The case demonstrates the strength of structured meshing over regular unstructured meshes for a seemingly simple fuel pin bundle configuration. The mesh generation is complex as it needs to model micron size tolerance gaps and line contact between fuel pin & spacer wires in a domain which can be 2 meters long.

Unstructured Tetra Mesh supplied by customer (left), Structured Mesh generated my Zeus Numerix (right)

The unstructured mesh generates large size disparity and yet numbers around half million for one pitch of 7 pin bundle. On the other hand, structured mesh is 0.1 million and places mesh points clustered all around the fuel pin, spacer and hexcan wall (an essential requirement if thermal hot spot are to be captured). The mesh quality is compared.

Pressure drop on 91-pin bundle (left), Temperature distributionfor 217 pin bundle (right)

The design of fuel pin bundle is an optimization process between pressure drop and mixing efficiency. The packing of fuel pins is determined by geometric parameters. The customized tool from Zeus Numerix enabled the user to scale the problem size from 7 pins to 217 pins.