Three dimensional CFD analysis of Cable-in-Conduit Conductors (CICCs) using porous medium approach

Thermohydraulic studies based on porous medium analogy, pertinent to dual channel Cable-in-Conduit Conductors (CICCs) used in International Thermonuclear Experimental Reactor (ITER), are explored in the present work. Dual channel CICC used in Toroidal Field (TF) Coil consists of a circular jacket in which superconducting cable bundles are placed in the annular channel separated from the central channel by a spiral. The cable bundle in the annular channel can be considered as saturated porous medium and the central channel can be viewed as clear region for thermohydraulic studies. In the present work, a 3D Computational Fluid Dynamics (CFD) analysis is performed on CICC by considering dual channel CICC as partially filled saturated porous medium. The 3D geometry was developed and meshed in GAMBIT-2.1.6, and exported to a commercial solver FLUENT -6.3.26 for further analysis. The effect of mass flow rate ( 6 - 10 g/s) of supercritical helium (SHe) on the velocity and pressure gradient distributions (axial and radial) in the transverse plane is presented. These studies resulted in estimating the mass flow repartition between the two channels and pumping power required to pump the SHe in CICC. In addition, the present CFD analysis brings a clear perspective of the phenomena of flow and heat transfer in complex geometries such as CICC.     

Using the HELIOS facility for assessment of bundle-jacket thermal coupling in a CICC

In a Cable In Conduit Conductor (CICC) cooled by forced circulation of supercritical helium, the heat exchange in the bundle region can play a significant role for conductor safe operation, while remaining a quite uncertain parameter. Heat exchange between bundle and jacket depends on the relative contributions of convective heat transfer due to the helium flow inside the bundle and of thermal resistance due to the wrappings between the cable and the conduit.In order to qualify this thermal coupling at realistic operating conditions, a dedicated experiment on a 1.2 m sample of ITER Toroidal Field (TF) dummy conductor was designed and performed in the HELIOS test facility at CEA Grenoble. Several methods were envisaged, and the choice was made to assess bundle-jacket heat transfer coefficient by measuring the temperature of a solid copper cylinder inserted over the conductor jacket and submitted to heat deposition on its outer surface.The mock-up was manufactured and tested in spring 2015. Bundle-jacket heat transfer coefficient was found in the range 300–500 W m⁻² K⁻¹. Results analysis suggests that the order of magnitude of convective heat transfer coefficient inside bundle is closer to Colburn–Reynolds analogy than to Dittus–Boelter correlation, and that bundle-jacket thermal coupling is mainly limited by thermal resistance due to wrappings. A model based on an equivalent layer of stagnant helium between wraps and jacket was proposed and showed a good consistency with the experiment, with relevant values for the helium layer thickness.     

Analysis of transverse heat transfer coefficient in CICC’s with central cooling channel

This paper describes a new method to determine the equivalent transverse heat transfer coefficient between the helium flow in the cable bundle and the flow in the central space of a CICC’s with parallel cooling channel. The method is based on the analysis of the temperature traces during a heat pulse experiment. The equations for the average temperature distribution in the cable are solved analytically to obtain a closed-form expression of the characteristic rise time of the temperature front as a function of the transverse heat transfer coefficient. The value of the equivalent transverse heat transfer coefficient from the bundle to the hole is then obtained as the best fit of the experimental rise time. We show the results of the method by application to short cable sample experiments in SULTAN.     

Effect of sub-cooling channel in an ITER 13 T-40 kA Nb3Sn coil (CS insert) on stability

This paper analyzes the stability of a center solenoid (CS) insert and describes the effects a sub-cooling channel (central channel) at the center cross section of a cable-in-conduit (CIC) conductor has on stability. First, the calculation results are compared with test results in a center solenoid (CS) insert, and the effectiveness of the calculation is verified. Next, the effects of central channel on stability are examined by comparing the energy margin between a case in which perforations exist between the bundle region and the central channel, such as a CS insert, to allow the fluid to migrate and a case in which perforations are blocked to prevent the coolant from migrating. The following points are clarified: (1) if most of the thermal disturbance is applied to the jacket, the case with perforations is more stable than the case without perforations, thus the central channel contributes to the improvement of stability; (2) if thermal disturbance is applied only to the cable and is produced for about 20 ms, the central channel hardly has any effect on the stability. If the thermal disturbance is produced for about 400 ms, the central channel contributes to the stability; (3) the effect of cooling the bundle region by means of heat conduction through the central channel wall is very small, and it hardly affects the stability at all.     

Transverse heat transfer coefficient in the dual channel ITER TF CICCs. Part III: Direct method of assessment

The data resulting from the thermal-hydraulic test of the ITER TF CICC are used to determine the flow partition and the overall effective heat transfer coefficient (h_BC) between bundle and central channel in a direct way, i.e. by analysis of the heat transfer between both flow channels, based on the mass and energy balance equations and the readings of thermometers located inside the cable. In cases without a local heat source in the considered cable segment the obtained h_BC values were consistent with those obtained in earlier studies by analysis of experimental data using indirect methods. It was also observed that the transverse heat transfer was strongly enhanced in a cable segment heated from outside. This phenomenon results from the mass transfer from the bundle region to the central channel. The experimental h_BC data obtained for the case without a heat source in the considered segment were also compared with those calculated using various heat transfer correlations.     

Transverse heat transfer coefficients on a full size dual channel CICC ITER conductor

Dual channel Cable-In-Conduit Conductors (CICC) provide low hydraulic resistance and faster central channel circulation, limiting superconductors temperature rise. The Poloidal Field Insert Sample (PFIS) was tested in the SULTAN facility to evaluate the thermal coupling between the CICC channels upon an experimental heat transfer coefficient assessment. Simple assumptions on the flow – homogeneous central and annular temperatures, no jacket conduction, no steel inertia and diffusivity – lead to a one-dimensional thermal model fully solved in its transient response to a Heavyside temperature evolution at the inlet, using a Laplace transformation. Transient temperature step data fitted with the analytical resolution provide heat transfer coefficients as a function of mass flow rate, compared to crude predictions. The transient measurements provided consistent measurements on the full range of mass flow rate in both vertical flow directions, whereas steady state homogenization characteristic length measures pursuing the same goal suffer from annular isothermal assumption. Recommendations are made for the thermohydraulic instrumentation of future conductor samples.     

Numerical study of flow and heat transfer of superfluid helium in capillary channels

A modified two-fluid model is adopted to study flow and heat transfer of superfluid helium in a microchannel with a diameter as small as that of a superleak in a fountain effect pump. Variable properties of superfluid helium and energy dissipations due to the two-fluid mutual friction and the friction at the channel wall are fully taken into consideration. It is found that the normal fluid component flow is not trivial even in a channel with diameter of a micrometre, and that there exists an optimum diameter for the maximum mass flow rate. The flow of superfluid helium through a channel with different temperatures at the ends differs considerably from that of a Newtonian fluid. The strong dependence of the thermodynamic properties on temperature and pressure, as well as the internal-convection mechanism are found to be the causes of the unique flows.     

中国先进研究堆(CARR)冷中子源装置设计

对中国在建的核功率为60 MW的中国先进研究堆中冷中子源系统的设计作了总体描述.该冷中子源采用液氢作为慢化剂,主要由两个分系统氢循环系统和氦制冷系统构成.氢循环系统中的冷包设计为带氦助冷通道的液氢层为月牙形的冷包.氢在连接冷包与氢氦换热器的单管内进行两相热虹吸循环.冷包材料和慢化剂氢的核发热通过氦制冷系统产生的冷氦带走,氦制冷系统采用带液氮预冷的逆布雷顿制冷循环.     

Evaluation of thermal gradients and thermosiphon in dual channel cable-in-conduit conductors

In an effort to optimize superconductor cryogenics of large coils, dual channel cable-in-conduit conductors (CICC) have been designed. The qualitative and economic rationale of the conductor central channel is here justified but brings high complexity to the conductor cooling characteristics. Temperature gradients in the cable must be quantified to guarantee conductor temperature margin during coil operation under heat disturbance and set adequate inlet temperature. A simple one-dimensional thermal model, with neither fluid nor strand or jacket conduction, allows to better understand and quantify the steady state behavior of CICC central and annular channels. This thermohydraulic model with homogeneous central and annular temperatures and no jacket conduction is summarized with explicit thermal coupling equations. Local convection coefficients chosen proportional to friction factors lead to a model of global interchannel heat exchange coefficient serving the bithermal model. A first stationary experimental evaluation of the internal heat transfer coefficient using the interchannel heat exchange space constant at various heat loads and mass flow rates is illustrated on two full size samples tested at cryogenic temperatures. Annular heaters experiments with low distributed power achieve pertinent model correlation. Discrepancy between model and experimental data may be linked to the simplistic homogeneous annular temperature hypothesis, to the estimate of CICC mass flow distribution among channels, and to gravitational effects at high heat loads. Perturbation due to the thermosiphon generated between the two channels is considered since neither the experiments nor the expected applications are free of gravity.     

液氦温区微小漏孔流动特性研究

低温环境下系统器件的密封性能至关重要,对于低温密封研究,了解低温下微小漏孔泄漏的流动特性很有必要。本文构建了具有随机粗糙度壁面的二维微通道,应用仿真软件研究液氦温区漏孔微通道内气流的流动特性,计算分析通道内各物理量的分布和变化。结果表明随机粗糙度壁面对速度分布的扰动在近壁面影响最大,随着远离壁面的方向减小,对中心区域流动几乎无影响;低温导致氦气粘滞系数下降,微通道内气体流速增加;压力分布也因通道特征尺寸变化而发生波动;液氦温区漏孔的质量流量比常温下大两个数量级,并且当进出口压力比超过临界值后漏孔质量流量不再变化。对于低温系统的漏率预测、泄漏预防和低温检漏、密封研究具有理论意义和实用价值。     

CFD model of ITER CICC. Part VI: Heat and mass transfer between cable region and central channel

Dual-channel cable-in-conduit conductors (CICC) are used in the superconducting magnets for the International Thermonuclear Experimental Reactor (ITER). As the CICC axial/transverse size ratio is typically ∼1000, 1D axial models are customarily used for the CICC, but they require constitutive relations for the transverse fluxes. A novel approach, based on Computational Fluid Dynamics (CFD), was recently proposed by these authors to understand the complex transverse thermal-hydraulic processes in an ITER CICC from first principles. Multidimensional (2D, 3D) Reynolds-Averaged Navier-Stokes models implemented in the commercial CFD code FLUENT were validated against compact heat exchanger and ITER-relevant experimental data, and applied to compute the friction factor and the heat transfer coefficient in fully turbulent spiral rib-roughened pipes, mimicking the central channel of an ITER CICC. That analysis is extended here to the problem of heat and mass transfer through the perforated spiral separating the central channel from the cable bundle region, by combining the previously developed central channel model with a porous medium model for the cable region. The resulting 2D model is used to analyze several key features of the transport processes occurring between the two regions including the relation between transverse mass transfer and transverse pressure drop, the influence of transverse mass transfer on axial pressure drop, and the heat transfer coefficient between central channel and annular cable bundle region.     

Numerical simulation of cavitating flow of liquid helium in venturi channel

The fundamental characteristics of the two-dimensional cavitating flow of liquid helium through a venturi channel near the lambda point are numerically investigated to realize the further development and high performance of new multi-phase superfluid cooling systems. First, the governing equations of the cavitating flow of liquid helium based on the unsteady thermal nonequilibrium multi-fluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two-dimensional structure of the cavitating flow of liquid helium though venturi channel is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid-to-gas phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase.     

Study on the heat transfer of high-vacuum-multilayer-insulation tank after sudden, catastrophic loss of insulating vacuum

One of the worst accidents that may occur in a high-vacuum-multilayer-insulation (HVMLI) cryogenic tank is a sudden, catastrophic loss of insulating vacuum (SCLIV). There is no doubt that the gases leaking into the insulation jacket have some influence on the heat transfer process of it. However, this issue has not been thoroughly studied so far. In this paper, a test rig was built up and experiments were conducted using a SCLIV cryogenic tank and with nitrogen, helium and air as the working medium, respectively. The venting rates of the tank and temperature in the insulation jacket were measured respectively after the three different gases leaking into the jacket. A heat transfer model describing the heat transfer process of a SCLIV tank was also presented. The calculated results using this model were compared against the experimental data. It is found that the heat transfer performance of the HVMLI cryogenic tank after SCLIV is strong relevant to the type of gas leaking into the insulation jacket.     

Simulation of the flow-reversal effect in dual-channel CICC for ITER

The discovery of an upward counter flow of helium in the outer annulus of the vertically oriented and top-to-bottom cooled ITER PF-FSJS (Poloidal Field Coil-Full Size Joint Sample) in 2002 led to closer investigations of the effect because it may lead to a reduction of the operational margin of the superconductor used in the ITER environment. Recently, further thermo-hydraulic experiments were carried out on the TFAS2 sample (Toroidal Field Advanced Strand sample 2) with the intent to asses the effect in detail. First investigations confirmed the initial assumption that the origin of the effect lies in the buoyancy of the heated, and thus less dense, helium in the outer annulus of the cable. The helium there is in good contact with the superconducting strands heated by neutron irradiation, ac losses or heat influx, but is thermally and hydraulically less well coupled to the downward flowing helium in the central channel. This paper presents an analysis of the TFAS2 experiments using the simulation program THEA™, specifically extended with a term for gravitational forces acting on helium of varying density. With the experience gained, the simulation of the thermo-hydraulic behavior of the Toroidal Field Coil inner leg shows the operational limits and boundary conditions of this coil in ITER.     

高真空多层绝热低温容器完全真空丧失后传热及绝热夹层内温度分布规律实验

在搭建了高真空多层绝热低温容器完全真空丧失传热研究实验台的基础上,分别利用干燥氮气、二氧化碳、氧气、氦气及空气为破空介质,进行了高真空多层绝热低温容器发生完全真空丧失事故后的传热实验研究.实验中通过流量计和温度采集系统测得了高真空多层绝热低温容器在发生完全真空丧失事故后的排放率和绝热夹层内的温度分布规律.实验结果表明,...     

Calibration of saturated liquid helium weir flowmeter at Fermilab

The flow of saturated liquid helium at 4.2 K was studied in a unique weir flowmeter mounted inside a duct. The objective was to develop a calibration for helium flow rate in the weir, and thereby, making it a viable flowmeter. The weir flowmeter was tested in a liquid helium system at Fermi National Laboratory used for cooling the superconducting magnets in the experimental areas of the site. Unlike for open channel weir devices, the calibration of this flowmeter was influenced by both the liquid flow through the weir notch and the vapor flow over it. The results of this study and the calibration are presented taking into account effects of both liquid and vapor.     

Quench analysis of an ITER 13T-40kA Nb3Sn coil (CS insert)

In order to analyze the quench characteristic of a cable-in-conduit (CIC) conductor that has a sub-cooling channel at the center of conductor cross section, an axisymmetric two-dimensional calculation model was developed. The tests and calculation results of the central solenoid (CS) insert were compared regarding the pressure drop and the behavior of the total voltage, temperature and normal zone propagation in the quench. They show good agreement. Therefore, the effectiveness of the calculation model is verified. It was also found that there is coolant convection between the central channel and bundle region even in a steady state. This makes the pressure drop in the central channel larger than that in a cylindrical pipe which has a smooth surface. In addition, it was found that the higher temperature of the coolant flowing through the central channel heats the coolant and the cable in the bundle region. It can be said that the hot coolant flowing through the central channel accelerates normal zone propagation.     

Analysis of laser-produced aerosols by inductively coupled plasma mass spectrometry: transport phenomena and elemental fractionation

The transport phenomena of laser-produced aerosols prior to analysis by inductively coupled plasma mass spectrometry (ICPMS) were examined. Aerosol particles were visualized over the cross section of a transport tube attached to the outlet of a conventional ablation cell by light scattering using a pulsed laser source. Experiments were carried out under laminar or turbulent in-cell flow conditions applying throughputs of up to 2.0 L/min and reveal the nature of aerosol transportation to strongly depend on both flow rate and carrier gas chosen. For instance, laser ablation (LA) using laminar in-cell flow and helium as aerosol carrier resulted in stationary but inhomogeneous dispersion patterns. In addition, aerosols appear to be separated into two coexisting phases consisting of (i) dispersed particles that accumulate at the boundary layer of several vortex channel flows randomly arranged along the tube axis and (ii) larger fragments moving inside. The occurrence of these fragments was found to affect the accuracy of Si-, Zn-, and Cd-specific ICPMS analyses of aerosols released by LA of silicate glass (SRM NIST610). Accuracy drifts of more than 10% were observed for helium flow rates of >1 L/min, most probably, due to preferential evaporation and diffusion losses of volatile constituents inside the ICP. The utilization of turbulent in-cell flow made the vortex channels collapse and resulted in an almost complete aerosol homogenization. In contrast, LA using argon as aerosol carrier generally yielded a higher degree of dispersion, which was nearly independent of the flow conditions applied. To illustrate the differences among laminar and turbulent in-cell flow, furthermore, the velocity field inside the ablation cell was simulated by computational fluid dynamics.     

Counterflow in rotating superfluid helium

We have observed a variety of effects involving thermal counterflow in rotating superfluid helium. In particular, the temperature and chemical potential gradients associated with the motion of vortex lines have been measured. The two-fluid equations for helium in rotation have been solved for channel flow in a channel of finite height and the results are compared with our data. Interesting effects associated with the onset of turbulence and with the onset of vortex line depinning are discussed.Supported by a grant from the National Science Foundation.     

Simulation of quench for the cable-in-conduit-conductor in HT-7U superconducting Tokamak magnets using porous medium model

A cable-in-conduit-conductor (CICC) consists of superconducting cable, copper, supercritical helium and conduit. To keep the operating temperature of superconducting cable lower than its current sharing temperature, the supercritical helium is forced flow through the CICC. The supercritical helium through the cable bundle has the complex directional changes due to the interaction between the supercritical helium and strands. The structure of CICC is characterized with the porous medium. The quench characteristics of CICC are analyzed by the model which the temperature difference between the strands and helium is assumed to be very small due to the heating induced flow to generate high heat transfer coefficient of supercritical helium. A moving mesh method is developed for the numerical solution of the problem with the steep drop for temperature and density of supercritical helium in the short front region of the normal zone. The computational mesh is obtained by equidistribution of a monitor function tailored for the functional variation of the arguments for density, temperature and velocity of supercritical helium. Existence and uniqueness of the discretised equations using a moving mesh are also established. The coupled equation for porous medium is solved using the finite element method with the artificial viscosity term. The validation of the code is tested by comparing it with the other codes with good accuracy. The converged properties of numerical solution due to quench in CICC are studied. We present preliminary estimates of the maximum conductor temperature rise and helium pressure during a quench in the inner layer of toroidal field (TF) magnet for HT-7U. The quench scenarios with different dump time constants of 6.25, 12, and 21.1 s are considered. The goal of such work is to guide the protection scheme and a detailed prediction of the quench evolution of magnet.