Single-phase density wave oscillation -A new mechanism of flow instability in inverted U-type steam generator

Highlights: • Single-phase density wave oscillation in UTSG was discovered. • Single-phase DWO could result in premature of flow reversal in UTSG. • Single-phase DWO in UTSG should only occur in a certain region. 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.