Numerical study on the influence of input power and tube shape on a single-phase natural circulation for passive residual heat removal heat exchanger

The passive residual heat removal heat exchanger (PRHR HX) plays an important role in cooling the reactor core under accident conditions. However, few studies have considered the exact conditions of the natural circulation loop (NCL) for the PRHR HX, such as different initial temperatures of fluid in the NCL and the cooler, and configuration of the heat exchanger tubes. In this study, analyses with these considerations are conducted. Influences of different heating power inputs and the two configurations (spiral-shaped and C-shaped heat exchanger tubes) on heat transfer performance are discussed. The CFD computational setting was qualitatively verified and transient computations of 600 s were conducted. The computational results suggest that the oscillation of the first 40 s is associated with difference of the initial temperatures of the fluid in the NCL and the cooler. It is found that the frequency increases first and then the amplitude with increase of heating power. The system reaches a neutrally-stable condition with heating power between 500 W and 750 W, and then it reaches a neutrally-stable condition earlier with an increasing heating power, which also results in a higher frequency and a larger amplitude. In addition, the loop flow rate is positively correlated with the average convective heat transfer coefficient. On the other hand, replacing a C-shaped tube with a spiral-shaped tube insignificantly affects the flow oscillation in the loop, and the more complex heat exchanger tube structure could enhance its heat transfer performance. The results of the study reveal the characteristics of the NCL for the PRHR HX.