低压自然循环系统流动闪蒸过程流型研究

本文采用高速摄影和网格电导传感器对低压自然循环系统垂直上升段内闪蒸诱发的两相流流型演变开展研究。针对不同的流动状态,分别给出了稳定和不稳定两相流动条件下上升段内的流型种类。基于上升段内流体温度沿轴向的变化规律,确定流体温度沿轴向位置的转折点为闪蒸发生的起始位置。采用无量纲过冷数和闪蒸数,对低压自然循环系统的流动状态进行了划分;在入口过冷数小于12、闪蒸数介于4～5之间时,系统处于稳定的两相自然循环流动状态。     

低压自然循环系统流动闪蒸过程流型研究

本文采用高速摄影和网格电导传感器对低压自然循环系统垂直上升段内闪蒸诱发的两相流流型演变开展研究。针对不同的流动状态，分别给出了稳定和不稳定两相流动条件下上升段内的流型种类。基于上升段内流体温度沿轴向的变化规律，确定流体温度沿轴向位置的转折点为闪蒸发生的起始位置。采用无量纲过冷数和闪蒸数，对低压自然循环系统的流动状态进行了划分；在入口过冷数小于12、闪蒸数介于4～5之间时，系统处于稳定的两相自然循环流动状态。     

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。     

两流体方法相间界面力模型参数敏感性分析研究

基于两流体方法对动量传递模型中各界面力模型和关键参数进行了参数敏感性分析,研究了界面力模型和关键参数对轴向和加热段末端径向参数分布的影响程度和规律,并借助整体效应实验结果对结算结果进行了对比分析。研究表明,在轴向参数影响方面,不同曳力模型由于基本假设不同,对轴向平均空泡份额会产生一定影响,非曳力模型对轴向参数分布基本无影响。在径向参数分布影响方面,曳力模型对其基本无影响,非曳力中湍流分散力和升力对于近壁区域的空泡份额峰值和对径向空泡份额分布具有显著影响,壁面润滑力对壁面空泡份额峰值具有一定影响。虚质量力对轴向和径向参数分布均无显著影响,在流动过冷沸腾计算研究中可予以忽略。     

竖直圆管内泡状流空泡份额径向分布实验研究

常温常压下,采用光学探针测量方法,对圆管（内径50 mm）内空气 水两相竖直向上泡状流空泡份额的径向分布特性进行了实验研究。结果表明,竖直圆管内泡状流空泡份额的径向分布随气液两相表观流速不同而变化。液相流速较高时空泡份额分布呈“壁峰型”,即中心区域变化平缓,近壁区出现峰值后迅速降低;液相静止时,随气相流速增加,空泡份额增加速度沿径向向外逐渐减小,气相流速较大时分布呈“核峰型”,即空泡份额随径向位置向外呈减小趋势;液相流速较低时分布呈现出过渡型。探针测量面积加权平均空泡份额与通过重位压降得到的空泡份额的相对偏差小于10%。     

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。     

竖直圆管内空泡份额径向分布特性形成机制

在常温常压下,采用光纤探针测量方法对垂直上升大圆管中空气-水两相流动的空泡份额径向分布特性及形成机制进行研究。实验选用圆管直径为100 mm,气相、液相折算速度的范围分别为0.004~0.053 m/s和0.071~0.213 m/s。结果表明空泡份额径向分布随着气-液流量的不同,呈现出"核峰"或"壁峰"型分布特点;通过分析气泡所受到的横向升力和壁面力,表明二者对气泡横向运动的综合作用是造成空泡份额径向分布呈现出"核峰"或"壁峰"型分布的主要原因。     

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。     

基于确定性抽样的过冷沸腾边界条件不确定性分析

本文使用Fluent软件构建数学物理模型，对DEBORA过冷沸腾基准实验进行了数值模拟，并采用确定性抽样方法对模拟沸腾流动的边界条件不确定性进行分析，计算得到了主要径向参数分布的期望值和置信区间，分析了边界条件不确定性的影响趋势。此外，还计算得到了不确定性源对部分径向参数的影响权重。结果表明，流体入口温度和壁面热流密度的不确定性对径向空泡份额的影响较大，而运行压力和流体入口温度的不确定性是影响径向液体温度计算的主要因素。     

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。     

Examination of transient characteristics of two-phase natural circulation within a Freon-113 boiling/condensation loop

Transient characteristics of two-phase natural circulation within a Freon-113 loop with a large condenser have been examined mainly focused on the flashing phenomenon. General behavior was described and parametric studies were performed. The items observed were the period and duration of flashing, peak flow rate, amount of flow carryover per flashing, lowest-peak liquid level within the condenser, and the peak void distribution in the riser section. The parameters considered were the heater power input, valve friction at the heater inlet (simulating the loopwise friction), condenser cooling, degree of subcooling at the heater inlet, and the heat loss to the surroundings. As a whole, the heater power input, valve friction, and the rate of condenser cooling played important roles in flashing while the other effects being marginal. In general, the flow appeared to be more unstable with the larger condensing surface which causes the condensation-induced flashing.     

Study theoretically on two-phase circulation flow characteristics under ERVC condition in advanced PWR

For the problem of two-phase natural circulation flow in gap clearance between reactor vessel lower head and insulator in the condition of severe accident, one-dimensional steady-state natural flow analysis code was written by utilizing FORTRAN. Based on the code, the effects of different correlations for friction coefficient and the number of nodes of heating section on mass flow rate of two-phase natural circulation flow were studied. And the results are compared with that of Chinese REPEC experiment and simulation using RELAP5 program so as to verify the rationality and correctness of the code. Based on the experiment data, simulation results and the model, friction coefficient and the void fraction condition under ERVC correlation are obtained by fitting. The results calculated by the model using fitting friction coefficient correlation agree well with ULPU V test data. Furthermore, the effect of power, pressure, inlet area, gap diameter, flooding level and inlet water subcooling on mass flow rate and void fraction of two-phase natural circulation were studied utilizing this code.     

Single rod experiments on transient void behavior during low-pressure reactivity-initiated accidents in light water reactors

A series of experiments for fast transient void behaviors during cold shutdown reactivity-initiated accidents in light water reactors has been performed with a single simulated fuel rod at the atmospheric pressure. A fast-response impedance technique was applied to the void fraction measurement. It was confirmed that, based on the comparison with X-ray technique, the impedance technique was capable of measuring void fraction within an allowable range of accuracy for fast transient two-phase flows. Experimental data was accumulated concerning the onset of net vapor generation, local void fraction, water temperature and pressure. The observed tendency on the onset of net vapor generation was qualitatively agreed with the Saha and Zuber model. However, the model overestimated local water subcooling at the onset of net vapor generation. Inlet water subcooling largely influenced the variation of local void fraction. It was observed that larger inlet water subcooling resulted in repeated growth and collapse of voids to suppress a continuous increase in void fraction.     

Detailed Evaluation of the Natural Circulation Mass Flow Rate of Water Propelled by Using an Air Injection

One-dimensional (1D) air-water two-phase natural circulation flow in the “thermohydraulic evaluation of reactor cooling mechanism by external self-induced flow—one-dimensional” (THERMES-1D) experiment has been verified and evaluated by using the RELAP5/MOD3 computer code. Experimental results on the 1D natural circulation mass flow rate of water propelled by using an air injection have been evaluated in detail. The RELAP5 results have shown that an increase in the air injection rate to 50% of the total heat flux leads to an increase in the water circulation mass flow rate. However, an increase in the air injection rate from 50 to 100% does not affect the water circulation mass flow rate, because of the inlet area condition. As the height increases in the air injection part, the void fraction increases. However, the void fraction in the upper part of the air injector maintains a constant value. An increase in the air injection mass flow rate leads to an increase in the local void fraction, but it has no influence on the local pressure. An increase in the coolant inlet area leads to an increase in the water circulation mass flow rate. However, the water outlet area does not have an influence on the water circulation mass flow rate. As the coolant outlet moves to a lower position, the water circulation mass flow rate decreases.     

Analytical modeling of flashing-induced instabilities in a natural circulation cooled boiling water reactor

A dynamic model for natural circulation boiling water reactors (BWRs) under low-pressure conditions is developed. The motivation for this theoretical research is the concern about the stability of natural circulation BWRs during the low-pressure reactor start-up phase. There is experimental and theoretical evidence for the occurrence of void flashing in the unheated riser under these conditions. This flashing effect is included in the differential (homogeneous equilibrium) equations for two-phase flow. The differential equations were integrated over axial two-phase nodes, to derive a nodal time-domain model. The dynamic behavior of the interface between the one and two-phase regions is approximated with a linearized model. All model equations are presented in a dimensionless form. As an example the stability characteristics of the Dutch Dodewaard reactor at low pressure are determined.     

Vapour void fraction in subcooled flow boiling

Experimental data on steam void fraction and axial temperature distribution in an annular boiling channel for low mass-flux forced and natural circulation flow of water with inlet subcooling have been obtained. The ranges of variables covered are: mass flux = 1.4 × 10⁴−1.0 × 10⁵ kg/hr m²; heat flux = 4.5 × 10³−7.5 × 10⁴ kcal/hr m²; and inlet subcooling = 10–70°C. The present and literature data match well with the theoretical void predictions using a four-step method similar to that suggested by Zuber and co-workers.     

Characteristics of Type-I Density Wave Oscillations in a Natural Circulation BWR at Relatively High Pressure

Experiments were conducted to investigate two-phase flow instabilities in a boiling natural circulation loop with a chimney at high pressure. The SIRIUS-N facility was designed to have non-dimensional values which are nearly equal to those of a typical natural circulation BWR. The observed oscillations are found to be density wave oscillations, since the void fractions in the chimney inlet and exit are out of phase. They belong to the Type-I category, since they occur at low flow qualities, according to the Fukuda—Kobori's classification. Moreover, the oscillation period correlates well with the passing time of bubbles in the chimney section regardless of the system pressure, the heat flux, and the inlet subcooling. Two distinct phenomena are found in relation between the oscillation period and liquid passing time in the chimney, indicating that the driving mechanisms of the instabilities are different between low and high pressures. Stability maps were obtained in reference to the inlet subcooling and the heat flux at the system pressures of 1, 2, 4, and 7.2 MPa. The flow became stable below a certain heat flux regardless of the channel inlet subcooling. The stable region enlarges with increasing system pressure. Thus, the stability margin becomes larger in a startup process of a reactor by pressurizing the reactor sufficiently before withdrawing the control rods. The obtained stability map demonstrates that the nominal operating condition of the ESBWR has a significant stability margin to the unstable region.     

Characteristics of two-phase condensing flow by visualization using computed image processing

The mechanics of the condensing behavior of vapor bubbles in a subcooled bulk flow is complicated and influenced by both heat and mass transfer. To examine the characteristics of such thermal-nonequilibrium two-phase flow, experimental and analytical researches have been made. In the experiment, the movement of each vapor bubble in a flowing channel was recorded on video tapes and analyzed by an image processing system. As result, the distributions of void fraction along the test section were obtained. In the analysis, a simple analytical model was introduced to predict the distributions of void fraction and liquid subcooling temperature. By considering the rate of vapor condensation along the flow direction, the differential equation of energy balance between two phases was obtained. Integration of this equation yielded the void fraction and bulk liquid subcooling at any position. The condensation rate was estimated as a function of the local liquid subcooling, interfacial area and mass velocity. Finally, a close fit between calculated results and experimental data was obtained.