Abstract Post critical heat flux (post-CHF) heat transfer may occur during loss of coolant accidents (LOCAs) in water-cooled nuclear reactors when makeup water quenches the uncovered portion of fuel rods in the reactor cores. Post-CHF flow regimes include inverted annular film boiling (IAFB), dispersed flow film boiling (DFFB), and a transition regime between the two, i.e., inverted slug film boiling (ISFB) for low mass fluxes or agitated inverted annular film boiling (AIAFB) for high mass fluxes. Flow regime transition is important for modeling heat transfer characteristics in the post-CHF flow regimes. Breakup of the liquid core in the IAFB regime represents the flow regime transition from the IAFB to ISFB/AIAFB regimes. This paper first presents a literature review of previous experimental and theoretical studies of the mechanism of breakup of the liquid core in the IAFB regime and flow regime transition in the post-CHF regimes. To physically explain the breakup of the liquid core in the IAFB regime, the Weber number is calculated from the experimental data obtained from steady-state post-CHF experiments at subcooled and low-quality conditions and a critical Weber number is obtained to describe the flow regime transition from the IAFB to ISFB/AIAFB regimes. Parametric effects of the inlet subcooling, mass flux, and system pressure on the critical Weber number are then studied, based on which a new correlation for the critical Weber number is proposed to predict the flow regime transition from the IAFB to ISFB/AIAFB regimes. Due to the different flow patterns in the transition regime, i.e., ISFB and AIAFB respectively for low and high mass fluxes, the heat transfer characteristics show different trends. The indication of the flow regime transition from the IAFB to ISFB/AIAFB regimes by the critical Weber number criterion matches well with the trend of the wall heat transfer coefficient.

中文翻译：

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过冷和低质量条件下后 CHF 流态中的流态转变

摘要 在水冷核反应堆的冷却剂损失事故 (LOCA) 期间，当补充水淬火反应堆堆芯中未覆盖的燃料棒部分时，可能会发生临界后热通量 (post-CHF) 热传递。后 CHF 流动状态包括倒环膜沸腾 (IAFB)、分散流膜沸腾 (DFFB) 和两者之间的过渡状态，即低质量通量的倒段塞膜沸腾 (ISFB) 或搅拌倒环膜沸腾。 AIAFB) 用于高质量通量。流态转换对于模拟后 CHF 流态中的传热特性很重要。在 IFB 状态下液芯的破裂代表了从 IFB 到 ISFB/AIAFB 状态的流态转变。本文首先介绍了以往关于 IAFB 状态下液芯破裂机制和后 CHF 状态下流态转变机制的实验和理论研究的文献综述。为了从物理上解释 IAFB 体系中液核的分解，韦伯数是根据在过冷和低质量条件下的稳态后 CHF 实验中获得的实验数据计算得出的，并获得了一个临界韦伯数来描述流动体制从 IAFB 过渡到 ISFB/AIAFB 体制。然后研究了入口过冷度、质量通量和系统压力对临界韦伯数的参数影响，在此基础上提出了临界韦伯数的新相关性，以预测从 IFB 到 ISFB/AIAFB 流态的流态转变。由于过渡区的不同流动模式，即对于低质量通量和高质量通量分别是 ISFB 和 AIAFB，传热特性表现出不同的趋势。通过临界韦伯数准则表明从 IFB 到 ISFB/AIAFB 流态转变的指示与壁传热系数的趋势很好地匹配。