自然循环流量漂移伴随动态振荡的机理分析

在 5MW核供热堆试验回路HRTL 5上观察到了一定条件下系统发生的静态流量漂移并同时伴随动态流量振荡的现象。本文在实验基础上 ,研究了流量漂移的发生、发展和向动态振荡演变的过程 ,以及欠热沸腾、冷凝、闪蒸在此过程中的作用机理。分析结果表明 :1 )自然循环流量漂移是一个长热工过程 ,动态振荡可发生在静态漂移过程中 ;2 )在流量漂移的过程中 ,先是欠热沸腾和冷凝占主导地位 ,然后渐变为闪蒸占主导地位 ,最终表现为自持振荡形式 ;3)最终的自持振荡主要表现为密度波振荡 ,又具备喷泉不稳定的特征。     

低干度自然循环流量漂移的特征曲线图谱分析

在5MW低温核供热堆全模拟试验回路(HRTL-5)上,实验观察到了低干度自然循环条件下的流量漂移现象.通过一个考虑了加热段欠热沸腾、上升段冷凝、闪蒸等物理过程的两相流动数学模型,编制了相应的计算程序,获得了自然循环特征曲线图谱及其运行曲线,确定了自然循环分岔图和静态不稳定边界图,进而提出了通过自然循环特征曲线图谱研究流量漂移的分析方法.分析表明:特征曲线图谱方法是研究自然循环静态不稳定的有效手段.增大系统压力、减小热流密度、增加入口单相阻力、减小出口两相阻力有利于避免自然循环流量漂移的发生.     

HRTL-5轴向空泡份额计算

基于一维两相四方程漂移流模型,并考虑加热段欠热沸腾、上升段冷凝以及上升段闪蒸的影响,采用数值模拟的方法对5 MW低温核供热堆热工模拟回路(HRTL-5)的轴向空泡分布进行模拟计算,分析HRTL-5轴向参数分布的特性及其参数效应。结果表明:1)在HRTL-5条件下,考虑欠热沸腾、冷凝、闪蒸等因素影响的计算结果更接近实际情况;2)是否考虑加热段的欠热沸腾对于加热段出口的空泡份额以及出口的状态确定有重要影响;3)冷凝过程对于闪蒸区以及自然循环驱动力的计算有一定的影响;4)闪蒸对低压低干度自然循环系统轴向空泡份额的分布有重要影响,尤其在低压情况下。     

低压低干度自然循环流量漂移和热虹吸现象的动态模拟

采用一维两相4方程漂移流模型编制程序,对5MW核供热堆模拟实验回路中发生的流量漂移现象进行数值模拟,同时分析热虹吸的机理,并对其进行动态模拟.结果表明,该程序可以对自然循环系统的静态不稳定性进行模拟,对流量漂移现象的模拟基本与实验相符.     

低压流动稳定性实验研究

在5MW核供热反应堆模拟热工水力学实验回路HRTL-5上研究了在低压(p=0.1,0.24MPa)自然循环各种进口过欠热度条件下,单相及两相流流动特性。研究了不同种类的流动不稳定性,如,欠热沸腾不稳定性,欠热沸腾引起的闪蒸不稳定性,纯闪蒸不稳定性,密度波-闪蒸耦合不稳定性及高频流动振荡。研究结果表明:低压自然循环系统在绝大部分进口欠热度条件下,其两相流流动是不稳定的,仅在小进口欠热度条件下可获得稳定     

低压流动稳定性实验研究(英文)

在5MW核供热反应堆模拟热工水力学实验回路HRTL-5上研究了在低压(P=0.1,0.24MPa)自然循环各种进口过欠热度条件下,单相及两相流流动特性。研究了不同种类的流动不稳定性,如,欠热沸腾不稳定性,欠热沸腾引起的闪蒸不稳定性,纯闪蒸不稳定性,密度波-闪蒸耦合不稳定性及高频流动振荡。研究结果表明:低压自然循环系统在绝大部分进口欠热度条件下,其两相流流动是不稳定的,仅在小进口欠热度条件下可获得稳定的两相流流动;在高进口欠热度条件下,加热段中的欠热沸腾控制流动不稳定,在中进口欠热度条件下,加热段中的欠热沸腾及上升段中的空泡闪蒸控制流动不稳定。     

自然循环两相漂移流理论分析模型的建立

以5MW核供热堆试验回路（HRTL－5）为物理原型，从理论上推导了加热段欠热沸腾，上升段入口附近的冷凝，上升段闪蒸以及气空间压力平衡等关系式，并考虑了热力不平衡等因素的影响，建立了一个完整的四方程漂移流模型，该模型适用于分析低压低干度自然循环系统的流动问题，通过引入凝边界层，首次解决了两相流动过程中汽泡在过冷水中的冷凝问题，对汽空间则推导了压力及液位的约束方程，方程组可采用积分方法求解，并利用Rung-Kutta-Verner方法求解所得的微分方程组。     

自然循环静态流动不稳定研究

研究了发生静态流量漂移时自然循环系统中流量和加热段进出口温度的变化规律及静态流漂移过程中发生的动态流量振荡现象。研究结果表明：在发生静肪流量漂移时，系统循环流量下降，、加热段的进口温度下降而出口温度上升；当静态流量漂移一段时间后，伴随着静态流量漂移，系统内同时发生动态流量振荡。在对现象描述的基础上，阐述了静态流量漂移发生的机理。     

过冷沸腾自然循环稳定性实验研究

报道了低压压水运行工况下自然循环系统中过冷沸腾引起的两相流流动稳定性研究结果。研究工作是在清华大学核能技术研究所的低温堆全尺寸、全参数热工水力学实验系统HRTL-5上进行的,采用了可视化研究与数据记录分析相结合的研究方法,研究了不同系统压力、不同进口过冷度条件下,加热段出口从单相流动、过冷沸腾,到低干度容积沸腾整个范围内自然循环系统的稳定性特征。研究发现了低压过冷沸腾自然循环不稳定现象。在较宽的过冷度范围内存在着多种不同模式的流量振荡,在特定条件下存在着自然循环流量静态漂移。研究结果对低压压水运行的自然循环系统,例如,壳式一体化自然循环供热堆和池式低温供热堆的热工设计和安全分析论证及启动方式研究都具有重要意义。     

过冷沸腾自然循环稳定性实验研究

报道了低压压水运行工况下自然循环系统中过冷沸腾引起的两相流流动稳定性研究结果。研究工作是在清华大学核能技术设计研究院的低温堆全尺寸、全参数热工水力学实验系统HRTL-5上进行的,采用了可视化研究与数据记录分析相结合的研究方法,研究了不同系统压力、不同进口过冷度条件下,加热段出口从单相流动、过冷沸腾,到低干度容积沸腾整个范围内自然循环系统的稳定性特征。研究发现了低压过冷沸腾自然循环不稳定现象。在较宽的过冷度范围内存在着多种不同模式的流量振荡,在特定条件下存在着自然循环流量静态漂移。研究结果对低压压水运行的自然循环系统,例如,壳式一体化自然循环供热堆和池式低温供热堆的热工设计和安全分析论证及启动方式研究都具有重要意义。     

5MW核供热堆自然循环两相流流动特性分析

实验研究在５ＭＷ核供热堆热工水力学模拟系统ＨＲＴＬ－５上进行。计算分析采用带有质量,蒸汽质量,能量及动量守恒方程的一维两相流漂移模型。用Ｃｌａｕｓｉｕｓ－Ｃｌａｐｅｙｒｏｎ方程计算上升段中闪蒸起始点。通过在过冷沸腾区,饱和沸腾区及上升段中推导守恒方程,得到可描述自然循环两相流系统特性的常微分方程组。用时域法求解。研究表明过冷沸腾及空泡的闪蒸对空泡分布,系统循环流量及流动稳定性都有很大影响,且系统压力越低,过冷沸腾及闪蒸的影响越大;在相当宽的两相流动条件下,加热段中只发生过冷沸腾;揭示了两相流不稳定时振荡的传播特性。在５ＭＷ核供热堆条件下理论分析与实验结果吻合得很好。     

Mechanism analysis on flow excursion of a natural circulation with low steam quality

Flow excursion was observed on the natural circulation test loop HRTL-5, which simulates the geometry and system design of the 5 MW nuclear heating reactor. By means of self-developed computational codes, a set of tools used for analyzing the flow characteristics of the natural circulation have been presented, including the characteristic curves, operational curve and the bifurcation curve. The two-dimensional disturbance analysis is adopted to explain the mechanism of the flow excursion. Analytical result shows: (1) flow excursion can occur in a natural circulation system at a suitable geometry and thermal–hydraulic conditions. (2) Characteristic curves, operational curve, bifurcation curve, and the two-dimensional disturbance analysis are the available method to analyze the flow excursion of the natural circulation. (3) The flow excursion is prior to the low steam quality density wave oscillation. (4) The onset of the flow excursion is the tangency point of the drive force curve and the flow resistance curve. (5) To operate at low heat flux to increase the inlet temperature is the effectual approach to transfer from the pressurized water state to the boiling water state, in which the flow excursion and the low steam quality density wave oscillation can be avoided.     

低温堆上空腔破口失水后期的流动振荡实验研究

在５ＭＷ低温堆的模拟试验台架ＨＴＲＬ－５上对该低温堆上空腔破口失水事故进行了实验研究。重点研究了事故后期的流动振荡现象,揭示了振荡的发展过程和发生机理。经实验研究,给出了小破口失水闪蒸过程中回路内压力、温度、空泡份额、循环流量等重要参数的变化规律,同时表明了小破口失水过程中由于压力下降引起闪蒸产生大量气泡并引起自然循环流量的变化,这些变化又反过来对各参数产生影响,使循环流量的传热情况发生周期性变化     

Assessment of Some Cross Section Libraries through Analyses of Coupled-Core Critical Experiments by Monte Carlo Calculations

An experiment was performed on the natural circulation test loop HRTL-5, which simulates the geometry and system design of the 5 MW full power natural circulation nuclear heating reactor. Different flow modes, including density wave oscillation and flow excursion et al., were observed in a wide range of inlet sub-cooling at 1.5MPa. By means of self-developed computational codes, the bifurcation chart has been obtained. Consequently the flow excursion boundary has been determined. Through the analysis on the excursion boundary, the method to avoid the flow excursion during startup has been presented. Analytical results show: (1) with the decreasing heat flux or the increasing system pressure, the static flow excursion occurs at higher inlet temperature and its range in the instability maps becomes narrower correspondingly; (2) to decrease the outlet two-phase resistance or increase the inlet single-phase resistance is beneficial to avoid the flow excursion; (3) by means of increasing the system pressure to start up the reactor with low heat flux, the flow excursion and low steam quality density wave oscillation can be successfully avoided. This investigation is meaningful to the reactor safety and the design of the nuclear heating reactors.     

Flow excursion phenomenon and its mechanism in natural circulation

An experiment was performed on the test loop (HRTL-5), which simulates the geometry and system design of a 5-MW nuclear heating reactor. In a wide range of inlet subcoolings, different flow modes, such as single-phase stable flow, subcooled boiling stable flow, subcooled boiling static flow excursion, density-wave oscillation and stable two-phase flow in the natural circulation system have been described. The phenomenon and mechanism of the static flow-excursion, which has never been studied well on this field, is especially interpreted. The experimental results show that, in the process of flow excursion, the mass flow rate and the inlet temperature decreases, while the exit temperature increases smoothly. As the process of the excursion continues for about 1 h, short period dynamic flow oscillation occurs, which can only be seen in the process of this static flow excursion, and has also never been studied well. These static and dynamic flow instabilities combine together and continue for about 2 h, then a point is reached, at which the static flow excursion disappears, but the dynamic flow oscillation continues. The mechanism of the static flow excursion is interpreted through two sets of curves for flow resistance pressure drop and driven head in natural circulation, and one curve for the natural circulation operation under special thermohydraulic condition. The study of the flow excursion and its concerned dynamic flow oscillation is of great significance for the development of the nuclear heating reactor under natural circulation.     

开式自然循环系统启动特性研究

针对开式自然循环系统启动特性进行了实验研究。实验表明:不同加热功率下,开式自然循环系统会经历不同的流动演化过程。低加热功率下,系统经历单相循环、喷泉不稳定,最终演化为闪蒸不稳定;中等以及高加热功率下,系统依次经历单相循环、喷泉不稳定和沸腾伴随闪蒸不稳定后,分别演化为稳定的汽液流动和密度波振荡。导致启动过程流动演化的主要原因是随着加热管入口水温的升高,管内沸腾现象持续增强,上升段内闪蒸现象则先增强而后减弱,两者相互作用,导致系统流量、相变位置及空泡份额等发生明显变化。最后,绘制了开式自然循环启动过程的无量纲化流动不稳定区域分布图,并拟合得到了喷泉不稳定及闪蒸主导的不稳定起始边界的经验关系式,拟合结果与实验结果符合良好。     

Experimental simulation study on start-up of the 5 MW nuclear heating reactor

An experimental simulation study on the start-up of a low temperature, natural circulation nuclear heating reactor (5 MW developed by the Institute of Nuclear Energy of Tsinghua University, Beijing) is presented. The experiment was performed on the test loop (HRTL-5), which simulates the geometry and system design of the 5 MW reactor. The manifestation of different kinds of two-phase flow instability, namely geysering, flashing instability and low steam quality density wave instability on the start-up are described. The mechanism of flashing instability, which has never been well studied in this field, is especially interpreted. Based on the study of these instabilities, it is suggested that the start-up process, from initial condition to boiling operation condition, should consist of three steps: increasing of initial pressure by means of a noncondensable gas (N₂), start-up of the reactor at this pressurized condition (single-phase regime operation), and transition to a lower pressure, boiling operation. Three transition methods are discussed. As a result of these studies, the method of transition with low heat flux and low inlet subcooling is proposed. A stable start-up process of the 5MW reactor is achieved by careful selection of the thermohydraulic parameters.     

低压自然循环复合流量脉动实验研究

在低压流动沸腾不稳定性实验中,研究了自然循环流动在不同入口过冷度下的演化过程。对实验中的流动沸腾不稳定性入口流量信号进行快速傅里叶变换,基于振幅和频率特性区分了3种流动脉动模式：小幅流量脉动、复合流量脉动和逆流。分析了加热功率和入口过冷度对自然循环不稳定性的影响。根据加热段出口水温变化得到了出口的流型变化,当流量波动振幅较小时加热段出口流体始终是饱和状态,而当流量波动振幅较大时,加热段出口为单相液体和两相混合物交替通过。给出了这3种流量脉动的边界图,分析了热流密度和入口过冷度对流量脉动模式的影响。结果表明：出口含气率大于0时发生流动不稳定性,热流密度达到间歇干涸型临界热流密度时发生逆流。