自然循环棒束形通道流动不稳定性起始点研究

针对自然循环条件下3×3棒束形通道内流动不稳定性起始点（OFI）进行了实验和RELAP5数值模拟研究。通过对实验数据进行处理，得出了计算自然循环条件下棒束形通道内OFI对应的热流密度的经验关系式，计算的最大相对误差为20.10%。运用驱动力方法分析了OFI的产生原因，计算结果表明：棒束形通道加热段出口处因过冷沸腾产生气泡，使得自然循环冷热段密度差大幅增大，进而使总驱动力增大，最终促使了OFI的产生。RELAP5对于低压自然循环OFI计算适用性好，其对OFI的计算结果较实验结果更不保守。

Investigation on Onset of Flow Instability in Natural Circulation with Rod Bundle Channel

The onset of flow instability (OFI) in a natural circulation loop with 3×3 rod bundle channels was studied experimentally and simulated by RELAP5 code. By fitting the experimental data, a new empirical equation for calculating the heat flux of OFI of the rod bundle channels in a natural circulation was obtained with the maximum relative error of 20.10%. The OFI was analyzed by the method of driving force in order to know its occurrence mechanism. Results indicate that the subcooling boiling at the outlet of the rod bundle channels leads to the increase of the density difference between the hot and cold parts in the natural circulation loop. Consequently, it contributes to the total driving force increase and the OFI occurrence. RELAP5 has good applicability to OFI calculation under low pressure natural circulation condition, and its result is less conservative than that of the experiment.