过冷度和粗糙度对铁铬铝平板淬冷沸腾影响的实验研究

为了探究过冷度和表面粗糙度对铁铬铝（FeCrAl ）平板淬冷沸腾的影响,对FeCrAl 平板在不同过冷度和表面粗糙度下的淬冷沸腾过程开展了可视化实验研究。采用热电偶测量平板内部温度,并利用导热反问题解析式求解平板表面温度和热流密度;通过对比分析实验现象,探究过冷度和表面粗糙度对平板淬冷沸腾过程的影响,并建立了过冷度与最小膜态沸腾温度的关系式。结果表明,淬冷沸腾过程中,FeCrAl 平板表面形成开尔文一亥姆霍兹（K-H）不稳定波,且气膜破裂后产生的骤冷前沿呈“抛物线”状;随着过冷度的增加,最小膜态沸腾温度增大,临界热流密度增大,平板表面冷却速率加快,淬冷沸腾过程的时长缩短;较大的表面粗糙度可以促进FeCrAl 平板表面淬冷沸腾的进行,但影响微小。      

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。     

5×5均匀加热棒束底部再淹没实验研究及热工安全分析程序评价

核反应堆安全分析中的冷却剂丧失事故（Loss of Coolant Accident,LOCA）是反应堆安全的重要研究对象之一，LOCA事故中的再淹没阶段棒束通道内的热工水力行为是其中一个十分关键的问题。利用西安交通大学核安全与运行研究室的膜态沸腾实验平台，本文开展了对5×5均匀加热棒束开展了底部再淹没实验研究。通过求解一维瞬态逆导热问题获得再淹没过程中加热棒束的表面参数，探究了不同实验条件对骤冷前沿推进速度的影响，使用热工安全分析程序RELAP5对实验结果进行对比计算，总结了其在模拟再淹没过程中存在的问题。结果表明：1）再淹没过程中高进口流量、高入口过冷度和低功率密度更有利于骤冷前沿的推进；2)RELAP5模拟的骤冷时间总均方根误差40.994 s；包壳峰值温度（Peak Cladding Temperature,PCT）总均方根误差61.465 K。模拟值在后临界热流密度（Critical Heat Flux,CHF）换热阶段与实验值相比误差较大，问题主要集中在沸腾模式判断和膜态沸腾换热模型上。本文中的实验数据可为再淹没过程的流动传热预测模型提供新的验证数据，也可用于评价和优化热工...     

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。     

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。     

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。     

DNB后过渡沸腾传热计算模型

提出过渡沸腾传热的物理模型,建立相应的数学模型,对偏离泡核沸腾(DNB)后过渡沸腾的传热特性进行分析。根据实验数据确定关系式中的系数,得到DNB后过渡沸腾传热计算模型。将提出的理论计算模型与现有不同工况范围内取得的实验数据及已有计算模型进行比较分析,并对计算偏差进行统计计算。结果表明,本文模型计算值与现有实验数据符合良好;相对于已有计算模型,模型具有良好的适用性。     

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%。     

矩形窄缝通道底部再淹没实验研究

为了解矩形窄缝通道在失水事故（LOCA）下底部再淹没过程中的热工水力特性,在不同实验条件下开展再淹没实验研究。矩形窄缝通道由2块因科镍合金焊接而成,本研究根据温度变化曲线分析底部再淹没过程,计算并对比不同实验工况下的骤冷前沿的推进速度（骤冷速度）,以及研究实验参数对再淹没过程的影响。实验结果表明,底部再淹没骤冷速度随着系统压力增大、进口流速增大、初始壁面温度降低以及冷却水过冷度的增大而增大。对比分析底部与联合再淹没工况,结果表明流量相同的情况下,底部再淹没的骤冷速度大于联合再淹没。本文研究为板状燃料元件反应堆事故预防以及事故缓解等研究奠定了基础。      

DNB后膜态沸腾传热计算模型

提出膜态沸腾传热的物理模型,建立相应的数学模型,对DNB后膜态沸腾传热特性进行分析,通过实验数据确定关系式系数,得到了DNB后膜态沸腾传热计算模型.本文提出的理论计算模型与现有不同工况范围内取得的实验数据和已有计算模型进行了比较分析,并对计算偏差进行了统计计算.比较结果表明,本文的计算结果与实验数据符合较好,且相对于已...     

Mechanisms of transitions to film boiling at CHFs in subcooled and pressurized liquids due to steady and increasing heat inputs

Experimental and theoretical studies published recently on the transitions to film boiling from non-boiling regimes or nucleate boiling at critical heat flux, CHFs, on a solid surface in water and wetting liquids such as ethanol, liquid nitrogen and liquid helium I due to steady and transient heat inputs for wide ranges of subcooling and pressure are reviewed. A large part of the transition processes to film boiling at CHF highly subcooled liquids at high pressure measured was not explained by the mechanism of heat transfer crisis based on the hydrodynamic instability (HI) model. Another mechanism assumed is that the transition occurs due to the explosive-like heterogeneous spontaneous nucleation (HSN) in originally flooded cavities on the solid surface in non-wetting and wetting liquids. The measured all transition processes for wide ranges of subcooling and pressure in the liquids due to steady and transient conditions were explained by the HI and HSN models for lower and higher subcooling, respectively. A HSN theoretical model based on the HSN surface superheat obtained experimentally was derived. The vapor behavior on the solid surface at the points in the processes measured was observed using a high-speed video camera. The assumption of the HSN model was verified theoretically and photographically.     

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.     

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。      

Applications of Artificial Neural Network for the Prediction of Flow Boiling Curves

An artificial neural network (ANN) was applied successfully to predict flow boiling curves. The databases used in the analysis are from the 1960's, including 1,305 data points which cover these parameter ranges: pressure P=100–1,000 kPa, mass flow rate G=40–500 kg/m²-s, inlet subcooling ΔT_sub =0–35°C, wall superheat ΔT_w = 10–300°C and heat flux Q=20–8,000kW/m². The proposed methodology allows us to achieve accurate results, thus it is suitable for the processing of the boiling curve data. The effects of the main parameters on flow boiling curves were analyzed using the ANN. The heat flux increases with increasing inlet subcooling for all heat transfer modes. Mass flow rate has no significant effects on nucleate boiling curves. The transition boiling and film boiling heat fluxes will increase with an increase in the mass flow rate. Pressure plays a predominant role and improves heat transfer in all boiling regions except the film boiling region. There are slight differences between the steady and the transient boiling curves in all boiling regions except the nucleate region. The transient boiling curve lies below the corresponding steady boiling curve.     

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.     

Photographic study on transitions from non-boiling and nucleate boiling regime to film boiling due to increasing heat inputs in liquid nitrogen and water

The behavior of vapor bubbles and vapor film during the transition from non-boiling regime such as natural convection or transient conduction regime to film boiling regime on a 1.2-mm diameter platinum horizontal cylinder in liquid nitrogen and in water due to exponentially increasing heat inputs, ranging from a quasi-steady state heat input to a very rapidly increasing one, were examined by photographs taken by a high-speed video camera. The experiments for water were performed for the two cases without and with pre-pressurization before each experimental run. It was confirmed by the observation of vapor behavior that the direct transitions in liquid nitrogen and in water which is pre-pressurized before each run occur due to the explosive-like heterogeneous spontaneous nucleation (HSN) in originally flooded cavities not only in the transient conduction regime, but also in quasi-steadily increasing natural convection regime without the vapor bubbles from active cavities entraining vapor. It was also confirmed that the semi-direct transition from conduction regime to film boiling with nucleate boiling due to the rapidly increasing heat inputs in water occurs due to the HSN with nucleate boiling at around the lower limit of HSN surface superheat in subcooled water even for the non-prepressure case. The lower limit of HSN surface superheat was measured as an initial boiling surface superheat caused by a quasi-steadily increasing heat input for the case with pre-pressurization before each experimental run.     

Effects of tube inclination on pool boiling heat transfer

Effects of tube inclination angles on nucleate pool boiling heat transfer of water at atmospheric pressure have been investigated experimentally. Experiments were performed for seven angles (0, 15, 30, 45, 60, 75, and 90°) with two tubes (12.7 and 19.1 mm in diameter) of 540 mm in length. Through the study, it can be concluded that tube inclination gives much change on nucleate pool boiling heat transfer. When the tube is near the horizontal and the vertical positions, the maximum and minimum of heat transfer coefficients are expected, respectively. The decrease in bubble slug formation on the tube surface and easy liquid access to the surface are thought to be the causes for the enhanced 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.