CFD modelling of subcooled boiling—Concept, validation and application to fuel assembly design

The paper describes actual Computational Fluid Dynamics (CFD) approaches to subcooled boiling and investigates their capability to contribute to fuel assembly design. In a prototype version of the CFD code CFX a wall-boiling model is implemented based on a wall heat flux partition algorithm. It can be shown, that the wall boiling model is able to calculate the cross sectional averaged vapour volume fraction of vertical heated tubes tests with good agreement to published experimental data. The most sensitive parameters of the model are identified. Needs for more detailed experiments are established which are necessary to support further model development. The model is applied for investigation of the phenomena inside a hot channel of a fuel assembly. Here the essential phenomenon is the critical heat flux. Although subcooled boiling represents only a preliminary state towards the critical heat flux occurrence, essential parameters like swirl, cross flow between adjacent channels and concentration regions of bubbles can be determined. By calculating the temperature of the rod surface the critical regions can be identified which may later on lead to departure from nucleate boiling and possible damage of the fuel pin. The application of up-to-date CFD with a subcooled boiling model for the simulation of a hot channel enables the comparison and the evaluation of different geometrical designs of the spacer grids of a fuel rod bundle.     

套管式直流蒸汽发生器动态实时仿真研究

基于基本的流体质量、能量和动量守恒原理,建立了套管式直流蒸汽发生器的动态数学模型。该模型采用集总参数和移动边界的建模方法,将直流蒸汽发生器流道分为欠热段、沸腾段和过热段。采用Gear算法求解,对建立的模型进行稳态和动态仿真,并利用RELAP程序进行校验。结果表明：所建立的模型是正确和有效的,能用来进行直流蒸汽发生器的动态特性实时仿真。     

过冷沸腾自然对流两相CFD模拟及应用

采用计算流体力学（CFD）方法,开展过冷沸腾自然对流两相模拟与应用研究。对侧壁加热圆柱水箱过冷沸腾自然对流实验采用两相CFD瞬态模拟,模拟时间为1 500 s,通过模型设置与模拟方法研究,再现了过冷沸腾发生后实验的温度阶跃,得到与实验较一致的温度分布、气泡产生时间与产生位置,确保了数值计算的合理性与准确性。在此基础上,对以欧洲ESBWR（经济简化沸水堆）非能动安全壳冷却系统（PCCS）为原型的ISP-42实验进行了两相CFD模拟,获得与实验一致的温度分布,确定采用两相CFD数值模拟对非能动安全壳冷却系统及非能动余热排出系统进行应用研究可行,为下一步计算传热系数、构建自然对流传热模型建立了良好基础。该项研究对工程应用中探寻非能动安全壳冷却系统及非能动余热排出系统的两相自然循环传热特性具有较大价值。     

Thermal analysis of EAST neutral beam injectors for long-pulse beam operation

Two sets of neutral beam injectors(NBI-1 and NBI-2) have been mounted on the EAST tokamak since 2014. NBI-1 and NBI-2 are co-direction and counter-direction, respectively. As with indepth physics and engineering study of EAST, the ability of long pulse beam injection should be required in the NBI system. For NBIs, the most important and difficult thing that should be overcome is heat removal capacity of heat loaded components for long-pulse beam extraction. In this article, the thermal state of the components of EAST NBI is investigated using water flow calorimetry and thermocouple temperatures. Results show that(1) operation parameters have an obvious influence on the heat deposited on the inner components of the beamline,(2) a suitable operation parameter can decrease the heat loading effectively and obtain longer beam pulse length, and(3) under the cooling water pressure of 0.25 MPa, the predicted maximum beam pulse length will be up to 260 s with 50 keV beam energy by a duty factor of 0.5. The results present that, in this regard, the EAST NBI-1 system has the ability of long-pulse beam injection.     

蒸汽发生器传热管束过冷沸腾区两相流动数值模拟

基于两流体欧拉数学模型结合RPI壁面沸腾模型,利用大型商用CFD软件ANSYS CFX 12.0对蒸汽发生器传热管束过冷沸腾区一次侧、壁面和二次侧耦合传热过程进行了数值模拟。研究了三叶梅花孔支撑板和不同入口过冷度条件下蒸汽发生器传热管束内的流动沸腾现象,得到一、二次侧流场与温度场,二次侧空泡份额分布,支撑板梅花孔局部的流动状况及不同入口过冷度对蒸汽发生器热工水力特性的影响。数值模拟结果表明,三叶梅花孔支撑板的存在及不同入口过冷度对蒸汽发生器传热管束过冷沸腾区域的热工水力特性影响显著。     

管内过冷流动沸腾CFD模型参数敏感性研究

采用CFD方法对燃料组件进行过冷流动沸腾数值模拟研究是反应堆热工水力分析的一项重要内容。本研究使用STAR CCM+基于欧拉双流体模型结合壁面沸腾模型对管内过冷流动沸腾进行数值模拟,得到了壁面温度、主流温度及空泡份额的分布。基于实验结果对网格模型、湍流模型、壁面沸腾模型及相间作用力模型的参数设置进行了敏感性分析。研究结果表明,对于欧拉双流体模型,并非网格量越多结果越准确,加热面第1层网格的高度对结果影响显著。湍流模型和曳力模型对计算结果影响较小,非曳力中的湍流耗散力及升力对结果影响较大。Li Quan或Hibiki Ishii汽化核心密度模型与Kocamustafaogullari气泡脱离直径模型组合对壁面温度及空泡份额的计算较准确。本研究可为反应堆燃料组件内过冷流动沸腾数值模拟提供参考依据。     

同轴型热虹吸系统轴向传热特性实验研究

对一种用于冷中子源系统的自然循环进行了轴向传热特性研究,建立了以氟里昂为工质的同轴型热虹吸管传热实验系统。研究表明:同轴型热虹吸系统可强化轴向热传输能力,其传热工况有过冷沸腾流动振荡工况、稳定工况和烧干工况3类。在低功率区,过冷沸腾和携带机制造成启动过程的两相流振荡;进入稳定工况区后,系统具有自调适能力,能够维持恒定的循环质量流率;在高功率区,蒸发器烧干产生传热极限。本研究显示出同轴型热虹吸系统的动力特性与普通自然循环系统有明显区别,其循环机制与普通热虹吸系统有本质区别。     

非能动余热排出热交换器半液位换热性能研究

非能动余热排出热交换器（PRHR HX）是先进反应堆非能动堆芯余热排出系统的关键设备之一,置于内置换料水箱中。水箱内水蒸发或水箱泄漏等故障失水后液位降低,PRHR HX与管外水传热面积和管外流场均发生变化,导致堆芯非能动余热排出能力下降。为分析半液位下PRHR HX换热性能,建立半液位PRHR HX实验装置和数值计算模型,获得稳态和瞬态升温过程的换热量、温度分布和流场特性。实验结果表明,稳态沸腾换热阶段半液位结构的总换热量较全液位时的下降12%～22%,仍具有较强的换热能力。瞬态升温过程中管外换热模式从纯单相对流换热开始,先后逐渐出现局部过冷沸腾和局部饱和沸腾,并最终迅速由局部饱和沸腾转变为全管长饱和沸腾。水箱内逐渐升温过程中出现显著的热分层现象,结合数值模拟分析发现此时管外流体在高度上呈现多层回流流动结构。     

自然循环欠热沸腾起始点特性

利用适于强迫循环的伯格尔斯和罗斯诺经验方法、Davis和Anderson理论方法，以及本课题组依据自然循环实验提出的预测欠热沸腾起始点的经验公式，对两种类型的欠热沸腾起始点的热力学平衡干度进行比较计算，研究自然循环的欠热沸腾起始点的基本特性。研究结果表明：自然循环欠热沸腾起始点的热力学平衡干度对加热量、进口温度、系统压力有着更大的敏感特性，在同等条件下，更早发生欠热沸腾现象。统计物理的微观角度研究进一步指出：产生这一现象的根本原因在于处于自组织状态的自然循环耗散结构的特点、动力学的涨落力和动量力对热力学平衡的共同影响。此研究结果为今后研究和应用自然循环的欠热沸腾奠定了基础。     

Analysis of Power Deposition on Heat Load Components for EAST Neutral Beam Injector

Neutral beam injection is recognized as one of the most effective means of plasma heating. The target values of EAST Neutral beam injector (NBI) are beam energy 50–80 keV, injection beam total power 2–4 MW, beam pulse width 10–100 s. The beam power will deposit on the beam collimator due to the beam divergence and it will cause heat damage to heat load components, or even destroy the entire NBI system. In order to decrease the risk, the beam power deposited on heat load components should be assessed. In this article, the percent of power deposition on each heat load components has been calculated using Gaussian beam transmission model. Comparison of the results measured with water flow calorimeter and calculated results shows the beam transmission model has relative good agreement with real distribution. The results can direct the operation parameter optimization of EAST NBI.     

Pool boiling heat transfer enhancement on porous surface tube

The passive residual heat removal exchanger (PRHR HX),which is a key equipment of the passive residual heat removal system,is installed in an elevated pool.Its heat transfer performance affects security and economics of the reactor,and boiling heat transfer in the liquid surrounding the exchanger occurs when the liquid saturation temperature exceeded.The smooth tubes,which are widely used as heat transfer tubes in PRHR HX,can be replaced by some enhanced tubes to improve the boiling heat transfer capability.In this paper,the pool boiling heat transfer characteristics of smooth tube and a machined porous surface tube are investigated by using high-pressure steam condensing inside tube as heating source.Compared with smooth tube,the porous surface tube considerably enhances the boiling heat transfer,and shortens the time significantly before reaching the liquid saturation temperature.Its boiling heat transfer coefficient increases from 68% to 75%,and the wall superheat decreases by 1.5oC.Combining effect of condensation inside tube with boiling outside tube,the axial wall temperatures of heat transfer tube are neither uniform nor linear distribution.Based on these investigations,enhance mechanism of the porous surface tube is analyzed.     

Analysis of critical heat flux during subcooled boiling for finned fuel elements

Experimental and analytical studies were performed to determine the critical heat flux (CHF) during subcooled boiling on finned fuel elements. Tests were conducted in a vertical, concentric-annulus test section consisting of a glass tube containing a finned heater element with either six, eight, or ten longitudinal fins. The phenomena leading to CHF are described and the parametric trends are discussed.A two-dimensional finite-element heat transfer model using the Galerkin method was used to analyse the experimental data to obtain CHF values. A dimensionless correlation was derived to predict the CHF values during subcooled boiling. Over 90% of the predicted CHF values agreed with those obtained from the two-dimensional analysis within ±30%.     

低压低流速条件下的过冷沸腾换热特性

为探究低压低流速条件下的过冷沸腾换热特性,开展本实验研究。通过分析实验中采集的热工参数和可视化图像,探究了沸腾滞后现象、沸腾失稳现象以及沸腾换热特性。实验发现沸腾起始点壁面过热度较高,而沸腾的发生大幅提高了换热系数,因此出现了显著的沸腾滞后现象。实验中较为光滑的加热面可达到较高的过热度,而低压下快速产生的气泡尺寸较大,在较低的热流密度下气液界面发生剧烈变化,使气泡破裂为多个小气泡并成为核化点。在过冷沸腾换热系数的预测中,Dittus-Boelter对流换热关系式不再适用,采用Hallman关系式和Gnielinski关系式计算对流换热系数,并引入壁面过热度对池式沸腾换热系数进行修正,可使过冷沸腾换热系数的预测精度大幅提高。     

螺旋管直流蒸汽发生器一、二次侧耦合传热特性分析

为获得螺旋管直流蒸汽发生器（HCOTSG）螺旋换热管内两相流动换热特征,以国际革新安全反应堆（IRIS）HCOTSG为研究对象建立了HCOTSG一、二次侧耦合热分析模型,分析了稳态工况下,不同二次侧给水流量对HCOTSG热工水力参数产生的影响,并将所建立的HCOTSG一、二次侧耦合热分析模型与计算流体力学软件（CFX）三维流动换热计算相结合,对HCOTSG稳态工况下螺旋管内精细的热工水力参数进行计算。通过HCOTSG一、二次侧耦合热分析模型计算得到HCOTSG稳态工作时沿管程的相关热工水力参数;通过CFX三维模拟发现螺旋管横截面流体流速和温度分布不均匀现象,得到螺旋内侧流体温度高于螺旋外侧,螺旋内侧流体速度低于螺旋外侧,螺旋内侧流体比螺旋外侧流体先开始沸腾的结论。因此,本研究对于HCOTSG稳态运行和螺旋换热管事故分析具有指导作用。      

螺旋管直流蒸汽发生器一、二次侧耦合传热特性分析

To study the flow and heat transfer characteristics of the primary and secondary sides of the helical coil once-through tube steam generator (HCOTSG) under steady state conditions, taking HCOTSG of International Reactor Innovative and Secure (IRIS) as the research object, a primary and secondary sides heat balance calculation model for steady state operation of HCOTSG is established. The influence of different secondary side feed water flow rate on HCOTSG thermal and hydraulic parameters under steady-state condition is analyzed, and the detailed thermal and hydraulic parameters in the helical tube under steady-state condition are calculated by combining the coupled thermal analysis model with the three-dimensional flow and heat exchange calculation of CFX. The relevant thermal and hydraulic parameters along the tube side of HCOTSG during steady-state operation are calculated by the thermal analysis model. The CFX simulation results show that the velocity and temperature distribution of the fluid in the cross section of the helical tube are not uniform. The temperature of the fluid inside the helix is higher than that outside the helix. The velocity of the fluid inside the helix is lower than that outside the helix. The boiling of the fluid inside the helix occurs earlier than that outside the helix. Therefore, this study has a guiding role in the accident analysis for HCOTSG steady-state operation and spiral heat exchange tube.     

Simulation of nucleate boiling under ANSYS-FLUENT code by using RPI model coupling with artificial neural networks

The present study is to develop a new user-defined function using artificial neural networks intent Computational Fluid Dynamics(CFD)simulation for the prediction of water-vapor multiphase flows through fuel assemblies of nuclear reactor.Indeed,the provision of accurate material data especially for water and steam over a wider range of temperatures and pressures is an essential requirement for conducting CFD simulations in nuclear engineering thermal hydraulics.Contrary to the commercial CFD solver ANSYS-CFX,where the industrial standard IAPWS-IF97(International Association for the Properties of Water and Steam-Industrial Formulation 1997)is implemented in the ANSYS-CFX internal material database,the solver ANSYS-FLUENT provides only the possibility to use equation of state(EOS),like ideal gas law,Redlich-Kwong EOS and piecewise polynomial interpolations.For that purpose,new approach is used to implement the thermophysical properties of water and steam for subcooled water in CFD solver ANSYS-FLUENT.The technique is based on artificial neural networks of multi-layer type to accurately predict 10 thermodynamic and transport properties of the density,specific heat,dynamic viscosity,thermal conductivity and speed of sound on saturated liquid and saturated vapor.Temperature is used as single input parameter,the maximum absolute error predicted by the artificial neural networks ANNs,was around 3%.Thus,the numerical investigation under CFD solver ANSYSFLUENT becomes competitive with other CFD codes of which ANSYS-CFX in this area.In fact,the coupling of the Rensselaer Polytechnical Institute(RPI)wall boiling model and the developed Neural-UDF(User Defined Function)was found to be useful in predicting the vapor volume fraction in subcooled boiling flow.     

基于OpenFOAM的过冷流动沸腾数值模拟

过冷流动沸腾现象被广泛应用于工业生产和动力系统中,对该现象的准确预测是两相流CFD模拟的重要研究方向。本文详细阐述了该模拟过程中的欧拉两流体模型及相关辅助模型,基于开源CFD平台OpenFOAM,模拟了4.5 MPa下竖直圆管内的过冷流动沸腾,得到了截面空泡份额、液相平均温度及壁面温度沿轴向的分布。计算结果与实验值符合良好,说明了模型的有效性和程序的正确性。本文可为在OpenFOAM中添加新的模型及开发新的求解器以模拟过冷流动沸腾问题提供参考。     

Modification and validation of the natural heat convection and subcooled void formation models in the code PARET

Two new modifications have been included in the current PARET code that is widely applied in the dynamic and safety analysis of research reactors. A new model was implemented for the simulation of void formation in the subcooled boiling regime, the other modification dealt with the implementation of a new approach to improve the prediction of heat transfer coefficient under natural circulation condition.     

Heat transfer enhancement of NBI vacuum pump cryopanels

Huge cryogenic pumps are installed inside neutral beam injectors in order to manage the typically very large gas flows. This paper deals with the aspect of passive cooling in NBI cryopump design development and discusses design considerations in two example areas. One is the design of cryopanels consisting of a pipe, centrally supplied with cryogenic helium, and a welded fin, passively cooled, to provide the necessary pumping surface below a given maximum temperature. The results of several parametric simulations in ANSYS are presented using different copper thicknesses and cryopanel geometries to discuss the thermal capability (heat transfer characteristics and heat capacities) of a number of design variants. The optimum design solution is based on copper-coated fins, using an electroplating technique, and thereby improving the heat transfer of the cryopanels while attaining an overall reduction in weight. The other area is the sound design of the manifold shielding system with a weld contact between copper and stainless steel. Weld samples were manufactured and investigated to raise awareness of the demands and risks during manufacturing and to demonstrate that readily applicable weld procedures exist.     

Structural dynamics: Mario Paz, (Van Nostrand Reinhold Company,New York, 1980) $29.95.

Stable film boiling heat transfer data have been obtained in an 8.9 mm ID tube at pressures from 2 to 9 MPa. These data were obtained at low-quality and subcooled conditions, over a mass flux range of 0.11 to 2.75 Mg m^?2 s^?1. Excessive film boiling surface temperatures were avoided by using the hot patch technique. Contrary to the high-quality data, the low-quality data showed a decrease in heat transfer coefficient with an increase in quality. The film boiling data were compared with existing film boiling correlations. None of these were found to be satisfactory.