Flow instabilities in a thimble-based heat transfer element

To understand natural circulation flow instabilities in a thimble-based heat transfer element, an experimental study was conducted on a monotube experimentation unit using different heating powers and inlet subcooling indices to analyze the unstable flow phenomena that may occur at low pressures, as well as on their driving mechanisms. The unstable region was confirmed and the geyser and the flashing instability boundary represented by dimensionless inlet subcooling and heating power were obtained. Results indicate that the geysering and flashing instability flow occurred successively, following the decrease in inlet subcooling. Compared with conventional natural circulation loops, the circulation flow of the thimble-based heat exchange element is unique in the fact that the fluid in the rising segment transfers heat to the fluid in the downcomer; therefore, the fluid temperature decreases in the rising segment and the condensation of steam bubbles becomes more serious. This effect easily causes geysering instability and delays the occurrence of flashing instability to some degree. When the flashing phenomenon occurs, the scope of the ratio between the flow oscillation period and the necessary time of fluid passing in the rising segment becomes 0.8~1.1. © 2018, Editorial Department of Journal of HEU. All right reserved.