameters Rayleigh number, Hartmann number and Prandtl number are considered. The effects of the Hartmann number and Rayleigh number on the streamlines, isotherms, velocity profiles and average Nusselt number are examined graphically. The local Nusselt number and the average Nusselt number of the heated portion of the enclosure with the semi-circular heater are presented in this paper. Finally, for the validation of the existing work, the current results are compared with published results and the auspicious agreement is achieved.

Utilization of Logistical Regression to the Modified Sine-Gordon Model in the MST Experiment

In this paper, a logistical regression statistical analysis (LR) is presented for a set of variables used in experimental measurements in reversed field pinch (RFP) machines, commonly known as “slinky mode” (SM), observed to travel around the torus in Madison Symmetric Torus (MST). The LR analysis is used to utilize the modified Sine-Gordon dynamic equation model to predict with high confidence whether the slinky mode will lock or not lock when compared to the experimentally measured motion of the slinky mode. It is observed that under certain conditions, the slinky mode “locks” at or near the intersection of poloidal and/or toroidal gaps in MST. However, locked mode cease to travel around the torus;while unlocked mode keeps traveling without a change in the energy, making it hard to determine an exact set of conditions to predict locking/unlocking behaviour. The significant key model parameters determined by LR analysis are shown to improve the Sine-Gordon model’s ability to determine the locking/unlocking of magnetohydrodyamic (MHD) modes. The LR analysis of measured variables provides high confidence in anticipating locking versus unlocking of slinky mode proven by relational comparisons between simulations and the experimentally measured motion of the slinky mode in MST.

Magneto-hydrodynamic flow of squeezed fluid with binary chemical reaction and activation energy

The present exploration is conducted to describe the motion of viscous fluid embedded in squeezed channel under the applied magnetics effects.The processes of heat and mass transport incorporate the temperature-dependent binary chemical reaction with modified Arrhenius theory of activation energy function which is not yet disclosed for squeezing flow mechanism.The flow,heat and mass regime are exposed to be governed via dimensionless,highly non-linear,ordinary differential equations (ODEs) under no-slip walls boundary conditions.A well-tempered analytical convergent procedure is adopted for the solutions of boundary value problem.A detailed study is accounted through graphs in the form of flow velocity field,temperature and fluid concentration distributions for various emerging parameters of enormous interest.Skin-friction,Nusselt and Sherwood numbers have been acquired and disclosed through plots.The results indicate that fluid temperature follows an increasing trend with dominant dimensionless reaction rate σ and activation energy parameter E.However,an increment in σ and E parameters is found to decline in fluid concentration.The current study arises numerous engineering and industrial processes including polymer industry,compression and injection shaping,lubrication system,formation of paper sheets,thin fiber,molding of plastic sheets.In the area of chemical engineering,geothermal engineering,cooling of nuclear reacting,nuclear or chemical system,bimolecular reactions,biochemical process and electrically conducting polymeric flows can be controlled by utilizing magnetic fields.Motivated by such applications,the proposed study has been developed.

Heat Transfer in MHD Flow of Maxwell Fluid via Fractional Cattaneo-Friedrich Model:A Finite Difference Approach

The idea of fractional derivatives is applied to several problems of viscoelastic fluid.However,most of these problems(fluid problems),were studied analytically using different integral transform techniques,as most of these problems are linear.The idea of the above fractional derivatives is rarely applied to fluid problems governed by nonlinear partial differential equations.Most importantly,in the nonlinear problems,either the fractional models are developed by artificial replacement of the classical derivatives with fractional derivatives or simple classical problems(without developing the fractional model even using artificial replacement)are solved.These problems were mostly solved for steady-state fluid problems.In the present article,studied unsteady nonlinear non-Newtonian fluid problem(Cattaneo-Friedrich Maxwell(CFM)model)and the fractional model are developed starting from the fractional constitutive equations to the fractional governing equations;in other words,the artificial replacement of the classical derivatives with fractional derivatives is not done,but in details,the fractional problem is modeled from the fractional constitutive equations.More exactly two-dimensional magnetic resistive flow in a porous medium of fractional Maxwell fluid(FMF)over an inclined plate with variable velocity and the temperature is studied.The Caputo time-fractional derivative model(CFM)is used in the governing equations.The proposed model is numerically solved via finite difference method(FDM)along with L1-scheme for discretization.The numerical results are presented in various figures.These results indicated that the fractional parameters significantly affect the temperature and velocity fields.It is noticed that the temperature field increased with an increase in the fractional parameter.Whereas,the effect of fractional parameters is opposite on the velocity field near the plate.However,this trend became like that of the temperature profile,away from the plate.Moreover,the velocity field retarded with strengthening in the magnetic parameter due to enhancement in Lorentz force.However,this effect reverses in the case of the temperature profile.

Approximate solution of the flow over a nonlinear magneto-hydrodynamic stretching sheet

The approximate solution of the magneto-hydrodynamic （MHD） boundary layer flow over a nonlinear stretching sheet is obtained by combining the Lie symmetry method with the homotopy perturbation method. The approximate solution is tabulated, plotted for the values of various parameters and compared with the known solutions. It is found that the approximate solution agrees very well with the known numerical solutions, showing the reliability and validity of the present work.

Hall Current Effects on Unsteady MHD Flow in a Rotating Parallel Plate Channel Bound-ed by Porous Bed on the Lower Half—Darcy Lapwood Model

We discussed the unsteady flow of an incompressible viscous fluid in a rotating parallel plate channel bounded on one side by a porous bed under the influence of a uniform transverse magnetic field taking hall current into account. The perturbations are created by a constant pressure gradient along the plates in addition to the non-torsional oscillations of the upper plate. The flow in the clean fluid region is governed by Navier-Stoke’s equations while in the porous bed the equations are based on Darcy-Lapwood model. The exact solutions of velocity in the clean fluid and the porous medium consist of steady state and transient state. The time required for the transient state to decay is evaluated in detail and ultimate quasi-steady state solution has been derived analytically and also its behaviour is computationally discussed with reference to different flow parameters. The shear stresses on the boundaries and the mass flux are also obtained analytically and their behaviour is computationally discussed.

微重力环境对MHD角速度传感器性能影响研究

磁流体动力学角速度传感器在轨测试期间,其内部流体部分运动特性在微重力下与实验室(常重力)环境不同,表现为静压梯度、沉浮分层与浮力对流减弱,且航天器姿态调整时传感器所受微重力方向会发生变化,故有必要分析航天器所在微重力环境对传感器输出性能的影响。由于在轨测试成本高,地面测试复现效果不理想,利用仿真软件Fluent,对比两种不同结构的传感器在常重力下和航天器不同姿态的微重力下流体运动特征与敏感元件输出特性。仿真结果表明,相较于常重力环境,在航天器不同姿态的微重力环境下,径向磁场结构敏感元件的标度相对误差不超过0.01%,最大值较轴向磁场结构减小1个数量级,低于标度重复性测试相对误差3个数量级,相位偏差小于10^(-5)°。

Computational analysis for fractional characterization of coupled convection-diffusion equations arising in MHD fows

The work is devoted to the fractional characterization of time-dependent coupled convection-diffusion systems arising in magnetohydrodynamics(MHD)flows.The time derivative is expressed by means of Caputo’s fractional derivative concept,while the model is solved via the full-spectral method(FSM)and the semi-spectral scheme(SSS).The FSM is based on the operational matrices of derivatives constructed by using higher-order orthogonal polynomials and collocation techniques.The SSS is developed by discretizing the time variable,and the space domain is collocated by using equal points.A detailed comparative analysis is made through graphs for various parameters and tables with existing literature.The contour graphs are made to show the behaviors of the velocity and magnetic fields.The proposed methods are reasonably efficient in examining the behavior of convection-diffusion equations arising in MHD flows,and the concept may be extended for variable order models arising in MHD flows.

基于MHD的小间隙真空电弧仿真研究

真空开关电弧电流过零前的电弧等离子体状态对真空开关能否顺利开断具有十分重要的影响。本文在建立小间隙真空电弧磁流体动力学(MHD)模型的基础上,采用通用计算流体力学仿真软件对电弧电流过零前0.5 ms时的电弧等离子体的特性进行数值仿真研究。计算得到了真空电弧等离子体压力、等离子体密度、离子纵向速度、阳极表面纵向电流密度和电弧电压等的空间分布。仿真结果表明:从阴极到阳极,真空电弧等离子体压力和密度逐渐增大,而等离子体速度逐渐减小;阳极表面电流密度存在较大径向梯度,最大值低于形成阳极斑点的极限收缩电流,阳极仍不活跃。仿真得到的等离子体密度分布与高速摄像机拍摄得到的CMOS图像光强基本吻合,计算得到的电弧电压与测得的电弧电压是一致的,从而验证了本模型的可行性。

喷口电弧二维磁流体动力学(MHD)数学模型

为了揭示电弧在开断过程中的复杂物理现象，建立的喷口电弧二维磁流体动力学数学模型，强调了电弧、气流、电磁场三者的相互作用，消除了因忽略Ｌｏｒｅｎｔｚ力和电流密度径向分量引起的物理意义失真．应用有限元法对模型中Ｎ－Ｓ（ＮａｖｉｅｒＳｔｏｋｅｓ）方程和二维电磁场方程进行数值求解．数值计算结果反映了Ｌｏｒｅｎｔｚ力以及电流密度径向分量对电弧特性的影响，同时揭示了电弧热边界区的存在．

MHD燃烧室煤粉喷嘴的冷态试验研究

对浓相输送条件下ＭＨＤ燃烧室的钝体型煤粉喷嘴进行了冷态试验研究，用极板电容棵针测量了钝体不同张角、不同轴向位置、不同宽径比下ＭＨＤ燃烧室内煤粉颗粒相对浓度分布，分析了上述因素对浓度场的影响。

星载MHD传感器灵敏度特性误差分析与模型验证

磁流体动力学(magneto-hydro-dynamic,简称MHD)传感器是测量航天器平台宽频微角颤振的关键器件之一。鉴于MHD传感器的误差与噪声特性会影响星载惯性导航系统的精度与分辨率,对所设计的MHD传感器进行误差分析与试验验证。采用样机数据分析与组件物理建模相结合的方法,分析与验证MHD传感器灵敏度特性与误差模型。结果表明:传感器的误差主要受到重力方向与温度条件的影响,并且该项误差对无温控MHD传感器的误差起决定作用;当温度变化较小时,变压器磁芯磁导率的工艺误差占总误差的一半;差分放大器的噪声对传感器噪声具有显著影响。

Effect of contact materials on the transient characteristics of vacuum arc plasma and anode erosion

In this study, the mechanisms of the anode phenomena and anode erosion with various contact materials were investigated. Arc parameters were calculated, and the anode temperature was predicted with a transient self-consistent model. The simulation results predicted a constricted arc column and obvious anode phenomena in Cu–Cr alloy contacts than in W–Cu alloy contacts.This observation could be the reason for the concentrated anode erosion in Cu–Cr alloys. For the contacts made by pure tungsten(W) and W–Cu alloy, the anode temperature increased rapidly because of the low specific heat of W. However, the maximum energy flux from the arc column to the anode surface was lower than in other cases. The simulation results were compared with experimental results.

火星沉降H-ENA的分布与特性

能量中性原子(Energetic Neutral Atom,ENA)是由能量离子与中性背景成分电荷交换产生.火星的外逸层扩展范围远高于弓激波,上游太阳风质子与其相互作用并转化为太阳风氢ENA(Hydrogen-ENA,H-ENA),新生的H-ENA可直接进入低层大气,成为新的物质与能量输运通道.本文基于单流体多成分MHD模型与中性外逸层模型,计算了火星200 km等高面沉降H-ENA通量的空间分布,统计了多种太阳风条件下沉降H-ENA的粒子与能量沉降率,分析影响因素.结果表明,在一定模拟条件下,产生于弓激波上游的太阳风H-ENA,呈cosZ分布(Z为天顶角),受磁异常影响较小,是沉降H-ENA的主要成分,占ENA总沉降量的59%,占能量沉降的81%;产生于磁鞘的磁鞘H-ENA受磁异常阻碍影响较大,在最强磁异常上方其沉降通量显著下降;沉降H-ENA与上游太阳风的通量呈正比例关系;2.1%~3.5%上游太阳风质子转化为太阳风H-ENA.

A New Class of Simple,General and Efficient Finite Volume Schemes for Overdetermined Thermodynamically Compatible Hyperbolic Systems

In this paper,a new efficient,and at the same time,very simple and general class of thermodynamically compatiblefinite volume schemes is introduced for the discretization of nonlinear,overdetermined,and thermodynamically compatiblefirst-order hyperbolic systems.By construction,the proposed semi-discrete method satisfies an entropy inequality and is nonlinearly stable in the energy norm.A very peculiar feature of our approach is that entropy is discretized directly,while total energy conservation is achieved as a mere consequence of the thermodynamically compatible discretization.The new schemes can be applied to a very general class of nonlinear systems of hyperbolic PDEs,including both,conservative and non-conservative products,as well as potentially stiff algebraic relaxation source terms,provided that the underlying system is overdetermined and therefore satisfies an additional extra conservation law,such as the conservation of total energy density.The proposed family offinite volume schemes is based on the seminal work of Abgrall[1],where for thefirst time a completely general methodology for the design of thermodynamically compatible numerical methods for overdetermined hyperbolic PDE was presented.We apply our new approach to three particular thermodynamically compatible systems:the equations of ideal magnetohydrodynamics(MHD)with thermodynamically compatible generalized Lagrangian multiplier(GLM)divergence cleaning,the unifiedfirst-order hyperbolic model of continuum mechanics proposed by Godunov,Peshkov,and Romenski(GPR model)and thefirst-order hyperbolic model for turbulent shallow waterflows of Gavrilyuk et al.In addition to formal mathematical proofs of the properties of our newfinite volume schemes,we also present a large set of numerical results in order to show their potential,efficiency,and practical applicability.

竖直平板嵌入非Darcy多孔介质时,数值研究流过平板的不稳定MHD自然对流中的Dufour和Soret效应

就竖直平板嵌入非Darcy多孔介质中,导电流体流过平板时作不稳定的二维磁流体(MHD)双扩散对流,数值研究了Dufour和Soret效应对流动的影响.用Crank-Nicolson型的隐式有限差分法,按三对角矩阵处理,求解无量纲的非线性控制方程.详细地研究了问题中出现的各种参数对不稳定无量纲的速度、温度和浓度曲线的影响.进一步地,给出并分析了表面摩擦因数、Nus-selt数和Sherwood数随时间的变化.研究结果表明,不稳定速度、温度和浓度分布曲线,受Dufour和Soret的影响十分显著.随着Dufour数的增加或者Soret数的减小,表面摩擦因数和Sherwood数都在减小,而Nusselt数在增加.研究发现,当磁场参数增加时,边界层中的速度和温度在减小.

MHD治疗的CKD-MBD患者行5E康复模式的延续性护理对iPTH的研究

目的探究维持性血液透析(MHD)治疗的慢性肾脏病矿物质与骨异常(CKD-MBD)患者行5E康复模式的延续性护理对甲状旁腺激素(iPTH)的作用。方法随机选择2018年7月—2020年6月开展MHD治疗的80例CKD-MBD患者为研究对象,按照选取顺序编号,利用抽签法平分为对照组、研究组,每组40例。对照组采取常规护理措施,研究组在前者基础上开展5E康复模式的延续性护理,比较病情恢复相关指标和自我管理能力。结果研究组护理后校正钙达标率、血磷达标率、iPTH达标率分别为72.50%、70.00%、67.50%,高于对照组37.50%、35.00%、32.50%,差异有统计学意义(χ^(2)=9.899、9.825、9.800,P<0.05);研究组护理后iPTH为(218.67±23.97)pg/mL,低于对照组(342.63±24.12)pg/mL,差异有统计学意义(t=23.055,P<0.05);研究组自我护理能力评分为(62.67±4.97)分,高于对照组(55.63±4.12)分,差异有统计学意义(t=6.897,P<0.05)。结论针对MHD治疗的CKD-MBD患者,进行5E康复模式的延续性护理利于其病情恢复。

Simulation of Fault Arc Based on Different Radiation Models in a Closed Tank

This paper focuses on the simulation of a fault arc in a closed tank based on the magneto-hydrodynamic （MHD） method, in which a comparative study of three radiation models, including net emission coefficients （NEC）, semi-empirical model based on NEC as well as the P1 model, is developed. The pressure rise calculated by the three radiation models are compared to the measured results. Particularly when the senti-empirical model is used, the effect of different boundary temperatures of the re-absorption layer in the semi-empirical model on pressure rise is concentrated on. The results show that the re-absorption effect in the low-temperature region affects radiation transfer of fault arcs evidently, and thus the internal pressure rise. Compared with the NEC model, P1 and the semi-empirical model with 0.7 〈 α 〈 0.83 are more suitable to calculate the pressure rise of the fault arc, where is an adjusted parameter involving the boundary temperature of the re-absorption region in the semi-empirical model.