Narrow rectangular channels are widely used in various fields because of their compact structure and other advantages.

This study aims to improve the prediction method of critical heat flux (CHF) in narrow rectangular channels for reactor safety and economy by conducting CHF visualization tests in narrow rectangular channels with different gap size to explore the CHF triggering mechanism.

Firstly, a high-temperature and high-pressure experimental loop with narrow rectangular channels was built, and the visualisation video and thermal-hydraulic data were collected simultaneously. It was found that the flow patterns correspond to bubble flow, slug flow, churn flow and annular flow when CHF occurs with the gap size of 5 mm, 3 mm, 2 mm and 1 mm, respectively.

Before the occurrence of CHF, bubble flow, slug flow and churn flow experience temperature fluctuations. In the annular flow, the CHF involves a gradual expansion of the area from the initial dry spot; in the churn flow, the CHF covers a smaller area; while the slug flow affected the widest area; in the bubble flow, the temperature fluctuations at the heating wall are the most frequent. Furthermore, when the system pressure is in the range of 1?4 MPa and the gap size is 1 mm, there is a non-linear relationship between the system pressure and the CHF, while in the other channels the CHF increases as the system pressure increases.

The narrow gap size has a very important effect on CHF in narrow rectangular channels, and the findings of this paper can lay the foundation for the establishment of a CHF mechanism model in narrow rectangular channel.