Vibration Characteristic of Heated Rod Induced by Subcooled Flow Boiling

A subcooled boiling loop with annular flow channel on an electrically heated rod at the channel axial center was used to make an experimental approach to investigate the vibration characteristic induced by subcooled flow boiling. The experiments were carried out to measure vibration induced in the rod under different conditions of incoming coolant subcooling temperature of 10–80 K, coolant flow rates of 0.1–0.35 × 10^?3 m³/s and imposed linear power densities of 17–600 W/cm. Distilled water was used as coolant in the loop. The vibration behavior of the rod was measured by using an accelerometer. Also, a high-speed motion analyzer system was used to analyze the bubble behaviors in the different subcooled boiling conditions.

The results show the intensive SBIV (subcooled boiling-induced vibration) which highly depends on dynamic force generated by rapid growth and collapse of vapor bubbles under high heat flux loading whilst they still attach to the heating surface. These behaviors were influenced strongly by the conditions of subcooling temperature, linear power density and flow rate.     

过冷度对蒸汽气泡破碎及微气泡喷射过程的影响

为研究过冷度对蒸汽气泡破碎及微气泡喷射过程的影响,利用高速摄像机记录不同过冷度下过冷池中蒸汽气泡凝结过程。实验结果表明:在低过冷度(ΔTsub=17K)下,蒸汽气泡界面波动发展缓慢,气泡不会破碎,而是逐渐分裂凝结;在高过冷度(40KΔTsub75K)下,蒸汽气泡表面上的波动剧烈发展,随后气泡会突然破碎,并形成大量微气泡;在ΔTsub=30K时,气泡突然破碎前会有小气泡分裂现象发生。40KΔTsub75K时气泡破碎形成的微气泡的直径和速度在量级上与气泡微细化沸腾区域的微气泡接近。随过冷度的升高,微气泡的直径减小,速度增加,且蒸汽气泡破碎前其表面上波动的波数迅速增加,波动的最大幅值先增加后减少。     

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。     

基于气泡动力学的过冷流动沸腾边界层模型研究

过冷流动沸腾中的流动和传热特性对反应堆的安全运行和经济性都具有重要意义。过冷流动沸腾起始于ONB点,结束于Tsat点,中间被OSV点划分为高过冷流动沸腾段和低过冷流动沸腾段,不同阶段流场情况以及气泡行为存在较大区别。目前关于过冷流动沸腾过程的研究主要基于宏观实验、理论研究和数值模拟,随着气泡动力学的发展,从微观层面揭示沸腾机理变得可行。本文基于气泡动力学和气泡边界层模型,提出了一套预测过冷流动沸腾的理论模型,采用分相模型,将流场在径向上划分为多个区域,通过1组准二维控制方程,将各区域内的气泡行为,区域间的质量、动量和能量交换以及两相参数沿轴向的变化情况考虑在内,利用获得的边界层流场信息,可确定ONB点和OSV点。模型与空泡份额和流体温度的实验结果符合良好,并成功应用于核反应堆燃料元件通道的过冷流动沸腾计算。     

冷却剂丧失条件下钠冷快堆棒束传热的三维蒙特卡罗模拟方法

采用蒙特卡罗方法分析钠冷快堆在假想冷却剂丧失条件下燃料棒束的钠两相流传热问题。以分子运动理论的基本定律为基础，开发出替代宏观经验模型来分析反应堆棒束中的钠蒸发率和冷凝率的微观模型，且采用三维蒙特卡罗方法模拟分子的运动轨迹，分子间的碰撞率以及分子与棒束、分子与棒束组件盒壁的碰撞率。对包壳干涸区的再浸润现象用动力膜模型描述，并计算了通过液膜的液体速度分布和平均液膜速度，对于从冷凝液膜蒸发的钠分子则被重新记为蒙特卡罗计算的源项。用微观和宏观模型相结合的方法数字模拟了德国卡斯鲁尔研究中心的堆外钠沸腾实验。     

Simultaneous measurement of void fraction and fundamental bubble parameters in subcooled flow boiling

Visualization was performed for the vapor bubbles in water subcooled flow boiling in a vertical heated tube to measure simultaneously the void fraction and the four fundamental bubble parameters: nucleation site density, bubble release frequency, bubble lifetime and bubble size. Using the mass flowrate and liquid subcooling as the experimental parameters, the changes of void fraction and bubble parameters with the wall heat flux were measured. The results of image analysis showed that the vapor void fraction could be approximated by the function of nucleation site density and bubble lift-off diameter; the bubble lift-off diameter was more influential than the nucleation site density. It was hence concluded that the bubble lift-off diameter could be regarded as the key parameter to determine the vapor void fraction under the present experimental conditions. The strong relation of bubble lift-off diameter to superheated liquid layer thickness was indicated for the future model development studies of bubble lift-off diameter.     

Development,computer simulation and performance testing in sodium of an eddy current flowmeter

Sodium is used as a coolant in Liquid Metal Fast Breeder Reactor (LMFBR). Sodium flow measurement is of prime importance both from the operational and safety aspects of a fast reactor. Various types of flowmeters namely permanent magnet, saddle type and eddy current flowmeters are used in FBRs. From the safety point of view flow through the core should be assured under all operating conditions. This requires a flow sensor which can withstand the high temperature sodium environment and can meet the dimensional constraints and be amenable to maintenance. Eddy current flowmeter (ECFM) is one such device which meets these requirements. It is meant for measuring flow in PFBR primary pump and also at the outlets of the fuel sub-assemblies to detect flow blockage. A simulation model of ECFM was made and output of ECFM was predicted for various flowrates and temperatures. The simulation model was validated by testing in a sodium loop. This paper deals with the design, simulation and tests conducted in sodium for the eddy current flowmeter for use in the Prototype Fast Breeder Reactor (PFBR).     

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%。     

Analysis of subcooled boiling flow with one-group interfacial area transport equation and bubble lift-off model

To enhance the multi-dimensional analysis capability for a subcooled boiling two-phase flow, the one-group interfacial area transport equation was improved with a source term for the bubble lift-off. It included the bubble lift-off diameter model and the lift-off frequency reduction factor model. The bubble lift-off diameter model took into account the bubble's sliding on a heated wall after its departure from a nucleate site, and the lift-off frequency reduction factor was derived by considering the coalescences of the sliding bubbles. To implement the model, EAGLE (elaborated analysis of gas-liquid evolution) code was developed for a multi-dimensional analysis of two-phase flow. The developed model and EAGLE code were validated with the experimental data of SUBO (subcooled boiling) and SNU (Seoul National University) test, where the subcooled boiling phenomena in a vertical annulus channel were observed. Locally measured two-phase flow parameters included a void fraction, interfacial area concentration, and bubble velocity. The results of the computational analysis revealed that the interfacial area transport equation with the bubble lift-off model showed a good agreement with the experimental results of SUBO and SNU. It demonstrates that the source term for the wall nucleation by considering a bubble sliding and lift-off mechanism enhanced the prediction capability for the multi-dimensional behavior of void fraction or interfacial area concentration in the subcooled boiling flow. From the point of view of the bubble velocity, the modeling of an increased turbulence induced by boiling bubbles at the heated wall enhanced the prediction capability of the code.     

Local Sodium Boiling behind Local Flow Blockage in Simulated LMFBR Fuel Subassembly

This paper deals with local sodium boiling in the downstream of a six-subchannel blockage in an electrically heated LMFBR fuel subassembly mock-up.

The first series of experiments were conducted to measure temperature distributions in the downstream of the blockage under non-boiling conditions. The measured temperature rise due to the blockage agreed fairly well with the calculation by the LOCK code.

The second series of experiments were performed to investigate local boiling phenomena. In the local boiling region, no flow instability was observed since the sub-channels near the wrapper wall were still filled with sub-cooled liquid. In the nearly bulk boiling region, however, considerable upstream voiding occurred and then the inlet flow decreased, leading to final dryout.

The boiling caused a considerable increase in acoustic noise intensity. The root-mean-square (RMS) noise level of approximately 20 mbar obtained in the present local boiling experiments with sodium was much higher than that (approximately 0.5 mbar) in the ordinary nucleate boiling experiments with water. The peak observed in the hertz ranges was due to the repetition of bubble formation and collapse. In the kilohertz ranges, however, resonance peaks were superposed on a smooth curve with a broad peak at approximately 7 kHz.

The frequency (2.9 and 20.2 sec^?1) of bubble formation decreased with the increase of the bubble size at its point of maximum development. The product of the bubble frequency and the equivalent diameter was found to be constant.     

窄通道内不凝结气体对过冷沸腾汽泡行为的影响

针对窄通道内不凝结气体对过冷沸腾汽泡行为的影响进行了实验研究,通过高速摄影仪记录了矩形窄通道内含和不含不凝结气体的汽泡行为并进行了分析。分析结果表明：在过冷沸腾条件下不凝结气体对汽泡行为产生了较大的影响,延缓了汽泡的生长和冷凝,使汽泡的直径变化呈震荡特性;从蒸汽的产生和冷凝角度分析,不凝结气体的存在有利于汽泡的生存,促进了汽泡之间的聚合过程,在一定程度上有利于换热。     

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.     

Direct Measurement of Neutron Flux

An experimental study was carried out on steady-state boiling of sodium flowing in the annular channel formed around an electrically heated simulation of a fuel pin. In the present experiment, the inlet temperature and flow rate were held constant, and the heat flux was gradually increased up to the inception of boiling. Thereafter, the heat flux was further increased step by step until the surface temperature of the heater pin marked a sharp rise, indicating the occurrence of dry-out. Records were obtained of the changes brought by the increasing heat flux to boiling phenomena, with particular reference to the behavior of the two-phase flow pattern and to the characteristics of boiling noise, as well as of the frequency of bubble formation.

It was made clear that there exists a region in which steady-state boiling will be established, and under these conditions the two-phase flow pattern changes sequentially from bubbly flow to slug flow and then to annular flow. This behavior of sodium boiling in a narrow channel is quite similar to the case of water.

With rising heat flux, the level of noise intensity associated with boiling first increased sharply to attain a maximum point, then decreased somewhat and remained constant thereafter until dry-out.

The frequency of bubble formation depended on the size of the bubble. The product of bubble frequency and equivalent diameter was found to be constant.     

Sodium boiling experiments in a 19-pin bundle under loss-of-flow conditions

An experimental study was conducted on transient sodium boiling in a 19-pin electrically heated LMFBR fuel subassembly mockup under loss-of-flow conditions. In each run the inlet flow was reduced or stopped at constant heater power. There was no strong effect of temperature ramp rate on incipient-boiling (IB) wall superheat. The observed coolant voiding was initially limited to the center subchannel because of steep temperature gradient in the bundle. The bulk pressure rise registered upon initial vaporization was markedly lower than the vapor pressure corresponding to the IB wall superheat. The pressure pulse generated at vapor bubble collapse correlated reasonably well with the re-entrant liquid velocity, but the measured value was very much smaller than the calculation by sodium hammer analysis.     

A unified model considering force balances for departing vapour bubbles and population balance in subcooled boiling flow

Population balance equations combined with a three-dimensional two-fluid model are employed to predict subcooled boiling flow at low pressure in a vertical annular channel. The MUltiple-SIze-Group (MUSIG) model implemented in CFX4.4 is extended to account for the wall nucleation and condensation in the subcooled boiling regime. A model considering the forces acting on departing bubbles at the heated surface is formulated. This model provides the capacity of complex analyses on the bubble growth and departure for a wide range of wall heat fluxes and flow conditions.Comparison of model predictions against local measurements is made for the void fraction, bubble Sauter mean diameter and gas and liquid velocities covering a range of different mass and heat fluxes and inlet subcoolings. Good agreement is achieved with the local radial void fraction, bubble Sauter mean diameter and liquid velocity profiles against measurements. However, significant weakness of the model is evidenced in the prediction of the vapour velocity. Work is in progress to circumvent the deficiency of the MUSIG boiling model by the consideration of additional momentum equations to better represent the momentum forces acting on the range of bubble sizes in the bulk subcooled liquid.     

Modeling of low-pressure subcooled boiling flow of water via the homogeneous MUSIG approach

Applying a three-dimensional two-fluid model coupled with homogeneous multiple size group (MUSIG) approach, numerical simulations of upward subcooled boiling flow of water at low pressure were performed on the computational fluid dynamics (CFD) code CFX-10 with user defined FORTRAN program. A modified bubble departure diameter correlation based on the Unal's semi-mechanistic model and the empirical correlation of Tolubinski and Kostanchuk was developed. The water boiling flow experiments at low pressure in a vertical concentric annulus from reference were used to validate the models. Moreover, the influences of the non-drag force on the radial void fraction distribution were investigated, including lift force, turbulent dispersion force and wall lubrication force. Good quantitative agreement with the experimental data is obtained, including the local distribution of bubble diameter, void fraction, and axial liquid velocity. The results indicate that the local bubble diameter first increases and then decreases due to the effect of bubble breakup and coalescence, and has the maximum bubble diameter along the radial direction. Especially, the peak void fraction phenomenon in the vicinity of the heated wall is predicted at low pressure, which is developed from the wall repulsive force between vapor bubbles and heated wall. Nevertheless, there is a high discrepancy for the prediction of the local axial vapor velocity.     

Fragmentation mechanisms based on single drop steam explosion experiments using flash X-ray radiography

Flash X-ray and high-speed regular photography were used to investigate the fragmentation processes during the vapor explosion of single drops of molten metal immersed in water. For relatively low ambient flow velocities (< 5 m/s), a comparison of the breakup of hot and cold drops shows that whereas cold drops breakup due to the stripping of fragments by the relative flow, the fragmentation of a hot drop is dominated by the growth and collapse of a vapor bubble. X-ray radiographs show during the growth of the bubble, that fine filaments of metal protrude from the drop and the drop surface becomes highly convoluted. Using a simple model for the bubble dynamics, an estimate of the energy budget shows that the majority (about 80%) of the thermal energy transfer occurs during bubble collapse. For hot drops at higher flow velocities (> 45 m/s), vapor bubble growth is diminished and high-speed motion of vapor within the bubble leads to an enhanced fragmentation rate.     

NHR—5负荷自跟随特性与机理的分析

用带有高过冷沸腾模型的ＲＥＴＲＡＮ－０２程序对５ＭＷ核共热核负荷增加５０％和失去全部负荷的自跟随实验进行了分析，其计算结果与实验结果符合得很好。并对额定工况下负荷增加３０％和失去全部负荷两种工况的安全性进行了分析，结果表明反应无所作为理安全的。