INTEGRATED STUDY OF HEAT TRANSFER AND LOCAL GAS DYNAMICS IN A PEBBLE BED VERY HIGH TEMPERATURE NUCLEAR REACTOR USING ADVANCE MEASUREMENT TECHNIQUES

Abstract

The aim of this study, pebbles of diameter of 5 𝑐𝑐𝑐𝑐 and a pebble-to-diameter ratio (aspect ratio) of 6 were used. In part I, the local gas velocities were studied at different radial and axial locations in the pebble bed using a sophisticated hot wire anemometry (HWA) technique, which was supported with a novel probe-protector case to protect the fragile probe, allowing the measurements to be taken between the pebbles for the first time. The local gas velocities were found to be significantly higher in the near wall region due to the higher void fractions in the near-wall region, compared to the center. In the part II, the local convective heat transfer coefficients were studied using a cartridge heated probe pebble, a microfoil heat flux sensor and a thermocouple at various radial locations, axial locations, and angular orientations and under different superficial inlet gas velocities that correspond to both laminar and turbulent flow conditions. The obtained results showed that the local heat transfer coefficients are higher in the near-wall region compared to the center of the bed, which is the same pattern as that observed for the local gas velocities. In part III, new heat transfer coefficients measurements were conducted by changing the orientation of the probe pebble and the location of the thermocouple, accordingly. This allowed us to confirm the reproducibility of the local heat transfer measurements using our measurement technique as well as to confirm the patterns of the variation of the local heat transfer coefficients along the diameter of the pebble bed. Data from all the parts of this study were compared with correlations from the literature and new correlations were developed for the predictions of the local gas velocity and local heat transfer coefficients within the experimental range of our study.