Enhanced heat transfer and reduced pressure loss with U-pattern of helical wire spacer arrangement for liquid metal cooled-fast reactor fuel assembly

Abstract

This paper proposes the new pattern of wire wrap spacer, called U-pattern to enhance coolant mixing and reduce pressure drop for liquid metal cooled reactor fuel assembly. It is designed by designating its winding direction and pin arrangement without any additional structures for fuel assembly. To evaluate its thermal-hydraulic performance, both experimental and numerical approaches are performed; flow visualization experiments using a combined particle image velocimetry (PIV) and matching index-of-refraction (MIR) technique and CFD simulation. The visualized flow field in both axial and lateral planes shows that U-pattern wire wrap spacer has a stronger swirl flow in center subchannel compared to the conventional case. To extend the application ranges to the normal LMFR operation condition, CFD analysis is conducted using ANSYS-CFX. Simulation results indicate 8.60 °C decrease of peak cladding temperature and 5.2% reduced pressure drop respectively. Rod bundle with U-pattern wire spacer has a similar range of bundle loss coefficient at Re < 8500, and a smaller than the conventional one at Re > 10,000. Measurement and calculation results for the U-pattern exhibit that flow is re-distributed to have flatter temperature profile and larger flow channel with the removal of the wire wrap spacer at the center pin brings a lower flow resistance. Therefore, the U-pattern wire wrap spacer for LMFR can enhance the thermal margin and improve economics by promoting thermal mixing and reduction in pressure loss.