【作者】 李佳；

【导师】 潘良明；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2013， 硕士

【摘要】 自然循环是无外加驱动机械驱动下的流动，它主要依靠沸腾产生的汽泡形成浮升作用，克服流动摩擦阻力和重位驱动流体流动。此过程中，系统流量与各个热工参数形成耦合，表现出强烈的流动不稳定性，其对核安全、锅炉水力系统以及热交换器等热力设备的安全、稳定运行有重要的影响。因此，对不稳定性的预测和影响因素的分析可以有效地避免系统在运行过程中出现事故。本文以水为工质，对影响自然循环过冷沸腾流动不稳定性起始点的影响因素进行了实验研究；通过对实验系统的理论分析，提出了可以预测不稳定性发生的半经验关系式及无量纲关系式，并计算分析了自然循环不稳定性发生的边界。为分析自然循环两相段内空泡份额与各个热工参数间的关系，首先对加热段内汽泡的成核规律进行了实验研究，得到了系统参数对加热段内汽化核心密度及汽泡脱离频率的影响情况。利用得到的成核规律的实验结果对自然循环流动不稳定性的机理进行了分析。发现入口过冷度和系统压力的增加均可提高系统的稳定性；随着界限热流密度的增大，密度波脉动的过冷度边界区域变大；并得到了可综合表示各参量对不稳定性影响情况的无量纲边界图。为研究系统两相段长度对不稳定性发生边界的影响，本文首先计算了稳定边界处ONB点的发生位置。结果表明热流密度和系统压力均对稳定边界处ONB点的出现位置有较大影响；本文考虑了非加热段内两相段长度对不稳定性的影响，计算得到了不同过冷度和系统压力下稳定边界处的两相段长度，其与过冷度的关系曲线形状呈开口向上的抛物线状，并发现低过冷度区增大系统压力或者高过冷度区减小系统压力均可使两相段长度变大。通过对两相微分方程的无量纲处理，本文提出了预测密度波型脉动起始条件的半经验关系式，预测误差在±10%内；通过联立求解预测关系式和系统能量守恒方程，得到了不稳定性发生边界图。本文最后对自然循环流动不稳定性的起始条件进行了理论研究，得到了可以预测流动不稳定性发生边界的无量纲关系式，并将计算得到的两相段长度与实验计算结果进行了对比，其误差在±30%以内；计算分析了各个热工参数对不稳定性边界的影响。计算得到的结果与半经验关系式预测结果及实验分析结果相符。更多还原

【Abstract】 The natural circulation refers to a common continuous flow without external drivenforce in a closed system. The buoyancy force, resulted by the bubbles generated duringthe boiling of fluid, drives the fluid flow by overcoming friction and gravity pressuredrop. In this process, due to system flow rate coupling each thermal parameters, systemflow shows the strong instability, which has an important effect on the safety andstability operation of nuclear reactor, boiler hydraulic system and other thermal devicesystems. hence, it is very important for the prediction and analysis of influent factors offlow instability. With water as the working medium, this paper has investigated thefactors which affect on the triggering points of flow instability in a natural circulationsystem at subcooled boiling conditions. According to the theoretical analysis of theexperimental system, an semi-empirical correlation and a dimensionless correlationhave been proposed which can predict the triggering conditions of density waveoscillation, and instability boundary has been calculated and analyzed.To analyse the relationship between the void fraction in the natural circulationtwo-phase length and various parameters, this paper studies the nucleation law ofbubbles in heating section in first. The influence of system parameters on nucleation sitedensity and bubble frequency is obtained.Using the experimental results of the nucleation law, this paper analyses themechanism of natural circulation instability. Study reveals that increasing inletsubcoolling and system pressure can improve the system stability. Increasing heat fluxmakes the subcooling boundary area of density wave oscillation bigger. And anon-dimension instability boundary map is obtained which shows the syntheticalinfluence of various parameters on the instability.To study the system-phase length on impact of instability boundary, this papercalculates the location of ONB points at the stability boundary. Result indicates thatboth of heat flux and system pressures have a high impact on the occurrence of ONBpoint at stability boundary. Consider the influence of two-phase length in non-heatedsection on instability, this paper evaluates the two-phase lengths at stability boundaryunder different subcooling and system pressure, which relationship with the subcoolingis a para-curve going upwards. This paper also finds that whether increasing systempressure at low subcooling region or reducing system pressure at high subcooling region may make the two-phase length larger.According to the dimensionless analysis of the two-phase flow differentialequations, an semi-empirical correlation has been proposed which can predict thetriggering conditions of density wave oscillation, the relative error of which is less than±10%. The instability boundary map are also obtained according to solving thecorrelation and system energy conservation equation.Finally, the paper has carried on the theoretical research on the onset conditions ofthe natural circulation flow instability, and obtain a dimensionless correlation whichcan predict the boundary of flow instability. The calculated two-phase length arecompared with the experimental calculation results, the relative error of which is lessthan±30%. This paper also calculated and analyzed the influence of various thermalparameters on the instability boundary. The calculation results are well consistent withthe prediction results of semi-empirical correlation and the experiment analysis results.更多还原