水基磁性流体竖直加热棒下的池沸腾传热实验研究

研究了磁性微粒浓度、外加磁场对水基磁性流体在竖直加热棒加热情况下的沸腾传热影响;实验结果显示,磁性微粒浓度和外加磁场对磁性流体的沸腾换热有很大影响：对于中低浓度的磁性流体,存在一个最优的磁粒浓度,在该浓度下沸腾传热的强化效果最显著,施加磁场时,该结论仍然成立。施加磁场能强化磁性流体的沸腾传热。     

过冷流动沸腾中汽泡脱离直径的理论研究

汽泡脱离直径模型是壁面沸腾计算中的一个重要子模型。为了正确预测过冷流动沸腾中的壁面传热情况,研究结合新改进的汽泡生长模型,采用力平衡方法对过冷流动沸腾中的汽泡脱离直径进行了模拟。汽泡生长模型同时考虑了微液层、过热层和汽泡顶部过冷液体层对汽泡生长所做的贡献,并采用饱和沸腾与过冷沸腾2个实验对其进行了验证,结果表明预测曲线与实验值吻合良好。另外,选取了3个过冷流动沸腾实验来验证汽泡脱离直径模型,模拟结果均在可接受的误差范围内。      

竖直窄矩形通道内过冷沸腾传热模型

通过引入池式沸腾-流动沸腾汽泡脱离直径比对沸腾抑制因子S进行了修正,并将修正后的S引入Lee-Mudawwar过冷沸腾CHF模型,通过结合竖直窄矩形通道内的汽泡行为进行分析,建立了适应于竖直窄矩形通道的过冷流动沸腾传热模型,探讨了影响过冷沸腾传热系数的主要因素,并通过将模型预测值与实验值进行对比,验证了模型的可靠,表明当前模型可用于计算竖直窄矩形通道内的过冷沸腾传热特性。     

低压下水欠热流动沸腾的两相CFD数值模拟研究

采用两流体(汽相和液相)基本数学模型,结合汽相和液相之间的界面传热、传质和动量交换封闭模型、汽泡平均直径模型、汽泡脱离直径模型、汽泡成核模型、汽泡脱离频率模型、欠热沸腾起始点模型和壁面热流密度分配模型,在CFD软件CFX4.4中采用用户自定义函数将相变引起的传热、传质和动量交换作为源项分别添加到汽相和液相的能量、质量和动量守恒方程中,对低压下内管加热外管绝热的环形通道内的欠热沸腾进行了数值研究,得到了欠热流动沸腾下汽相体积份额、液相速度、汽相速度分布等。采用Lee等的环形通道内低压下欠热沸腾体积份额实验数据对计算结果进行了验证,吻合良好。     

多孔球层内沸腾现象与传热特性研究

采用池式沸腾实验系统,在常压底部加热条件下分别对由直径4、6、8mm玻璃球构建的多孔结构内沸腾过程进行了可视化研究.结果表明,过冷沸腾时,加热壁面上产生孤立汽泡,小汽泡可聚合为主汽泡,主汽泡脱离频率较低,汽相以分散的小汽泡为主;饱和沸腾初期,汽泡生长变快,主汽泡体积变大,连续汽相范围广阔;主汽泡形成频率随热流密度增加而增加;膜态沸腾时,底面被汽膜包围,液相占据球层空间.球体直径越大,产生同类现象需要的热流密度越大,传热系数的极值越大.饱和沸腾存在传热强化区和抑制区.直径4、8mm玻璃球构建的多孔介质传热系数随热流密度的增加而增加,6mm多孔介质则相反.     

强制对流过冷沸腾流体中汽泡脱离频率

在垂直上升环形通道内进行强制对流过冷沸腾实验。实验工质为水，实验压力为大气压力。由一台高速摄像仪对汽泡核化的动力学过程进行图像捕捉。通过影像获取总共58个工况下的汽泡脱离频率。对本实验的数据和从文献中获取的数据进行无量纲分析。将已有的模型和关系式与汽泡等待时间、生长时间以及脱离频率等实验数据进行比较。由池式沸腾改进的关系式未能很好地应用于强制对流过冷沸腾，同时，由过冷沸腾提出的模型也不能预测宽实验参数范围内的汽泡脱离频率。无量纲汽泡脱离频率与无量纲泡核沸腾热流密度相关。新的关系式与现有的低壁面过热度下的实验数据吻合良好。     

微尺度核态沸腾汽泡聚合特性研究

从微观角度深入探究汽泡的生长、聚合及传热规律,采用数值仿真的方法,建立相变物理模型。将建立的模型嵌入计算流体动力学(CFD)软件,利用流体体积函数(VOF)相界面追踪方法,对汽泡的生长、运动、聚合及脱离进行探究。设立1个尺寸为3 mm×2 mm×2 mm的三维微型通道,底部壁面设置5个单元,用于加热并对汽泡底部热流密度进行监测。获得微通道加热条件下汽泡的合并运动形态和底面热流密度分布,并与实验观测值比较。通过对汽泡形态的模拟和底部热流密度的监测,建立汽泡动力学规律与底面热流密度分布的关系,进一步深化对汽泡合并的动力学规律和沸腾传热机理的认识。     

管流中汽泡的运动

管内沸腾传热决定于汽泡的运动状况,而汽泡的运动状况,则决定于汽泡所受的作用力。文章根据作用于管壁生成汽泡上的力,导出了汽泡脱离壁面的临界直径的计算公式,从理论上解释了汽泡脱离壁面与汽流速度及密度差的关系。分析了在管流边界层中作用于脱离壁面的汽泡上的力,导出了汽泡运动相对速度的计算公式及汽泡走向管子轴心速度的计算公式。还进一步分析了汽泡的合并、变形和分裂的机理,并导出了相应的计算公式。     

沸腾换热强化特性实验研究

沸腾换热具有传热温差低、换热系数高等优点,对提高设备的紧凑性、经济性和安全性具有重要意义.目前,核动力装置中主要以光管作为沸腾换热元件,沸腾换热强化的空间很大.本文针对核动力装置非能动余热排出换热器沸腾换热工况,以水为工质,对光管及3种强化管的管外沸腾换热特性进行实验研究,得出了4种管型的沸腾换热强化特性,并分析了各强化管的强化机理.整体针翅管表面大量螺旋排列的三维翅片增大了换热面积、延缓了汽泡在壁面附近聚合形成大汽膜,使沸腾换热得到强化;多孔管采用机械方法在壁面加工出大量微细小孔,汽化核心数量和汽泡脱离频率均大幅提高,因此,沸腾换热强化效果显著;绕丝针翅管是一种复合强化手段,兼有整体针翅管和多孔管的优点,沸腾换热强化效果也较好.     

窄缝通道内过冷沸腾区域汽泡脱离频率研究

采用高速摄像仪对矩形窄缝通道内垂直上升流过冷流动沸腾区域汽泡脱离频率进行可视化实验研究。结果表明,汽泡脱离频率随质量流速的增大而减小,随入口过冷度的增大而减小,随热流密度的增大而增大。将实验数据与文献中汽泡脱离频率计算模型进行比较,发现基于池式沸腾和饱和流动沸腾开发的计算模型不能准确预测过冷沸腾区域汽泡脱离频率。本文以无量纲参数的形式,分别用液相雷诺数、过冷雅各布数和核态沸腾热流密度表示质量流速、主流过冷度和热流密度对汽泡脱离频率的影响,获得矩形窄缝通道内过冷沸腾区域汽泡脱离频率预测关系式,关系式的平均预测误差为±17.1%。     

Assessment of the MARS code for design of water pool-type reactor cavity cooling system in HTGR

This paper reports an experimental and numerical study on the assessment of the MARS code as a tool for analyzing the water pool-type reactor cavity cooling system (RCCS), which was developed by Seoul National University (SNU). A series of experiments were performed to determine the heat removal capability of the proposed RCCS and assess the capability of MARS code to predict the forced convective, natural convective and radiative heat transfer under normal operation conditions and boiling heat transfer during accident conditions in the RCCS. In the loss of forced convection (LOFC) accident experiment performed at the integral effect test facility called RCCS-SNU, the MARS code underestimated the vapor generation rate at the inner wall of the water pool. Therefore, the newly developed models of the bubble departure and lift-off diameters were implemented into the MARS code to make a better prediction of the vapor generation rate. The improved MARS code was assessed again using the experimental data of the LOFC accident conditions in the RCCS-SNU facility.     

Study on characteristics of vapor–liquid two-phase flow in mini-channels

To explore the mechanism of boiling bubble dynamics in narrow channels, two types of channels are investigated which have I- and Z-shaped with width of 2 mm. Using VOF model and self-programming, the whole flow field is simulated with two different kinds of media, namely, water and ethanol. The influence of wall contact angle on the process of bubble generating and growth is studied, and the relationship between different channel shapes and the pressure drop is also investigated taking into account the effects of gravity, viscosity, surface tension and wall adhesion. The bubble generation, growth and departure processes are analyzed through numerical simulation and self-programming, and the influence of interface movements and changes on internal pressure difference and average surface heat transfer coefficient is investigated by using geometry reconstruction and interface tracking. It is found that wall contact angle has a great influence on the morphology of bubble. The smaller the wall contact angles are, the more round the bubbles are, and the less time the bubbles take to depart from the wall. The variation of contact angle also has effect upon the heat transfer coefficient. The greater the wall contact angle is, the larger the bubble-covered area is, thus the wall thermal resistance gets higher, and bubble nucleation is suppressed, and the heat transfer coefficient becomes lower. The role of surface tension in the process of boiling heat transfer is much more important than the gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance the heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of I-shaped ones, while the pressure drop of the former is obviously higher. In addition, surface tension and viscosity significantly impact the pressure drop of boiling system, and different specific heat and boiling point values result in different heat transfer coefficients. The simulation results in this paper match well with the experimental data revealed in other sources, both show that the heat transfer coefficient of water is higher than that of ethanol and Z-shaped channels have better heat transfer capability.     

Experiments on microgravity boiling heat transfer by using transparent heaters

To clarify the relation between the liquid–vapor behavior and the heat transfer characteristics in the boiling phenomena, the structures of transparent heaters were developed for both flow boiling and pool boiling experiments and were applied to the microgravity environment realized by the parabolic flight of aircraft. In the flow boiling experiment, a transparent heated tube makes the heating, the observation of liquid–vapor behavior and the measurement of heat transfer data simultaneously possible. The heat transfer coefficient in the annular flow regime at moderate quality has distinct dependence on gravity provided that the mass velocity is not so high, while no noticeable gravity effect is seen at high quality and in the bubbly flow regime. The measured gravity effect was directly related to the behavior of annular liquid film observed through the transparent tube wall. In the pool boiling experiment, a structure of transparent heating surface realizes both the observation of the macrolayer or microlayer behavior from underneath and the measurements of local surface temperatures and the layer thickness. It was clarified in the microgravity experiments that no vapor stem exists but tiny bubbles are observed in the macrolayer underneath a large coalesced bubble at high heat flux. The heat flux evaluated by the heat conduction across the layer assumes less than 30% of the total to be transferred. The evaporation of the microlayers underneath primary bubbles just after the generation dominates the heat transfer in the microgravity, not only in the isolated bubble region but also in the coalesced bubble region.     

纳米流体饱和池沸腾传热及CHF模型研究

纳米流体传热是一种新兴的换热方式,目前研究多集中在单相研究领域,而纳米流体沸腾传热特性的相关研究较少。本文采用热通量拆分方法,将壁面传热分为4种模型(微液层蒸发、气泡脱离前的瞬态导热、气泡脱离后的瞬态导热以及微对流换热),对这4种模型的传热量分别进行计算,结合壁面核化中心密度等参数,计算了壁面平均传热系数和CHF。结果表明,本文计算结果与国际上已发表的实验数据符合较好,充分验证了所建立模型的合理性。     

Numerical simulation on single bubble behavior during Al2O3/H2O nanofluids flow boiling using Moving Particle Simi-implicit method

In this study, regression analysis on the thermal properties of Al₂O₃/H₂O nanofluids was made firstly. The growth and departure of a single bubble behavior in Al₂O₃/H₂O nanofluid and pure water flow boiling process were then numerically simulated by an improved Moving Particle Semi-implicit method in different flow boiling conditions. The results indicate that the bubble in Al₂O₃/H₂O nanofluids grows faster and the bubble departure frequency of Al₂O₃/H₂O nanofluids is greater than that in pure water. The flow boiling heat flux is also improved by dispersing nanoparticles of Al₂O₃/H₂O in pure water. This work initially reveals that nanofluids can enhance flow boiling heat transfer from the point of view of bubble dynamics behavior. The effects of nanoparticle concentrations and diameters of Al₂O₃/H₂O nanofluids on the bubble behavior were also investigated and compared under the same flow conditions. It is found that the increasing of nanoparticle volume concentration may increase the bubble departure frequency and departure diameter, while the increasing rates of departure frequency and departure diameter are lessened with the increasing of nanoparticle volume concentration. It is suggested that the suitable nanoparticle volume concentration of nanofluid for flow boiling heat transfer enhancement should not be too large, especially regarding the negative effect of flow resistance increase due to the increasing of nanoparticle volume concentration. The interesting finding is that in the same nanoparticle volume concentration condition, the bubble departure frequency for the nanofluid with nanoparticle diameter of 29 nm shows a maximum value. The increasing of nanoparticle diameter leads to the decreasing of bubble departure diameter. It is boldly to predict that an optimal nanoparticle diameter range between 20 and 38 nm should be beneficial to flow boiling heat transfer enhancement of Al₂O₃/H₂O nanofluids.     

常压下ATF锆合金包壳Cr涂层表面饱和池式沸腾气泡行为实验研究

铬（Cr）涂层锆合金包壳是最有前途的耐事故燃料（ATF）的新型包覆材料之一,对其表面的气泡动力学进行研究有助于评估是否具有更好的传热性能。在常压下的Cr涂层锆合金包壳池式沸腾实验装置中对不同工艺方法下制备的Cr涂层锆合金包壳进行实验,研究了粗糙度等表面状态对气泡产生、长大以及脱离等气泡行为的影响。结果表明,气泡接触角与Cr涂层表面粗糙度有关,粗糙度越大,表面气泡接触角越小;不同涂层工艺下制备的4种Cr涂层锆合金包壳样件表面的气泡脱离直径范围为1.256～1.446 mm,气泡脱离频率范围为29.99～50.97 Hz;气泡脱离直径与粗糙度呈负相关,脱离频率与粗糙度呈正相关;气泡脱离直径预测模型与实验数据之间的偏差为±6%,脱离频率预测模型与实验数据之间的偏差为±3%。