Study on the influence of heating power on reflooding process in rectangular narrow channels

It is well known that the reflooding process involves particularly complex two-phase flow and heat transfer characteristics, making both experimental studies and the development of analytical models challenging. Over the past few decades, extensive experimental and simulation research had been conducted internationally on the reflooding characteristics in rod bundles, circular tubes, and so on. However, the flow and heat transfer characteristics of reflooding process in rectangular narrow channels still needs further investigation. The most severe challenge is whether the cooling capacity is sufficient to remove decay heat, thereby preventing the wall temperature from continuously rising, especially in the precursory cooling region where the quench front has not yet arrived. This paper focused on experimental studies of reflooding process in rectangular narrow channels. The main emphasis were on the differences in key parameters such as the quench front velocity, the wall temperature changes under different heating power. Additionally, visualization experiments were conducted to observe droplets and changes of flow patterns, which would help to understand the impact mechanism of heating power on the reflooding process. It was found that the effect of heating power was very complex. If the heating power was too low, steam generated by vaporization was insufficient to carry enough liquid to the downstream of the quench front, leading to poor precursory cooling capacity. If the heating power was too high, the overall cooling rate would be slower due to the rapid evaporation of droplets and the difficulty of droplets contacting with the wall. In the high superheat region, heat transfer coefficient increased with the increase of heating power under the same wall superheat. It should be pointed out that the convective heat transfer coefficient of single-phase steam was very small, the heat transfer coefficient was even less than 100W/m²K at the beginning of reflooding process within the parameter ranges we were studying. In addition, the visualization research results showed that Inverted annular flow rather than annular flow or annular dispersed flow was the main flow pattern in the quench front region during the reflooding process of rectangular narrow channels. And the higher the heating power, the greater the probability of droplets appearing in the precursor cooling region at the beginning of reflooding process.The findings of this study will enhance our understanding on the reflooding characteristics of rectangular narrow channels. Further experimental research especially focusing on on droplet size and motion characteristics will be conducted in the future, and on this basis, a set of numerical analysis models for the reflooding process in rectangular narrow channels will be developed.