Thermo-hydraulic characteristics of high-temperature molten salt microchannel heat exchangers

<bold>Background</bold>The small modular molten salt reactor (MSR) has broad developing prospects. However, due to the limitation of space, heat transfer efficiency and high temperature working environment, extreme performance of molten salt heat exchanger is required.<bold>Purpose</bold>This study aims to investigate the flow and heat transfer characteristics of various microchannel heat transfer structures for MSR in high-temperature environments.<bold>Methods</bold>The numerical simulations were conducted to analyze the thermo-hydraulic characteristics of a molten salt (FNaBe) - supercritical carbon dioxide (SCO₂) heat exchanger. The zigzag and wavy channel structures were applied to the FNaBe molten salt side, while the elliptical-fin and airfoil-fin structures were utilized on the SCO₂ side. The global and local flow and heat transfer characteristics of these four channel structures were examined and compared.<bold>Results</bold>Simulation results show that the zigzag channel has better heat transfer performance at the molten salt side whilst the wavy channel has better hydraulic performance. When the inlet mass flow rate is less than 0.2 kg·s^-1, the overall hydraulic and heat transfer performance of the zigzag channel is better. When the mass flow rate is greater than 0.2 kg·s^-1, the wavy channel performs better. The elliptical-fin channel on the SCO₂ side has better heat transfer and overall performance whilst the airfoil-fin channel has better hydraulic performance. Based on those simulation data, the heat transfer and flow empirical correlations, i.e., fitting relationship between Nusselt number and friction factor, of zigzag and wavy channels on the molten salt side, elliptical and airfoil-fin channels on the SCO₂ side are derived respectively.<bold>Conclusions</bold>The findings of this study provide valuable insights into the design of microchannel heat exchangers for small modular reactors. The proposed empirical correlations contribute to the optimization of heat exchanger performance, enhancing the efficiency and applicability of molten salt heat exchangers in high-temperature environments.