中文摘要：

对自然循环系统矩形通道内临界热流密度进行实验研究。研究发现：实验本体增加功率后,环状流液膜蒸干,壁温出现持续快速上升,实验本体出口发生沸腾临界。根据壁温的上升趋势和出口处流体的临界含汽率可以判断自然循环系统出现的临界热流密度（CHF）类型为干涸（Dryout）型。当自然循环系统沸腾临界出现时,自然循环流量出现明显的上升。根据理论分析可知：沸腾临界发生时导致自然循环流量上升的主要原因是环状流转变成弥散流,附在加热壁面的液膜消失,摩擦压降迅速减小。

英文摘要：

The critical heat flux （CHF） under natural circulation condition in rectangular channel was investigated in this paper. It was found that the dryup of a liquid layer in annular flow caused the insufficient cooling of heating wall and the rise of wall surface temperature. Then the boiling crisis occurred at the outlet of test section. Accord to the trend of the wall temperature rise and steam quality, the category of boiling crisis is considered to be dry-out. In addition when the boiling crisis occurs, the natural circulation flowrate was found to＇be increased. Based on the theoretical analysis, the sharp decreasing of the friction drop results in the rise of natural circulation flowrate when annular flow turns to dispersed flow.