Abstract
Enhancing heat transfer in the pebble bed reactor could reduce the surface temperatures and lower the possibility of forming hot-spots. The effectiveness of inserting a smaller sphere into a structured pebble bed on optimizing the heat transfer has been confirmed, and yet, the mechanism of heat transfer enhancement is still not fully understood. The impact of the quantity and size of the small spheres on the heat transfer characteristics has been investigated in this study and the mechanism of enhancement was analyzed. It was found that: (1) When the volume or the surface area of the inserted sphere was kept the same, the overall heat transfer coefficients (HTC) of the pebble bed in case 2 or case 3 respectively demonstrated 1.4% or 2.8% higher than that of the bed in case 1; (2) the overall HTC showed an increasing trend with the decreasing ratio of the surface area to the volume; (3) the varying trends of local HTCs along the designated direction were similar among 3 cases and the strongest heat transfer positions were found near pebble-sphere contact points. Such findings will help to design a better pebble bed core.
Highlights
The pebble bed in a Pebble-bed type High Temperature Gas-cooled Reactor (HTGR) is composed of a large number of randomly stacked spherical fuel elements, and the contact between any two fuel elements may cause hot-spots appearing on the pebbles’ surfaces [1]
The volume, and the surface area of the spheres, the local and average heat transfer coefficients of the pebble bed in the middle plane and diagonal plane were studied in detail under 3 conditions (D=0.03 m of two spheres for case 1, D=0.0378 m of one sphere for case 2, and D=0.0424 m of one sphere for case 3)
It can be seen that 1) the HTC values at the diagonal planes were smaller than those at the middle planes for all 3 cases; 2) case 2 demonstrated a 1.4% higher average heat transfer coefficient than case 1 even though two cases had the same sphere volume; 3) case 3 showed an average HTC increase of 2.8% compared to case 1 when the surface area was kept the same
Summary
The pebble bed in a Pebble-bed type High Temperature Gas-cooled Reactor (HTGR) is composed of a large number of randomly stacked spherical fuel elements, and the contact between any two fuel elements may cause hot-spots appearing on the pebbles’ surfaces [1]. 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 increase the average heat transfer coefficient (HTC); besides, it was found that the larger the inserted sphere was, the stronger the enhancement would be. The mechanism of how inserted sphere(s) affected the heat transfer characteristics of the pebble beds and what was the dominant parameter were investigated in details using this validated numerical model. The small spheres were made of the material as the pebble but without internal heat generation
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