Flow and temperature developments in a wire-wrapped fuel pin bundle of sodium cooled fast reactor during low flow conditions

Laminar flow of sodium in the entrance region of a seven pin nuclear fuel bundle with helically wound spacer wires is numerically investigated. Pin diameter, pin pitch, helical wire pitch and Reynolds number are varied as parameters. Evolutions of cross-stream velocity, friction factor and Nusselt number are critically studied. Nusselt number and friction factor exhibit periodic spatial oscillations in the entrance region, contrary to monotonic reduction observed in straight ducts. For identical Reynolds number, the friction factor exhibits a strong dependence on helical pitch of wire wrap where it increases as the pitch shortens. On the contrary, the Nusselt number does not exhibit such a strong dependence on helical pitch. A strong non-zero cross-stream velocity prevails in the fully developed region, contrary to that observed in straight channels. The mean value of non-dimensional cross-stream velocity is found to scale as πd/H, where, d is the pin diameter and H is the helical pitch. The friction factor and cross-stream velocity are relatively high in tight pin bundles. Based on parametric studies, a correlation is proposed for fully developed friction factor in pin bundle.