Flow reversal under low mass flow conditions is harmful to the performance of inverted U-type steam generator (UTSG) and existing research attributes the phenomenon to be flow excursion instability. In this paper, a new mechanism of flow instability that could also induce flow reversal, namely single-phase density wave oscillation (DWO), was analyzed based on numerical methods. As a primary study, the analysis was done in parallel channels consisted of two identical inverted U-type tubes. It was found that for UTSG at low mass flow conditions, the compressibility of single-phase flow would be a key factor to trigger the positive feedback of mass flow-temperature-pressure drop in DWO. Single-phase DWO should result in premature of flow reversal compared to that caused by flow excursion (FE), and analysis showed that it could occur only if the primary side temperature difference and average temperature of UTSG were both large enough, and for large-scale UTSG DWO would disappear due to the increase of total pressure drop. Parameter effects on single-phase DWO in UTSG were also carefully studied and it was found that the trends could be well explained by the variation of system compressibility.

低质量流量条件下的流动反转对倒U型蒸汽发生器（UTSG）的性能有害，现有研究将此现象归结为流动偏移不稳定。在本文中，基于数值方法分析了一种新的流动不稳定机制，该机制也可能导致流动逆转，即单相密度波振荡（DWO）。作为主要研究，分析是在由两个相同的倒 U 型管组成的平行通道中进行的。结果表明，对于低质量流量条件下的UTSG，单相流的可压缩性将是触发DWO质量流量-温度-压降正反馈的关键因素。与由流动偏移 (FE) 引起的相比，单相 DWO 应导致过早的流动逆转，分析表明，只有当UTSG的一次侧温差和平均温度都足够大时才会发生，而对于大规模的UTSG，DWO会因总压降的增加而消失。还仔细研究了参数对 UTSG 中单相 DWO 的影响，发现该趋势可以用系统压缩率的变化很好地解释。