窄矩形通道内自然循环过冷沸腾的机理研究

The plate-type fuel assembly is widely used in the design of the advanced reactor and military reactor, and natural circulation is also the important feature of those reactor. The research background of the project is the thermal-hydraulic design of the reactor under natural circulation conditions. The mechanism of heat transfer in the narrow rectangular channels in natural circulation is studied using the combination method of theoretical analysis, numerical simulation and experimental study. At first, the preliminary mechanistic model is built based on the theoretical analysis using the existing experimental data of forced circulation and natural circulation subcooled boiling. Secondly, the experimental study for the subcooled boiling in narrow rectangular channels under natural circulation condition is carried out. For the unilateral heating experiment, the bubble generation and growth process are observed from the front and side directions using visual approach. The effects of those parameters, such as gap width, inlet subcooling degree, system pressure and heat flux, to bubble generation and growth process are analyzed. For bilaterally heating experiment, the effect of one-side heat flux to the heat transfer of the other side is also studied. And, the importance degree order of those parameters is identified by the sensitive analysis. Then the numerical analysis model is built using the computational fluid dynamics software. The boundary conditions and supplementary equations in the numeral simulation are given by the experiment. At last, the mechanistic analysis model is improved again. This study can provide a theoretical basis for the subcooled boiling in natural circulation of the nuclear reactor.

平行板燃料组件在先进反应堆及军用反应堆上有着重要用途，自然循环又是先进反应堆的重要特征。本项目以反应堆自然循环工况下的热工水力设计为应用背景，采用理论分析、数值模拟和实验研究相结合的方法研究窄矩形通道内自然循环过冷沸腾的传热机理。首先依据已有的强迫循环和自然循环过冷沸腾的实验数据，开展理论分析，建立初步的机理模型。然后进行实验研究；单面加热时采用可视化的方法从正面和侧面两个方向测量汽泡的生成和成长过程，分析窄缝空间、入口过冷度、系统压力和热流密度等参数对气泡生成和成长的影响，研究汽泡成核模型、脱离直径和频率等；双面加热时研究一侧热流密度对另一侧换热的影响；并采用敏感度分析确定这些参数的重要度次数。接着采用数值模拟分析软件建立数值计算模型，数值模拟中由实验给出所需要的边界条件和补充方程。最后根据实验研究和数据模拟结果进一步完善机理分析模型。本研究可以为反应堆的自然循环过冷沸腾提供理论基础。

本项目以自然循环工况下的反应堆热工水力设计为应用背景，目的是对自然循环情况下窄矩形通道内的过冷成核沸腾进行研究。在实验方面，进行了矩形流道中过冷成核沸腾的研究，得到了相应的计算关系式。还研究了矩形流动中自然循环情况下，流量与压力、加热量的关系，也得到相应的计算关系式。对于自然循环情况下上升的加热通道内的单相对流换热，因工质浮升力作用引起的流动层流化和传热能力减弱的现象，目前正在采用数值计算的方法进行模拟。在数值计算方面，采用了ANSYS FLUENT 14.5中的VOF两相模型，以文献中的实验作为参照实验。计算了二维条件下处于过冷度为10K的静止流体中的单气泡的成长过程。计算通过在气液交界面附近网格添加额外质量源项和能量源相，来实现两相之间的相互转化。同时还考虑了气液表面张力，气泡与壁面之间的接触角，气泡底部微层的蒸发等因素。通过计算得到了气泡在脱离前不同时刻的轮廓，并将结果与可视化实验的结果进行比较，以验证数值计算的正确性。数值模拟还得到了实验中无法直接测得的温度场与速度场分布，以及不同时刻气泡底部，过热液层中，过冷主流中的质量转化流，并可以通过理论分析的方法论证其合理性。目前，正在进行可视化实验，接下来数值模拟的结果将与的可视化实验结果进行比较，以验证理论模型和数值模拟的准确性。