具有强迫射流的大型腔体中的速度场和热分层实验分析(英文)

本文介绍在模拟实际非能动安全壳冷却系统及接近实际安全壳分隔区域尺寸的条件下,大型矩形腔体中由强迫射流所造成的速度场和热分层现象。推导了强迫射流动量何时打破分层现象的判断准则。分析了射流对混合与分层的影响。基于测量结果讨论了大型腔体中因射流与自然对流传热所造成的速度场。     

安全壳模型装置内氢气分布特性及影响因素分析

采用计算流体力学方法,首先利用THAI HM-2实验对CFX分析模型的适用性进行验证,通过与实验数据的比对,表明计算结果与实验数据基本吻合,从而验证选用的模型适合对安全壳模拟装置氢气分布特性的分析。之后,建立待研究中等规模安全壳模型实验装置的三维几何模型和网格模型,采用基准工况+单因素对比的方式,分别模拟湍流浮力射流中心喷射和近壁面喷射工况以及考虑蒸汽壁面冷凝情况下安全壳模型内的氦气(氢气替代工质)流动扩散分布,讨论喷射位置因素、壁面蒸汽凝结效应对氦气分布的影响。分析结果表明,喷射位置对氦气分布的影响主要体现在壁面引流现象上,即氦气流更倾向于沿着安全壳壁面进行流动和扩散;而与安全壳壁面的换热和蒸汽的冷凝会进一步促进大空间自然对流的建立,从而较为显著地提高氦气在安全壳内的扩散和混合效果。     

加热上升混合对流传热实验研究

在加热上升混合对流中,浮升力的存在显著改变了速度分布和切压力分布,使边界层趋于层流化和充分发展湍流起始点的延后,并使传热系数表现出由弱化、恢复再强化的过程。本研究在中压下不同直径的竖直加热圆管上进行了纯蒸汽的强迫对流传热实验,也在超临界压力下不同直径的加热圆管上进行水的强迫对流和自然循环传热实验。研究表明:随着浮升力参数的增大,传热逐渐弱化;在浮升力参数达到一定值时传热系数达到最小值,随后逐渐恢复,并最终出现强化。在实验数据基础上提出了加热上升混合对流的传热关系式。     

破口事故比例模拟中安全壳破口源项参数评估研究

采用基于破口总焓相似、强迫射流及浮力羽流流场相似及传热传质过程相似的多约束分析体系,归纳与质能源项相关的传热传质过程、耗散过程以及自然循环过程的时间尺度,确定模拟实验源项满足的各种模拟工况所必须遵循的设计约束条件。分析表明,自然循环过程时间常数是约束不同物理过程最重要的基础参数,也是模拟装置设计的基本约束参数。给出适用于确定安全壳破口源项试验参数的计算关系式,用于计算获得试验装置的几何参数和试验边界条件。     

竖直圆管内超临界二氧化碳强迫对流传热实验研究

为分析超临界二氧化碳强迫对流传热特性,开展竖直圆管内超临界二氧化碳强迫对流传热特性实验研究。实验结果表明:实验参数范围内存在明显的浮升力效应和流动加速效应;对于向上流动工况,随浮升力效应增强超临界二氧化碳从强迫对流传热过渡至混合对流传热,最后发展为自然对流传热,传热能力有弱化到逐渐恢复直至强化;对于向上流和向下流动工况,流动加速效应皆弱化传热。基于实验数据建立新的超临界流体传热关联式,在实验工况范围内95.03%的预测值与实验值偏差为±30%以内。     

不同倾角射流时近壁区流动与传热特性研究

针对反应堆冷却剂系统中不同倾角三通构件射流时近壁区流动与传热特性,在试验研究的基础上,比较分析了流速比在0～2范围内,射流以45°、90°倾角入射主流时三通构件不同区域的流动特性、近壁区混合函数及无量纲换热系数。结果表明,不同入射角对混合函数及无量纲换热系数的变化趋势影响不大,控制变化趋势的关键参数依然是流速比,入射角仅对混合函数及无量纲换热系数的幅值大小有一定影响。     

典型严重事故非能动安全壳冷却系统效果分析

先进压水堆采用非能动安全壳冷却系统(PCCS)在事故下维持安全壳完整性,包括重力喷洒形成安全壳外部水膜冷却和空气冷却流道中空气对流传热。针对严重事故下PCCS效果研究,建立了非能动压水堆安全壳及非能动安全壳冷却系统的传热分析模型(包括对流传热及蒸发/冷凝传热),并耦合反应堆主系统模型及专设安全设施模型。通过与西屋公司PCCS大尺度试验结果的比对验证了模型的可用性,进而针对非能动先进压水堆选取全厂断电、热段小破口失水始发事故作为典型严重事故序列,模拟了事故进程、主系统响应及安全壳的响应,分析了PCCS对安全壳的降温、降压作用。结果表明,安全壳压力72h内未超过安全限值,保持安全壳完整性。     

直接安注反应堆压力容器下降环腔内射流传热试验研究

失水事故工况 (LOCA)下反应堆下降环腔内的流动和传热研究 ,对反应堆压力容器 (RPV)的安全具有重要的意义。通过对一种直接安注的反应堆压力容器内流动和传热的研究 ,将流动分为横穿射流和冲击射流 ,比较了在两种射流下下降环腔内流动和传热的特点 ,分析了流速比和对流换热系数及温度的关系 ,当流速比在 1～ 1 0时 ,流动属于横穿射流 ,对流换热主要由环腔流速决定 ;流速比大于 1 0后 ,属于冲击射流 ,环腔内对流换热主要决定于安注流速 ,此时局部对流换热能力随安注流速的增加而增加     

中低雷诺数条件下单侧加热矩形通道内混合对流换热的实验研究

非能动安全壳冷却系统是先进压水堆非能动安全系统的重要组成部分,其中空气对流换热的能力较差,对安全影响较大,因此本文主要研究了在大尺寸垂直单侧加热矩形通道内空气自下而上流动时的混合对流换热,用于模拟核电厂非能动安全壳冷却系统的换热情况。研究结果表明在较小雷诺数条件下自然对流的影响不能忽略且自然对流会占据主导作用;随着空气流量的增加,强迫对流换热的作用越来越明显。当前学者所用经验关系式都不能很好地体现出自然对流在混合对流中起的作用,因此本文还通过实验数据拟合了一个新的计算混合对流换热的关系式,该公式在一定雷诺数范围内与实验值能很好地符合。     

厚壁三通管件非等温横向射流的数值模拟研究

为了研究压水堆核电站冷却剂系统中厚壁三通管件的热应力问题，模拟了速度比分别为0.05和0．5条件下的非等温横向射流，并通过固体区域与流体区域的耦合传热计算，获得了厚壁管件的温度分布。计算基于有限容积法，采用κ－ε紊流模型，计算结果与实验吻合良好。通过研究流速比横向射流的流动和传热特性，从流动机理上考察了流动结构对构件温度分布及热应力的影响，指出在实际运行中可能对厚壁三通管件产生热冲击，从而引起构件热疲劳破坏的主要因素。定性地讨论了不同流速比下构件承受的热冲击状况，为运行参数及构件的优化设计提供了理论依据。     

Investigation of mixed convection in a large rectangular enclosure

This experimental research investigates mixed convection and heat transfer augmentation by gaseous forced jets in a large enclosure, at conditions simulating those of passive containment cooling systems for Gen III+ passively safe reactors. The experiment is designed to measure the key parameters governing heat transfer augmentation by forced jets, and to investigate the effects of geometric factors, including the jet diameter, jet injection orientation, interior structures, and enclosure aspect ratio. The tests cover a variety of injection modes leading to flow configurations of interest for mixing and stratification phenomena in containments under accident conditions. Correlations for heat transfer augmentation by forced jets are developed and compared with experimental data. The characteristic recirculation speed inside the enclosure is introduced and analyzed. Steady stratified temperature distributions are compared with model simulations of the BMIX++ code.     

Investigations of Electra KrF laser hibachi foil cooling with small obliquely impinging jets

Electra is a large electron beam pumped KrF laser with transmission foils that separate the vacuum diodes from the laser gas. Active cooling of these foils is important when Electra operates at a rep-rate of 5 Hz because the attenuation of the electron beam by the foil produces a volumetric heating source. A method adopting a large number of locally injected high-speed circular gas jets was explored experimentally and numerically for the purpose of cooling and protecting the hibachi foil with an enhanced convection heat transfer rate. The jets were issued from the openings on two stainless-steel jet tubes parallel to the foil in a fashion such that the two straight lines of jets impinge on the foil obliquely and form a staggered coverage of the foil. CFD simulations were performed to identify the best jet impingement configuration. Bench-top experiments were performed at various jet diameters and jet velocities for a single foil span between two neighboring supporting ribs with a surface heat flux over 20.0 kW/m². Heat transfer enhancement of 60–700% over the case with jets turned off was observed depending on jet Reynolds numbers. Experimental Nusselt numbers were well-correlated with jet Reynolds number and the normalized jet-to-foil distance. The study shows that the impinging jets enhance heat transfer from the surface and offer an improved average cooling for a much larger foil area.     

Modeling of turbulent condensation heat transfer in the boiling water reactor primary containment

A condensation heat transfer model is developed for the purpose of predicting the atmosphere temperature response within the primary containment of a boiling water reactor during the initial forced convection heat transfer period following a postulated loss-of-coolant accident. The model utilizes simultaneous heat and mass transfer for the process of condensation in the presence of a non-condensible gas. The gas-vapor diffusion layer formed is in the mode of turbulent, forced convection. The predicted heat transfer is determined to be diffusion controlled with negligible resistance being contributed by the condensate film. The model is qualified through the analysis of the response of a containment test facility; the results compare favorably with experimental observations made by the General Electric Co. Predicted temperature responses for a typical containment are also shown and compared with those obtained through use of the Uchida heat transfer correlations.     

Numerical and experimental studies of Electra's scalloped transmission foil cooling with small impinging jets

The 5 Hz rep-rate operation of the Electra KrF laser necessitates the cooling and protection of the transmission foil that is subject to the pulsating electron beam bombardment. The pulsed volumetric heating from the e-beam attenuation heats up the foil (～2.54 × 10^?5 m thick) rapidly and often causes the foil to fail, increasing the operation cost and down time for the laser. Various methods have been investigated forheat transfer enhancement. While elevated heat transfer was achieved, the previous methods assume a flat foil shape. The actual foil shape is scalloped due to the pressure difference across the foil during the laser operation. Also a new “scalloped” foil design was proposed for thermal stress reduction. This paper investigates the applicability of small locally impinging jets to cooling the scalloped-shaped foil. The jets were formed through a line of small circular openings on two stainless-steel jet tubes aligned with the foil edges having the two columns of jets impinging on the foil obliquely in a staggered pattern for improved coverage. CFD simulations were used to optimize jet configurations. Experiments were performed that utilize a scalloped foil strip which matched the foil shape between two neighboring supporting ribs in the Electra hibachi. Jet diameters and jet velocities were varied at a surface heat flux greater than 20.0 kW/m². Substantial heat transfer enhancement with impinging jets was observed. Average Nusselt numbers were correlated with jet Reynolds number and the normalized jet-to-foil distance. The study indicates that the impinging jets can effectively enhance heat transfer for the scalloped foil and can be a promising method for actual foil coolingof KrF lasers, including Electra.     

Passive containment cooling by natural air convection and thermal radiation after severe accidents

One essential feature of composite containment is its potential to remove decay heat by natural air convection coupled with thermal radiation. Experimental and numerical investigations have been carried out to determine the coolability of such a passive cooling system and to study the contribution of thermal radiation to decay heat removal. A data base has been provided for validating advanced multi-dimensional computer codes and for developing physical models. It has been found that the passive containment cooling by natural air convection coupled with thermal radiation is a promising concept. Both the experimental data and the numerical results show that for intermediate and high wall emissivities, respectively, thermal radiation significantly enhances the entire heat transfer, even at low temperatures of the containment wall. The FLUTAN code combined with the new radiation model developed has been proven to be an accurate and efficient numerical tool for investigating flow and heat transfer behaviour in the system considered.     

Analysis of large-scale tests for AP-600 passive containment cooling system

All next-generation light water reactors utilize passive systems to remove heat via natural circulation and are significantly different from past and current nuclear plant designs. One unique feature of the AP-600 is its passive containment cooling system (PCCS), which is designed to maintain containment pressure below the design limit for 72 h without action by the reactor operator. During a design-basis accident (DBA), i.e., either a loss-of-coolant or a main-steam-line break accident, steam escapes and comes in contact with the much cooler containment vessel wall. Heat is transferred to the inside surface of the steel containment wall by convection and condensation of steam and through the containment steel wall by conduction. Heat is then transferred from the outside of the containment surface by heating and evaporation of a thin liquid film that is formed by applying water at the top of the containment vessel dome. Air in the annular space is heated by both convection and injection of steam from the evaporating liquid film. The heated air and vapor rise as a result of natural circulation and exit the shield building through the outlets above the containment shell. All of the analytical models that are developed for and used in the COMMIX-1D code for predicting performance of the PCCS will be described. These models cover governing conservation equations for multicomponents single-phase flow, transport equations for the k − two-equation turbulence model, auxiliary equations, liquid-film tracking model for both inside (condensate) and outside (evaporating liquid film) surfaces of the containment vessel wall, thermal coupling between flow domains inside and outside the containment vessel, and heat and mass transfer models. Various key parameters of the COMMIX-1D results and corresponding AP-600 PCCS experimental data are compared and the agreement is good. Significant findings from this study are summarized.     

Conceptual design of helium cooling circuit for irradiation target

Small metal specimens of about 20 mm × 20 mm and 0.4 mm thick are irradiated in cyclotron facilities for radiation damage studies. Cooling of these specimens is an important factor which decides the intensity of irradiation. In this paper helium is used for the cooling of irradiation target specimen. In order to have enhanced heat removal from the specimen jet cooling is employed. The cooling scheme and the conceptual helium cooling circuit has been arrived at based on the empirical correlation available in the literature. The heat removal rate has been estimated for various jet velocities. Experiments with impinging air jets have been carried out to compare the empirical predictions. Numerical predictions have also been carried out using commercial Computational Fluid Dynamic (CFD) code. Experimental predictions are 35%–55% higher compare to empirical correlation. The empirical correlation is 30% higher compare to CFD predictions.     

Experimental study on convective boiling heat transfer in narrow-gap annulus tubes

Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are required to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteristics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the transition between various regimes. Some significant differences from unconfined forced convection boiling were found, and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.