锆-4在冷却水中的骤冷沸腾传热实验研究

为了研究锆-4在冷却水中的骤冷行为与沸腾传热特性,本文采用可视化方法,并测量了锆-4在骤冷过程中的温度变化。基于一维导热反问题求解,计算得到锆-4表面的热流密度和温度。在骤冷过程中锆-4会依次经历膜态沸腾、过渡沸腾、核态沸腾以及单相对流换热4个阶段,并且分析了轴向高度和冷却水过冷度对骤冷行为以及沸腾传热的影响。结果表明,随着过冷度的增大,骤冷时间减小,最小膜态沸腾温度增大,并且核态沸腾与过渡沸腾传热受加热表面局部特性影响显著,并建立了锆-4表面最小膜态沸腾温度的关系式,对反应堆的安全分析具有重要的意义。      

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%。     

平板滞止区过冷水喷流沸腾临界热流密度实验研究

对高温平板滞止区内过冷水圆形喷流冲击沸腾的临界热流密度进行了系统的稳态实验研究。考察了水过冷度、流速、喷流直径等流动条件对喷流沸腾临界热流密度的影响。建立了一个预示临界热流密度的经验型方程。研究结果证明,过冷水喷流冲击沸腾的临界热流密度取决于过冷度、滞止冲击速度及喷流直径、过冷度的影响尤为强烈。经验式能较好的预示本实验和他人结果。     

矩形流道中的传热与压降特性研究

随着高性能电子芯片的发展以及电路和其它紧凑系统的小型化，迫切要求开发与之相适应的高热流密度下高效的传热技术。为此，在矩形通道内以FC－84为工质，进行了单相强制对流、过冷沸腾及饱和泡核沸腾实验。实验段由五个平行的水平通道组成。各通道参数如下：水力直径Dh=0.75mm，长径比（L／Dh)=409.8，通道两面的热流密度相等。实验中主要调节参数包括质量流量、入口过冷度和热流密度。实验中测量了沿流动方向不同位置处的液相温度和壁面温度。基于测量向出的温度、压降和整个试验段的热平衡，计算出单相强制对流和流动沸腾的传热系数。实验中同时测量了单相及两相工况下的实验段压降，并推出了一个计算过冷沸腾及饱和泡核沸腾压降的关系式。此外本文还提出了适用于对冷沸腾及饱和沸腾的两个新的传热关系式。     

微细化沸腾传热实验研究

气泡微细化沸腾是沸腾到达某个临界热负荷后,加热面温度升高不大,与该临界热负荷相比,热流密度大幅提高的沸腾现象。本文在设计完成一可视化实验装置的基础上,通过高速摄影仪观察并结合采集的壁温数据,对常压下直径为10 mm铜加热面上的池式气泡微细化沸腾现象进行了研究,并讨论了液体过冷度对其的影响。实验发现,气泡微细化沸腾状态下,加热面上生成1层极其不稳定的气膜,气液交界面上不停地有大量微小气泡生成并以极高速度射入过冷液体中。随加热面热流密度的增大,气膜厚度波动周期缩短,气膜最大厚度减小,所生成微小气泡的直径也明显减小。实验中获得的最高热流密度达9 MW/m²。     

事故容错燃料包壳表面液滴碰撞行为及Leidenfrost现象实验研究

为了研究事故容错燃料包壳表面的液滴Leidenfrost现象，本研究采用高速相机对液滴与事故容错燃料包壳SiC和FeCrAl的碰撞行为进行可视化观测，并与常规包壳材料Zr-4对比。结果表明，液滴碰撞方式有沉积、带二次液滴散射的反弹、带二次液滴散射的碎化、反弹和碎化5种；沉积属于核态沸腾换热，反弹和碎化属于膜态沸腾，带二次液滴散射的反弹和带二次液滴散射的碎化属于过渡沸腾换热；液滴的临界热流密度（CHF）温度与韦伯数（We）无关，而Leidenfrost温度随着We和固体表面蓄热系数的增大而增大；在膜态沸腾阶段，液滴的铺展行为与温度无关，随着We的增大，液滴铺展的更快，且能达到更高的铺展因子。      

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.     

人工神经网络在预测流动沸腾曲线中的应用

选用20世纪60年代以来的实验数据，应用人工神经网络分析入口欠热度、质量流速、压力等主要参数对沸腾曲线的影响。在整个传热区内，热流密度随入口欠热度的增加而增大；在过渡沸腾和膜态沸腾区，热流密度随质量流速的增加而增加；压力起重要的作用，除膜态沸腾区外，增加压力能强化传热。除泡核沸腾外，稳态和瞬态的流动沸腾曲线的差异很小。     

竖直圆管下降流过渡沸腾传热实验研究

采用热块骤冷实验技术、非稳态一维数值分析方法和多元回归分析技术对竖直圆形元件管内下降流过渡沸腾传热特性进行了研究，建立了一组以ＣＨＦ点和最小膜态沸腾点（ＭＩＮ）为基础的过渡沸腾传热特性两点模型，和可以反映压力、流量和入口过冷度对过渡沸腾曲线影响的多变量数据分析模型。采用一维非稳态数值分析方法建立热块及试验管的精细温场分布，推导管内壁温度和热流密度，采用本文的模型和多元回归分析技术整理实验数据，得到一组过渡沸腾传热特性的半经验关系式，其适用于：Ｇ＝７０－２２５６ｋｇ／ｍ２·ｓ；Ｐ＝０．３－１６ＭＰａ；ΔＴｓｕｂ＝６℃－７５℃；预测结果与实验数据吻合较好。对主要流动参数对过渡沸腾传热特性的影响作了趋势分析和机理浅析。     

核化点间距对过冷沸腾中汽泡聚并特性的影响

过冷沸腾是反应堆热工过程中会出现的重要物理现象,汽泡聚并是核态沸腾过程中一种常见的汽泡相互作用形式。本文对核态沸腾过程中的汽泡聚并现象做了研究,研究了不同核化点间距下汽泡聚并特性和热流密度特征。利用微型加热片阵列,控制核化点的间距,并且详细记录汽泡底部的热流密度,与此同时用高速CCD相机从底部拍摄汽泡运动形态。将汽泡图像与热流密度结合分析,全面总结核化点间距对汽泡聚并和热流密度特性的影响。     

Collapsing criteria for vapor film around solid spheres as a fundamental stage leading to vapor explosion

Following a partial fuel-melting accident, a Fuel-Coolant Interaction (FCI) can result with the fragmentation of the melt into tiny droplets. A vapor film is then formed between the melt fragments and the coolant, while preventing a contact between them. Triggering, propagation and expansion typically follow the premixing stage.In the triggering stage, vapor film collapse around one or several of the fragments occurs. This collapse can be the result of fragments cooling, a sort of mechanical force, or by any other means. When the vapor film collapses and the coolant re-establishes contact with the dry surface of the hot melt, it may lead to a very rapid and rather violent boiling. In the propagation stage the shock wave front leads to stripping of the films surrounding adjacent droplets which enhance the fragmentation and the process escalates. During this process a large quantity of liquid vaporizes and its expansion can result in destructive mechanical damage to the surrounding structures. This multiphase thermal detonation in which high pressure shock wave is formed is regarded as “vapor explosion”. The film boiling and its possible collapse is a fundamental stage leading to vapor explosion. If the interaction of the melt and the coolant does not result in a film boiling, no explosion occurs.Many studies have been devoted to determine the minimum temperature and heat flux that is required to maintain a film boiling. The present experimental study examines the minimum temperature that is required to maintain a film boiling around metal spheres immersed into a liquid (subcooled distilled water) reservoir. In order to simulate fuel fragments that are small in dimension and has mirror-like surface, small spheres coated with anti-oxidation layer were used. The heat flux from the spheres was calculated from the sphere's temperature profiles and the sphere's properties. The vapor film collapse was associated with a sharp rise of the heat flux during the cooling process—from values typical for film boiling to much higher values typical for nucleate boiling. Correlations for the minimum temperature and the minimum heat flux necessary to maintain film boiling were established in terms of the subcooling level, the size of the spheres and their material.The minimum temperature to maintain film boiling was used as the principle criteria for the occurrence of vapor explosion. Other criteria, for the intensity of the vapor film collapse was derived from the maximum heat flux following the vapor film collapse, and the audible sound (which is generated by the shock wave). It is assumed that a high intensity of the vapor film collapse will result in a more efficient propagation stage and enhancement of the vapor explosion.     

下沉加热面上气泡微细化沸腾实验研究

为分析加热面相对位置对气泡微细化沸腾（MEB）的影响，对下沉加热面上的过冷沸腾进行了实验研究，并与齐平加热面实验结果进行了对比。25～50 K过冷度范围内，在下沉3 mm加热面上观察到了MEB现象。在50 K过冷度下，MEB时的热流密度可达5.55 MW/m²。可视化结果表明：在MEB区域，下沉加热面上形成的蒸汽气膜会频繁地膨胀收缩；随过冷度的升高，膨胀收缩的周期增加，而幅值变化较小。此外，相比于齐平加热面条件，下沉加热面周围的壁面可显著限制蒸汽气膜的横向膨胀。     

An investigation of transition boiling mechanisms of subcooled water under forced convective conditions

A mechanistic model for forced convective transition boiling has been developed to investigate transition boiling mechanisms and to predict transition boiling heat flux realistically. This model is based on a postulated multi-stage boiling process occurring during the passage time of the elongated vapor blanket specified at a critical heat flux (CHF) condition. Between the departure from nucleate boiling (DNB) and the departure from film boiling (DFB) points, the boiling heat transfer is established through three boiling stages, namely, the macrolayer evaporation and dryout governed by nucleate boiling in a thin liquid film and the unstable film boiling characterized by the frequent touches of the interface and the heated wall. The total heat transfer rates after the DNB is weighted by the time fractions of each stage, which are defined as the ratio of each stage duration to the vapor blanket passage time. The model predictions are compared with some available experimental transition boiling data. The parametric effects of pressure, mass flux, inlet subcooling on the transition boiling heat transfer are also investigated. From these comparisons, it can be seen that this model can identify the crucial mechanisms of forced convective transition boiling, and that the transition boiling heat fluxes including the maximum heat flux and the minimum film boiling heat flux are well predicted at low qualities/high pressures near 10 bar. In future, this model will be improved in the unstable film boiling stage and generalized for high quality and low pressure situations.     

The quenching behavior of aqueous nanofluids around rods with high temperature

The quenching behavior of aqueous nanofluids containing various volume fractions of Al₂O₃, SiO₂, TiO₂ and CuO nanoparticles is experimentally investigated around high temperature brass rod (diameter 20 mm × 75 mm). The experiments are performed at saturated conditions under atmospheric pressure. The results show that the quenching process is strongly dependent on the kind of nanoparticle, as well as its volume fraction. Although it is not observed from the first run in nanofluids, the quenching time is considerably shortened during the repetitive quenching tests. After the repetition tests in nanofluids, a nanoparticles porous layer occurs on the quenched surface and thus, the film boiling vanishes. The surface contact angles and the surface roughness of the quenched surfaces are measured. The results show that contact angles decreases and the surface roughness increases. It can be concluded that the primary reason of critical heat flux enhancement is the change of the surface characteristics due to the porous layer. In addition, the results also showed that there is no significant change in the nucleate boiling heat transfer.     

Film boiling heat transfer and vapour film collapse on spheres, cylinders and plane surfaces

An experimental study of transient film boiling was conducted, with different coolant velocities, on two spheres with different diameters, two cylindrical specimens of different lengths in parallel flow, a cylinder in cross flow and two flat plates with different lengths. A frame by frame photographic study on the nature of the vapour/liquid interface and the collapse modes has revealed a new mode for film collapse, in which an explosive liquid–solid contact is followed by film re-formation and the motion of a quench front over the hot surface. Steady state tests were carried out on a plate similar to the short plate used in the transient experiments and the heat transfer, film stability and collapse results are compared with those of the transient investigation.Heat transfer coefficients and heat fluxes during film boiling were found essentially to depend on specimen temperature and water subcooling. In contrast, the influences on heat transfer of specimen size and water velocity were relatively small for the ranges studied. A theoretical model predicted heat transfer coefficients to within 10% of experimental values for water subcoolings above 10 K and within 30% in all cases.     

Effects of Coolant Flow on Light Water Reactor Fuel Behaviors during Reactivity Initiated Accident

This paper describes the in-pile experimental results to study the influences of coolant flow on fuel behaviors under reactivity initiated accident (RIA) conditions performed in the Nuclear Safety Research Reactor (NSRR). A single PWR type test fuel rod was irradiated by a large neutron pulse in the NSRR to simulate a prompt power excursion of RIA's. The effects of coolant flow were studied at a coolant flow velocity of 0.3~1.8m/s and a coolant temperature of 20~90°C under the atmospheric pressure. It was found that the cooling conditions had considerable influences on fuel thermal behaviors under prompt heat-up. The increase of coolant flow velocity and subcooling enhanced heat transfer coefficient at cladding surface during film boiling, which resulted in large decrease of maximum cladding temperature and film boiling duration, and consequently in the increase of fuel failure threshold energy. The data tendencies are summarized and the influences of coolant flow are discussed with some computer analyses.     

Film Boiling Characteristics of Potassium Droplets on Heated Plate

For providing background information on the possible vapor explosion in the event of a core disruptive accident of LMFBRs, an experiment was conducted on the film boiling characteristics of liquid metal potassium in association with the Leidenfrost phenomenon. In a steel container filled with Ar gas, K droplets were put on a joule-heated plate of 316-SS or Ta. The behaviors of droplet were observed by a camera and a color VTR through viewports. The experimental conditions were the Ar pressure 1 bar, the initial K temperature 350~760°C, and the plate temperature 900~1,250°C for 316-SS and 1,100~1,600°C for Ta.

Stable film boiling known as Leidenfrost phenomenon was observed for a high temperature condition of the plate, whereas an instantaneous break-up of droplet with extensive vaporization occurred for a low temperature. The heat transfer characteristics of film and transition boiling regions were obtained by estimating the heat flux from the volumetric reducing rate of droplet due to vaporization. The results in the film boiling region showed an appreciably good agreement with the prediction based on Bromley's expression combined with the theory of Baumeister et al. The minimum film boiling temperature and heat flux were found to be about 1,300°C and 15 W/cm², respectively, for a droplet size of 0.15 cm³.