特定扰动下蒸汽发生器两相流热力型不稳定性研究

Load rejection is a special accident condition occurred during operation of the steam generator, moreover it is a significant and specific turbulence that consequentially influences thermal instability for two-phase flow in a steam generator. Therefore we investigate the impacts of the load rejection on the flow instability, the thermal oscillation and the alternating thermal stress in order to built the theoretical foundation for preventing the damages of steam generator tubes. The thermal instability for vapor-liquid two-phase flow within the steam generators during load rejection is revealed, which is considered as the starting point of the subject. By using the comprehensive experimental equipments at high temperature and pressure developed by our group, experimental study of load rejection is performed. The disturbance characteristics resulted from different ways of load rejection are planned to obtained. In addition, the impacts of load rejection that are subjected to different methods of doing work on thermal instability for two-phase flow and alternating thermal stress is to be revealed. Then, a function is fitted through experimental data and the disturbance boundary conditions for load rejection are modeled. Mathematical model of two-phase flow that can deal with the disturbance of load rejection is to be established to carry out numerical simulations. A quantitative method portraying flow pattern is proposed through image processing technology, which can reveal the influence of flow pattern on two-phase flow instability and the alternating thermal stress. Ultimately, the predictive model and method of two-phase flow thermal instability and alternating thermal stress on the steam generator tubes ought to be proposed.

甩负荷是发生在蒸汽发生器运行过程中的特殊事故工况，同时也是一种重要的特定扰动，该扰动必然会影响到蒸汽发生器内汽液两相流不稳定性。本项目研究甩负荷扰动对蒸汽发生器内两相流不稳定性、热力型脉动及交变热应力的影响规律，进而为防止蒸汽发生器传热管破损提供理论保障。 本课题以揭示甩负荷引起的特定扰动下蒸汽发生器汽液两相流热力型不稳定性为切入点，利用课题组开发的蒸汽发生器高温高压综合实验台进行蒸汽发生器甩负荷扰动的实验研究，得到不同甩负荷方式造成的扰动特征，进而揭示不同做功方式引起的甩负荷扰动下汽液两相流热力型不稳定性及交变热应力规律；引入基于实验数据拟合的函数，确立甩负荷扰动边界，得到表征甩负荷扰动的两相流数学模型而进行数值研究；结合图像处理技术提出刻画流型的量化方法，研究流型对两相流热力型不稳定性及交变热应力的影响。最终综合提出甩负荷扰动下汽液两相流热力型不稳定性及传热管交变热应力预测模型与方法。

甩负荷扰动是蒸汽发生器运行过程中一种重要的特定扰动，该扰动必然引发蒸汽发生器内的汽液两相流不稳定性。针对这一问题进行了如下工作：.实验方面，搭建流型可视化实验平台并自主研发全自动图像处理系统，开展气泡群图像灰度与体积含气率的影响因素研究。发现空气的体积流量范围为20.9-27.93 L/min时，灰度值-时间曲线和体积流量-时间曲线之间有较好的规律性，两者呈中等相关并且灰度值随高度分层。实验所得结果为气液两相流结构和流型的刻画提供了一定的基础。.数值模拟方面，首先基于漂移流理论建立蒸汽发生器流动不稳定性的动态模型，对比有无缓冲罐对传热管内两相流不稳定性的影响。发现对于无缓冲罐的系统，运行参数发生不同幅度扰动时，预热段高度、压力等发生周期为10 s的脉动；对于有缓冲罐的系统，传热管压降的响应时间减少5.3 s，振幅降低0.95 kPa，截面含汽率及质量含汽率的响应时间分别减少14 s和12.5 s。.然后，在前述模型基础上考虑并联管间的流量脉动，得到了并联管间流量漂移规律。发现当起始工作点处于不稳定区域且系统受到小扰动时，管内流量分配从均匀变为极度不均匀，分流比可达0.971，且有一定的延时性，延迟时间为0.3 s。而当工作点从稳定区向不稳定区线性变化时，变化路径与扰动线性变化的速度有关：变化时间为0.5 s时，压力和流量变化范围较大，流量漂移现象发生更早；变化时间为5 s时，扰动速度变慢，工作点提前发生流量漂移。.最后，基于两相流动与换热以及弹性力学基础建立特定扰动下传热管交变热应力的数学模型，得到了传热管交变热应力的分布。发现传热管在温度载荷作用下变形较小，在压力载荷作用下向第三象限偏移。机械应力与总应力关于中心截面对称分布且沿高度方向先增加后减小。沿圆周方向，机械应力与热应力关于35°与215°所连的直线对称分布。.本项目的研究对蒸汽发生器的安全、稳定和高效运行具有重要的指导价值，为汽液两相流不稳定性机理的深入挖掘提供了新的技术支撑。