Effect of pebble diameters on the heat transfer characteristics of a structured pebble bed in an HTGR

Investigation on the mechanism of enhancing heat transfer in a pebble bed by adding in the non-fixed number of smaller-sized spheres

“…However, thus far, the research on the heat transfer characteristics of beds packed with multi-sized spheres has barely been conducted. In our previous study, Chen and Lee [4,7] have analyzed the impact of the diameters of the pebble and the inserted sphere on the heat transfer characteristics of a face-centeredcubic (FCC) structured pebble bed and concluded that compared to the pebble bed without a small sphere, placing a sphere in the bed could reduce the surface temperature of certain pebbles and therefore increase the average heat transfer coefficient (HTC); besides, it was found that the larger the inserted sphere was, the stronger the enhancement would be. Although the reason for this heat transfer enhancement was partly described as the expansion of effective heat convection area caused by the inserted sphere contacting the adjacent pebbles, still, more and deeper mechanisms need to be discovered in order to apply this approach to a real pebble bed reactor.…”

“…By applying 5 boundary layers on both sides of the fluid-pebble interfaces, the thermal field can be analyzed more accurately. The total numbers of nodes and elements are around 3 million and 14.79 million, respectively, and the meshindependence test was already done elsewhere [4].…”

“…Once the temperature of hot spots gets too high, the structural integrity of the fuel elements may get affected, which may result in a break-apart of the spheres and lead to a severe accident like a release of fission products [2]. To reduce the possibility of hotspots appearance, researchers have studied the heat transfer characteristics of different pebble beds [3][4][5]. Moreover, Forsberg and Peterson [6] proposed that packing a bed with multi-sized pebbles rather than mono-sized pebbles could help the reactor reach a higher power density and also enhance the heat transfer of the bed.…”

Numerical Study on the Thermal Field and Heat Transfer Characteristics of a Hexagonal-Close-Packed Pebble Bed

“…Such correlation is plotted in Figure 12. Furthermore, a correlation of the average HTC of the FCC-structured bed is plotted as the dotted line [8] in Figure 11 to compare with the heat transfer characteristics in the HCP pebble bed. It was found that the average HTC of the FCC pebble bed was higher.…”

“…The spherical fuel elements in a high-temperature gas-cooled reactor (HTGR) core are randomly packed [1], resulting in uneven heat transfer on the surface of the pebbles and hotspots being formed [2], which will affect the integrity of the fuel spheres and may cause serious accidents [3,4]. The formation of local hotspots is closely related to the porosity [5][6][7], the diameter of the pebble [8][9][10], and the flow state of the coolant in the pebble bed [11,12]. Analyzing the flow patterns, temperature distribution, and heat transfer efficiency in the pebble bed is of great significance to the design of high-temperature gas-cooled reactors and the safety of the reactor.…”