Determination of Convective Heat Transfer Coefficients in the Core of the Pebble-Bed Fluoride-Salt-Cooled High Temperature Reactor Using Frequency Response Techniques

Frequency response techniques have been historically used to characterize various reactor parameters. In this paper, we apply these techniques to the Pebble-Bed Fluoride-Salt-Cooled High Temperature Reactor (PB-FHR) in order to extract experimental values for the interfacial convective heat transfer coefficient in the pebble-bed reactor core. A test section is filled with randomly packed copper spheres and a simulant fluid is passed through it. The temperature of the fluid is made to vary at a given frequency, thereby affecting the temperature of the spheres in the test section as well. Using this data the heat transfer coefficient between the surface of the spheres and the fluid can be extracted. The preliminary results from these tests are shown in this paper along with the predicted heat transfer coefficients using the Wakao correlation for heat transfer in packed beds. The experiments were carried out for a range of Reynolds numbers from 250–500 and Prandtl numbers from 20 to 50. The Wakao correlation predicts the experiment fairly well. Generally, the experimental Nusselt numbers were found to exceed the predictions by up to 32%. An analytical solution for the solid and fluid temperatures in the test section is presented, and used to show the optimal frequency of oscillation for this experimental setup. This methodology can be used in the future to better characterize the dynamic response of coolant-boundary structures in the PB-FHR.