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。

Experimental Investigation of Startup Characteristics in Open Natural Circulation System

An experimental investigation on startup characteristics of open natural circulation system was performed. The results show that heating powers have significant influence on the flow characteristics evolvement during startup process. Namely, the system undergoes single-phase natural circulation, geysering oscillation and flashing oscillation under low power conditions; it undergoes single-phase natural circulation, geysering oscillation and boiling accompanied with flashing oscillation, and then develops into the steady circulation of two-phase flow under the medium power conditions, the system under the high power conditions undergoes the same evolvement process as that under the medium power conditions at beginning, and finally develops into density wave oscillation. The main reason leading to the evolution of flow characteristics during startup process is that the boiling in the heated tube continues to enhance, and the flashing phenomenon in the upflow-leg enhances at first, and then weakens, with the increase of the entrance temperature of the heated tube. The two factors interact with each other, which bring about the obvious changes of the system circulation flow rate, the phase transition positions and the void fraction. Finally, a non-dimensional flow instability map of the open natural circulation system during startup process was drawn. And empirical correlations of the instability boundaries for geysering oscillations and flashing dominated oscillations were established. The fitting results fairly well agree with experiment data.