超临界压力锅炉水冷壁倾斜并联内螺纹管内两相流不稳定性的试验研究

针对国产首台600 MW超临界机组在亚临界压力条件下的水动力特性,进行了倾斜并联内螺纹管内的汽液两相流不稳定性试验研究,观察到压力降型和密度波型两种类型的脉动。探讨了影响不稳定性的主要参数。并给出了发生不稳定性的阈值。试验表明:在超临界锅炉滑压运行设计参数条件下,水冷壁管内不会发生两相流不稳定性。图11表1参7     

超临界循环流化床锅炉垂直并联内螺纹管内两相流不稳定性的研究

针对600 MW超临界循环流化床(CFB)锅炉水冷壁管在低质量流速条件下的水动力特性,进行了垂直并联内螺纹管内两相流不稳定性的试验研究,得到了不同条件下的水动力特性曲线和压力降、密度波2种脉动的典型脉动曲线.在试验参数范围内,分析了系统压力、质量流速、进1:2过冷度和上游可压缩容积对两相流不稳定性的影响.根据试验结果,采用均相流模型得到了不稳定发生的界限关系式,为超临界CFB锅炉垂直并联内螺纹管水冷壁的设计与安全运行提供了依据.     

并联蒸发管内两相流密度波型脉动线性分相模型

两相流密度波型脉动理论分析方法可以分为两大类：一类是数值解析法,另一类是近似分析法。数值分析法已经有很多学者进行了大量研究,而近似分析法研究相对少一些,特别是运用系统控制原理的方法来分析两相流密度波型脉动就更少,本文运用系统控制原理的方法来研究两相流密度波型脉动,提出了描述并联沸腾管内两相流密度波型脉动的线性分相模型,导出了描述系统稳定性的状态空间表达式,计算了质量流速、热负荷和系统压力的变化对系统特征方程式的特征根的影响。结果表明：各个参数对密度波型脉动界限值的影响规律与实验值是一致的。     

600MW超临界CFB垂直并联内螺纹管两相流压力降不稳定试验研究

在600 MW超临界CFB水冷壁变负荷实际运行条件下,以水冷壁实际采用的Φ28.6×5.8 mm的4头内螺纹管为研究对象,在高压汽水两相流回路上对垂直并联管中汽液两相流压力降型不稳定进行了试验研究.确定了压力、质量流速、进口过冷度以及上游可压缩容积对垂直并联内螺纹管两相流压力降型脉动的影响.结果表明,随压力增大,发生脉动的临界热负荷增加,界限干度逐渐升高,系统稳定性增强,脉动周期先变长后变短,脉动的振幅逐渐减小.本试验中,当压力P＞6 MPa时,就不再有压力降脉动发生;随着质量流速的增加,脉动发生的界限热负荷升高,而脉动的周期减小;进口过冷度对密度波脉动呈现单值性影响,随进口过冷度增加,界限热负荷单调增加,界限干度的变化表现出不同的趋势,在较低的质量流速下,随着过冷度的增加,界限干度单调下降;在较高的质量流速下,随着过冷度的增加,界限干度单调上升;上游可压缩容积对界限热负荷的影响较小,随充气比的增大,脉动的周期和幅值逐渐增大.     

基于反问题理论的汽液两相上升流流型预测与实验研究

汽液两相流流型的测量在两相流研究中占有重要地位。应用均相流模型建立了圆管内汽液两相上升流压力分布,基于反问题理论反演了汽液两相的物性参数,将反演结果与流型图结合,精确地预测了管内流型,计算结果与实验结果进行了对比,误差小于5%。提出的反演流型的方法,可以推广到水平管、螺旋管,为工程上的汽液两相流设备安全性分析、稳定性分析等提供了一种简单可靠的技术方法。     

沸腾系统中两相流体的不稳定(一)

本文归纳了几年来对两相流体不稳定研究的理论和试验成果,论证并说明了在各种沸腾流道系统中通常发生的三种不同类型的两相流体脉动,即密度波型,压降型和热力型脉动。首先概述两相流不稳定的分类,即三种两相流体脉动的区别和它们的机理。自从50年代原子反应堆商业化以来,西方国家开始研究不稳定性问题,接着苏联和中国也进行了研究。本文叙述了在单流道、双流道、交叉连接的双流道,四平行流道和四交叉平行流道向上流动系统中暂态的和持续的不稳定性的实验结果。指出了热流量变化,进口欠热,流量,进口和出口阻力的影响。同时包括了在垂直的单流道中强化传热对两相流不稳定的影响。综述了以均相两相流和两相热力平衡为假设的数学模型,用来预报在单流道中强迫对流沸腾向上两相流动稳定的和暂态的特性,并叙述了这些解的结果。提出了在单流道向上流动的沸腾系统中两个不同的两相流模型,即用来预报压降型不稳定界限的常参数均相流动模型和用来预报密度波不稳定界限的变参数漂流模型,并列出了一些解。     

并联多通道系统密度波不稳定问题多变量频域分析

基于多变量自动控制理论,本文提出一种新的分析方法,用于研究带有复杂边界条件的并联多通道系统的密度波不稳定性问题.该方法克服了目前广泛采用的等效通道单变量方法不能考虑系统中耦合效应的重要缺点,将密度波不稳定性问题的分析研究从单变量推广到多变量.根据本文方法编制的计算机程序可用于预测系统的稳定性以及系统参数的变化对稳定性的影响,从计算和分析得到的结论与有关文献的实验观测结果相一致.     

汽液两相流自调节液位控制器在高压加热器上的应用

通过汽液两相流自调节液位控制器在解决汽轮机高压加热器无水运行状况的成功应用实例,找出了无水运行状况的原因,介绍了汽液两相流自调节液位控制器原理、特点及安装要点,并分析了应用技术经济效果。     

两相流条件下的喷油系统中压力波时频特性的研究

利用Wigner-Ville分布算法对液-汽两相流状态下的四冲程柴油机喷油系统中的压力波的时频特性进行了研究.研究结果表明：采用Wigner-Ville分布算法进行分析时,可以更好地显示出液-汽两相流状态下的柴油机喷油系统中压力波在时间和频率范围内的能量密度,能更清晰地反映出这种非稳态信号频率随时间变化的关系.当喷油系统存在短时间存活汽泡时,其压力主波对应的频率较低,但能量密度比较大,而其压力次波具有一定量的高频率能量;当喷油系统存在长时间存活汽泡时,压力波的能量密度主要集中在主波上,除了低频率的能量外,还包含有相当一部分较高频率的能量.     

气液两相流相空间复杂网络非线性动力学特性分析

气液两相流动作为一个具有混沌特征的非线性动力学系统,其流型演化动力学特性尚未取得清楚的认识。以垂直上升管内空气-水两相流为研究对象,在实验获取气液两相流流型压差波动时间序列的基础上,将空气-水两相流的压差波动时间序列映射到流型相空间复杂网络对其非线性动力学特性进行了分析。通过分析发现在相空间不稳定周期轨的吸引特性作用下,不同流型的相空间复杂网络现呈出明显不同的网络结构,并且网络密度的演化趋势与流型的转化过程相吻合,较好地反映了垂直上升管内空气-水两相流的非线性动力学特性。     

Two-Phase Flow Patterns and Heat Transfer in Parallel Microchannels

MicroChannel heat sinks with two-phase flow can satisfy the increasing heat removal requirements of modern micro electronic devices. One of the important aspects associated with two- phase flows in microchannels is to study the bubble behavior. However, in the literature most of the reports present data of only a single channel. This does not account for flow mixing and hydrodynamic instability that occurs in parallel microchannels, connected by common inlet and outlet collectors. In the present study, experiments were performed for air- water and steam- water flow in parallel triangular microchannels with a base of 200-300μ m. The experimental study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. In air-water flow, different flow patterns were observed simultaneously in the various microchannels at a fixed values of water and gas flow rates. In steam-water flow, instability in uniformly heated microchannels was observed.     

Experimental Investigation of Two-Phase Flow Instabilities in Low-Mass-Flux Water Wall Tubes of Supercritical Circular Fluidized Bed Boilers

For the purpose of disclosing the hydrodynamic flow characteristics, under the low mass velocity conditions of the 600-MW supercritical circular fluidized beds boilers, experimental studies on instability of two-phase flow in parallel vertical internally ribbed tubes were conducted. Two kinds of oscillations, pressure-drop oscillation and density-wave oscillation, have been observed. In the range of test parameters the effects of pressure, mass flux, inlet subcooling, compressible volume, exit throttle, and asymmetric heat flux to the two-phase instability were explored and analyzed. Indications from experiment data are: To increase system pressure, mass flux and inlet subcooling will intensify the stability of water wall tubes. To increase exit throttle will intensify the instability of water wall tubes. The bounding pressure and bounding mass flux of density-wave oscillations and the bounding pressure of pressure-drop oscillation have been obtained. Based on the results of testing and using a homogeneous model, the threshold relational expressions of instability were obtained. The results may be used for the design and safe operation of parallel vertical rifled water wall tubes of supercritical circular fluidized beds boilers.     

Transient convective heat transfer of steam-water two-phase flow in a helical tube under pressure drop type oscillations

Experiments for subcooled water flow and for steam-water two-phase flow were conducted to investigate the effects of pulsation upon transient heat transfer characteristics in a closed-circulation helical-coiled tube steam generator. The non-uniform property of local heat transfer with steady flow was examined. The secondary flow and the effect of interaction between the flow oscillation and secondary flow were analyzed on basis of the experimental data. Some new phenomena were observed and explained. Correlations were proposed for average and local heat transfer coefficients both under steady and oscillatory flow conditions. The results showed that there exist considerable variations in local and peripherally time-averaged Nusselt numbers for pulsating flow. Investigations of pressure drop type oscillations and their thresholds for steam-water two-phase flow in a uniformly heated helical tube were also reported.     

Research of two-phase density wave instability in reactor core channels with rolling motion

In this study, two-phase density wave instability in parallel-twin rectangular channels was investigated with axially nonuniform heat profiles in the reactor core combined with static and rolling conditions. A parallel-channel thermal-hydraulic model was built using the method of two-phase homogeneous flow developed in previous work, while the drift-flux approach for void fraction and profile-fit model for subcooled boiling were implemented. Although the rolling condition was chosen as the typical motion in this work, the additional force caused by the motion with six degrees of freedom was derived. The theoretical analysis was performed based on the method of small power perturbation for parallel-twin rectangular channels. The flow oscillation caused by rolling was studied for different system parameters, including inlet resistance, exit resistance, pressure, and axial heating profile. The influence on flow instability of rolling parameters such as period, amplitude, and distance between channels was analyzed. The results showed that it would destabilize the system if a larger additional force was generated by rolling parameters. The influence of different axial heat profiles on flow instability was also studied under inlet-peaked, cosine-shape, and outlet-peaked heat fluxes. The coupling effect of rolling motion and axial nonuniform heating was finally studied. The stability boundaries under different conditions were compared to the inherent boundary under the static condition with uniform heating. The results indicated that the influence of nonuniform heating was more evident and should be paid more attention to.     

System instability of evaporative micro-channels

In parallel evaporative micro-channels, system instability may occur in terms of cyclical fluctuations at a long period. This is due to the co-existence of the liquid phase flow at high mass flux and the two-phase flow at a lower mass flux among different parallel channels under the same total pressure drop. For a system at constant flow rate pumping, with a pressure regulating tank and a constant heating pre-heater, alternations between these two states of boiling and non-boiling could happen with a period of minutes. This cyclical system instability has been modeled, where the liquid phase flow occurs at conditions of high inlet subcooling and low surface heat flux that the boiling inception is hard to initiate. The system instability criteria are established in terms of a system binary states parameter, S, and a non-dimensional surface heat flux. This model has been validated experimentally.     

Experimental Investigation of Forced Convective Boiling Flow Instabilities in Horizontal Helically Coiled Tubes

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Linear Analysis of the In-Phase and Out-of-Phase Stability Behavior of a Natural Circulation Reactor: Upscaling Effects

This study focuses on the stability behavior of a natural circulation chemical reactor. The reactor contains many heat-generating ethylene oxide catalyst tubes where the spaces between the tubes constitute parallel boiling channels connected between two plenums. In-phase and out-of-phase flow oscillations can occur in these channels. In this paper, these oscillations were analyzed under natural circulation conditions using a linearized frequency domain model. The results indicate that the out-of-phase mode of instability is more dominant than the in-phase mode instability. The model predictions for the in-phase mode oscillation were compared with those of the time-domain code MONA. The effect of the radial non-uniformity in the flow distribution in the parallel boiling channels on the flow stability was investigated. The model was further applied to investigate the effects of upscaling the reactor (increase in reactor area with constant power density) on the channel flow distribution and stability. It was found that with an increasing reactor diameter, the channel flow distribution is greatly affected even though the power density remains constant. This increases the out-of-phase instability of the reactor. The results are important for the design of future reactor systems.