Experimental analysis of the thermal field and heat transfer characteristics of a pebble-bed core in a high-temperature gas-cooled reactor

Identifying the locations of hot spots in a pebble-bed reactor core and analyzing the thermodynamic characteristics in the core are critical tasks. Although various numerical simulations have been conducted in this regard, experimental analyses are rarely found in the literature. Therefore, in this study, experiments are conducted with the pebbles packed in a face-centered-cubic (FCC) structure inside the test section. The local heat transfer characteristics of a pebble are analyzed, and it is found that the heat transfer varies with the location; areas with φ = 36° and 117° are the strongest heat transfer zones (φ is the circumferential angle from the z-axis to the hole), while areas with φ = 0°, 90°, and 180° are the weakest heat transfer zones. The experiments are performed under five different conditions of air inlet velocity, and the corresponding surface thermal profiles and maximum temperature differences between two pebbles are obtained. It is found that the maximum surface temperature difference between two pebbles is 15.3 °C when the Reynolds number is 1.46 × 104 and 7.8 °C when the Reynolds number is 3.30 × 104. In addition, the heat transfer coefficients and the Nusselt number are also obtained and they are correlated with the Reynolds number as hAVG = 0.03677Re0.8, Nu = 0.194Re0.8Pr0.4, respectively. These findings can not only provide a deeper understanding of the thermodynamics in a pebble-bed reactor core but also facilitate safer reactor design.