Abstract: To facilitate the IVR (in-vessel retention) of molten core debris strategy at severe reactor accident, the capacity of ERVC (external reactor vessel cooling) is a key parameter, i.e. the CHF (critical heat flux) of the external reactor vessel should be higher than the related location heat flux. In this paper, an analysis model of CHF on the downward facing curved surface for pool boiling has been proposed, which adopts the Helmholtz instability analysis of vapor-liquid interface of the vapor jets which penetrating in the thin liquid film underneath the elongated bubble adhering to the lower head outer surface. When the heat flux closing to the CHF point, the vapor-liquid interface becomes highly distorted which resulted in obvious vapor blanket, it will block liquid to feed the thin liquid film underneath the vapor blanket from the bulk region. As a result, the thin liquid film will dry out gradually. As a consequence, the CHF occurs. Based on the aforementioned mechanism and the energy balance between the thin liquid film evaporation and water feeding, and taking the subcooling of the bulk water into account, the mathematic model about the downward facing curved surface CHF has been proposed. The CHF of the downward facing curved surface for pool boiling increases along with the downward facing orientation, and subcooling has significant effects on the CHF. Comparing the result of this model with the published experimental results, it shows good agreement under the both of saturated and subcooled boiling conditions.

Key words: IVR, Helmholtz instability, liquid film evaporation, CHF

摘要： 堆内熔融物滞留（IVR）策略得以实施的关键在于压力容器下封头外部冷却（ERVC）能力，即压力容器下封头外部临界热通量（CHF）高于下封头壁面对应的热通量。通过结合Helmholtz不稳定性与液膜蒸发，提出了池沸腾下朝向曲面加热面临界热通量的分析模型。由于表面张力作用，内部嵌有汽柱的薄液膜附着在下封头壁面外，Helmholtz 不稳定性作用于薄液膜与汽柱的交界面；随着加热表面热通量的增大，汽柱与液膜之间相对速度达到一定时，在Helmholtz 不稳定性的作用下，汽液交界面产生畸变，并形成汽膜，阻碍主流液到达加热表面；当加热热通量接近CHF时，液膜逐渐蒸发直至CHF触发。通过该模型计算得到了不同过冷度下，CHF随加热曲面方位角的变化，计算结果与现有的大量实验数据一致性较好。

关键词: IVR, Helmholtz 不稳定性, 液膜蒸发, CHF