含不凝性气体的蒸汽气泡凝结过程研究

利用高速摄像仪对高过冷度下含不凝性气体的蒸汽气泡冷凝及破裂过程进行可视化研究,以分析不凝性气体对气泡微细化沸腾(MEB)过程的影响。实验结果表明:初始不凝性气体体积份额x0小于2.5%时,气泡突然破碎成大量微小气泡;x0在2.5%~7.5%之间时,较大气泡只会分裂成数个小气泡;x0大于7.5%时,气泡界面非常稳定,不会发生破碎和分裂现象。此外,当蒸汽气泡中含有较多不凝性气体时,气泡凝结过程减弱,液体对气泡的惯性冲击减小,气泡不易破裂。由此可表明,在气泡微细化沸腾发生时,不凝性气体的存在会阻碍加热面上气泡的破碎,从而降低传热能力。     

高压下含不凝性气体的冷凝换热模型研究

少量的不凝性气体会在很大程度上削弱蒸汽凝结的热传递,现有的含不凝性气体的冷凝换热模型大都建立在理想气体状态方程的基础上,但高压条件下气体受到压缩作用,基于理想气体建立的传热模型预测值与实际值偏差较大。为建立高压下含不凝性气体的冷凝预测方法,对一体化堆安全分析提供技术辅助手段,本研究基于实际气体方程,在扩散层理论的基础上引入液膜波动因子、抽吸因子、雾化因子等修正系数,建立了高压下含不凝性气体的冷凝换热模型。本文模型和Kim试验数据进行了对比分析,改进后的模型比基于理想气体的模型预测值偏差范围更小,相对偏差大部分到±25%以内,充分体现了高压条件下模型的适用价值。     

含有非凝性气体的蒸汽凝结数值模拟研究

非凝性气体对蒸汽的凝结过程有明显影响,导致凝结流量减小,进而导致凝结传热系数降低。本文采用CFD方法模拟含有非凝性气体的蒸汽凝结,并与现有实验数据进行比较。结果表明CFD数值计算结果精度较好,证明了用多相流模型的方法处理非凝性气体影响凝结问题的可行性。本文还讨论了蒸汽分压、非凝性气体的质量浓度等对蒸汽凝结过程中传热系数的影响及凝结过程对温度场、速度场分布的影响。     

钠热管中气液交界质量调节系数的分子动力学研究

钠热管是热管堆内的关键热量传输部件,深入理解内部工质的蒸发与冷凝换热机理对于钠热管的设计优化具有重要意义。本研究用分子动力学软件LAMMPS,结合钠热管的启动和运行工况,模拟了600、700、800、900 K下液态钠薄膜的平衡态蒸发,求解了质量调节系数（MAC）。并在模拟中引入了氩气作为非凝结性气体,研究了非凝结性气体对MAC的影响。研究结果表明,4组工况下的MAC分别为0.388 6、0.211 9、0.261 5、0.241 6;非凝结性气体存在时,MAC分别为0.282 9、0.254 3、0.129 5、0.107 2。本研究为钠热管的数值模拟提供了参考借鉴和理论支撑。     

过冷水中单个蒸汽气泡凝结动力学过程研究

对高过冷度条件下单个蒸汽气泡的凝结过程进行了可视化研究,并与现有的计算模型进行了对比。实验结果表明：高过冷度条件下,运动速度较高的气泡脱离后,其底部产生的液体射流现象会加剧气泡的变形和凝结过程;而现有的计算关联式均无法较好地预测该条件下的气泡动力学行为,凝结末期的相对误差超过50%。此外,通过Sobol方法对气泡凝结模型进行了敏感性分析,并定量评估了不同实验范围内Reynolds数、Jacob数以及Prandtl数对气泡凝结的影响程度。     

核供热堆不凝结气体排放份额试验研究

叙述了核供热堆上空腔不凝结气体在发生排放事故工况下排放特性的试验研究。试验模拟了核供热堆排放工况的主要参数，着重研究了在排放过程中的氮气排放份额，及氮气对排放背压的影响。本试验是在５ＭＷ核供热堆热工水力学试验回路（ＨＲＴＬ－５）和排放回路上进行的，系统压力为１．５ＭＰａ，初始氮气分压为０．３４ＭＰａ。采用静态校验法，成功地获得了氮气的排放份额。这些试验数据为核供热堆的安全分析提供了重要的试验数据。     

全压堆芯补水箱内饱和蒸汽凝结特性分析及数值模拟

基于对全压堆芯非能动注水系统中饱和闪蒸蒸汽在补水箱（CMT）深度过冷厚筒壁和液面层凝结特性分析，引入液相导热系数倍增因子和均温混合液层来反映汽流冲入CMT上部液层对蒸汽直接接触凝结过程的强化作用，进而提出了相应的传热简化模型。借助数值模拟方法，初步预测了不同工况下CMT中各时刻闪蒸蒸汽的凝结率，并与试验结果进行了比较。     

非凝性气体于竖直壁面处冷凝传热压力特性影响机理研究

非凝性气体于竖直壁面处冷凝传热的研究对一体化压水堆汽-气稳压器的瞬态调节以及紧凑型安全壳余热排出进程具有重要影响,当前对含有非凝性气体的蒸汽竖直壁面冷凝传热中压力的影响特性研究较少。基于传热传质比拟方法,采用适用于高压的改进扩散层模型对汽-气竖直壁面冷凝传热的压力影响进行研究。研究发现,基于传热传质比拟方法改进的扩散层模型与已有的实验结果基本一致,适用于较高压力汽-气竖直壁面冷凝传热系数的预测;总压的增加对存在非凝性气体的冷凝传热具有促进作用,这种促进作用随总压的增加逐渐减弱;在一定压力范围内(0.1~7.0 MPa),存在压力分界点pc,在压力影响分界点以下的低压力区域(0.1 MPa~pc)为压力影响敏感区,在压力影响分界点以上的高压力区(pc~7.0 MPa)为非敏感区。同时,本文还对非凝性气体的种类和含量对蒸汽在竖直壁面处冷凝传热过程的影响进行了分析,从气体扩散系数方面进一步分析了造成影响差异的原因。     

雷诺数关系式对膜状凝结湍流区传热计算的影响

膜状凝结现象广泛存在于核电站安全壳和稳压器中。关于膜状凝结液膜湍流区的传热模型,目前未明确辨析基于质量和能量关系的两种雷诺数关系式的差别。本文针对管外纯蒸汽自然对流膜状冷凝传热,定量地分析雷诺数关系式对膜状凝结液膜湍流区传热计算的影响。基于液膜湍流区修正项的一般性假设,推导了膜状凝结湍流区传热系数的表达式。同时,分别与雷诺数关系式Remass和Reenergy联立,求解得到不同雷诺数关系式之间以及对应的膜状凝结传热系数之间的关系。分析表明:受普朗特数Pr的影响,在膜状凝结液膜湍流区,雷诺数关系式Remass和Reenergy差别明显,并存在关于Pr的分界点。基于Remass和Reenergy得到的膜状凝结平均传热系数及其相对偏差是Re和Pr的非线性函数。当0.1Pr4.0且Re1 600时,基于Reenergy和Remass得到的膜状凝结平均传热系数相对偏差在-60%和+60%之间。通过实验和理论验证,在膜状凝结液膜湍流区基于Reenergy得到的膜状凝结传热系数更加准确。     

严重事故下水蒸气凝结对气溶胶扩散泳影响研究

基于模拟AP系列非能动安全壳冷却的气溶胶迁移机理试验平台,开展了在干燥及潮湿气氛下的气溶胶沉积试验。结果表明:在潮湿气氛下,气空间内气溶胶浓度会由于扩散泳沉积作用而衰减得更快,且水蒸气的凝结质量流量越大,气溶胶浓度衰减越快。证实了非能动安全壳冷却设计能够通过提高扩散泳沉积作用而强化气空间的气溶胶衰减。利用该特点,可针对性地采取措施增强水蒸气凝结,强化气溶胶扩散泳作用,提高安全壳内的放射性去除效果。     

安全壳冷却系统蒸汽冷凝传热研究综述

为保证事故条件下核电厂安全壳的完整性,新一代核电厂广泛采用安全壳冷却系统对事故中释放的蒸汽进行冷却,达到持续稳定导出堆芯蓄热与衰变余热的目的。含不凝性气体（空气、氢气等）的蒸汽在安全壳换热壁面上的冷凝传热成为疏导安全壳内部热量的重要手段。本工作对核电厂事故条件下含不凝性气体蒸汽的冷凝传热进行综述研究,深入全面分析传热过程,提出针对性意见,为核电厂安全壳冷却系统的热工水力研究奠定了基础。     

非凝性气体对汽-气稳压器稳压的影响研究

通过分析相间的传热传质过程以及非凝性气体存在时壁面蒸汽冷凝过程,建立了汽 气稳压器模型,研究了非凝性气体对稳压过程的影响,描述了稳压器的稳压特性,并将模型计算结果与MIT稳压器实验数据进行了对比。结果表明：当不含非凝性气体时,计算精度高,相对偏差在0.8%内,压力峰值为0.647 MPa;当非凝性气体含量从0增至20%时,计算精度相对减小,最高相对偏差为15.4%;压力峰值从0.647 MPa增至1.02 MPa。研究表明非凝性气体对稳压器稳压过程具有重要影响作用,随着非凝性气体的种类和含量的变化,稳压器内稳压过程发生显著变化。     

Condensation heat transfer characteristic in the presence of noncondensable gas on natural convection at high pressure

The steam-gas pressurizer in integrated small reactors experiences very complicated thermal-hydraulic phenomena. Especially, the condensation heat transfer with noncondensable gas under natural convection is an important factor to evaluate the pressurizer behavior. However, few studies have investigated the condensation in the presence of noncondensable gas at high pressure. In this study, therefore, a theoretical model is proposed to estimate the condensation heat transfer at high pressure using the heat and mass transfer analogy. For the high pressure effect, the steam and nitrogen gas tables are used directly to determine the density of the gas mixture and the heat and mass transfer analogy based on mass approach is applied instead of that based on the ideal gas law. A comparison of the results from the proposed model with experimental data obtained from Seoul National University indicates that the condensation heat transfer coefficients increase with increasing system pressure and with decreasing mass fraction of the nitrogen gas. The proposed model is also compared with other conventional correlations proposed in the literature. The proposed model demonstrates the capability to predict the condensation heat transfer coefficients at high pressure better than any other correlation. Finally, the condensate rate is compared to verify the application of the heat and mass transfer analogy at high pressure. The comparison results confirm that the heat and mass transfer analogy can be applied to evaluate the condensation heat and mass transfer at high pressure.     

振荡区下蒸汽冷凝过程的热工水力模型建立及验证

蒸汽射流冷凝过程具有强烈的凝结换热能力,广泛应用于先进非能动反应堆安全系统中,但该过程会产生强烈的压力振荡现象。为研究蒸汽浸没射流冷凝振荡现象的本质,从基本守恒方程式出发,建立了气泡边界层质量交换模型、气泡控制方程模型、气泡内蒸汽压力计算模型、水池中任意位置处压力计算模型等关键模型,构建了模拟水池中蒸汽气泡冷凝振荡过程的热工水力模型。运用建立的气泡半径和水池内压力的计算模型获取气泡半径和压力随时间变化的规律,并与Chun实验和Fukuda实验的实验数据进行比对,验证了模型的有效性,为后续开展冷凝振荡机理研究打下理论基础。     

Effect of noncondensable gas in a vertical tube condenser

An experimental study is performed to investigate the effects of noncondensable (NC) gas in the steam condensing system. A vertical condenser tube is submerged in a water pool where the heat from the condenser tube is removed by boiling heat transfer. The design of the test section is based on the passive condenser system in an advanced boiling water nuclear power reactor. Data are obtained for various process parameters, such as inlet steam flow rate, noncondensable gas concentration, and system pressure. Degradation of the condensing performance with increasing noncondensable gas is investigated. The condensation heat transfer coefficient and heat transfer rate decrease with noncondensable gas. The condensation heat transfer rate is enhanced by increasing the inlet steam flow rate and the pressure. The condensation heat transfer coefficient increases with the inlet steam flow rate, however, decreases with the system pressure. For the condenser submerged in a water pool with saturated condition, the strong primary pressure dependency is observed.     

Evaporation and condensation heat transfer with a noncondensable gas present

To evaluate the system pressure of an external water wall type containment vessel, which is one of the passive systems for containment cooling, the evaporation and condensation behavior under a noncondensable gas presence has been experimentally examined. In the system, steam evaporated from the suppression pool surface into the wetwell, filled with noncondensable gas, and condensed on the containment vessel wall. The system pressure was the sum of the noncondensable gas pressure and saturated steam pressure in the wetwell. The wetwell temperature was, however, lower than the supression pool temperature and depended on the thermal resistance on the suppression pool surface. The evaporation and condensation heat transfer coefficients in the presence of air as noncondensable gas were measured and expressed by functions of steam/air mass ratio. The evaporation heat transfer coefficients were one order higher than the condensation heat transfer coefficients because the local noncondensable gas pressure was much lower on the evaporating pool surface than on the condensing liquid surface. Using logal properties of the heat transfer surfaces, there was a similar trend between evaporation and condensation even with a noncondensable gas present.     

A model for the performance of a vertical tube condenser in the presence of noncondensable gases

Some proposed vertical tube condensers are designed to operate at high noncondensable fractions, which warrants a simple model to predict their performance. Models developed thus far are usually not self-contained as they require the specification of the wall temperature to predict the local condensation rate. The present model attempts to fill this gap by addressing the secondary side heat transfer as well. Starting with a momentum balance which includes the effect of interfacial shear stress, a Nusselt-type algebraic equation is derived for the film thickness as a function of flow and geometry parameters. The heat and mass transfer analogy relations are then invoked to deduce the condensation rate of steam onto the tube wall. Lastly, the heat transfer to the secondary side is modelled to include cooling by forced, free or mixed convection flows. The model is used for parametric simulations to determine the impact on the condenser performance of important factors such as the inlet gas fraction, the mixture inlet flowrate, the total pressure, and the molecular weight of the noncondensable gas. The model performed simulations of some experiments with pure steam and air-steam mixtures flowing down a vertical tube. The model predicts the data quite well. The model described also provides a basis under which the presence of aerosol particles in the gas stream could be analyzed.     

几何参数对含空气蒸汽冷凝影响的数值分析

含空气蒸汽冷凝是反应堆失水事故时安全壳内重要的热工水力现象。已有研究多关注气体压力、温度等热工参数对传热特性的影响,而对几何参数的影响及其作用原理分析较少。采用三维CFD数值模拟方法,基于扩散边界层冷凝机理模型研究了管径(4~60 mm)、管长(0.1~7 m)及倾角(0°~90°)对含空气蒸汽冷凝传热特性的影响。结果表明,管径、管长及倾角均对含空气蒸汽冷凝传热特性有显著影响。平均冷凝传热系数随管径的增大而减小;随管长的增长先减小后增大,3 m左右达到最小值;随倾角的增大而增大。局部冷凝传热系数沿管长方向先迅速减小后缓慢增大。倾斜布置时,迎流面产生明显传热强化,向两侧逐渐减弱,背流面存在一定的传热抑制。