CFD study and PCHE design for secondary heat exchangers with FLiNaK-Helium for SmAHTR

• CFD study was carried out for SHX with FLiNaK-Helium working fluids for AHTR.
• PCHE designs with various banking configurations were considered for the SHX.
• Heat exchanger size, pressure drop, and cost were considered for the PCHE decision.

Printed Circuit Heat Exchangers (PCHEs) were selected from a variety of heat exchanger candidates as a starting point for a Secondary Heat Exchanger (SHX) design with FLiNaK-Helium working fluids in Advanced High Temperature Reactors (AHTRs). Since experimental data are not currently available for the PCHEs with FLiNaK-Helium, a numerical study using a Computational Fluid Dynamics (CFD) code was performed to investigate thermal-hydraulic performance on the PCHE under the expected SHX condition. A local pitch-averaged method was used to develop local dimensionless parameters from CFD results. The Fanning factor and Nusselt number provided from the current CFD study were compared to the correlations that were previously developed for several different working fluids. A large discrepancy and unusual behavior were observed for the Fanning factor multiplied by Reynolds number on the FLiNaK side. Based on our suggested design procedure, a cost analysis was performed to combine pros and cons of the heat exchanger size and the associated pressure drop. Finally, an appropriate Reynolds number operating range where the PCHE is to meet the design requirements was suggested for the SHXs with different banking configurations. Furthermore, considering the heat exchanger size, pressure drop, and cost comprehensively, we proposed a reasonable SHX design for the 125-MWth SmAHTR.