Study on coupled heat transfer characteristics and performance optimization for a helical coil heat exchanger in the lead-cooled fast reactor

Helical coil heat exchangers (HCHEs) are extensively applied in thermal engineering, petrochemical engineering, nuclear reactors and refrigeration engineering etc. due to the remarkable benefits of high heat transfer efficiency with compact structures. The coupled heat transfer characteristics and performance optimization of HCHE are paramount for long-lasting operational safety of the miniaturized lead-cooled fast reactors (LFRs), especially using liquid lead and supercritical carbon dioxide (s-CO2) as the working mediums. The present work conducts a numerical study on the flow and heat transfer characteristics of lead and s-CO2 in HCHE under different mass flow and inlet temperature. Then the Non-dominated Sorting Genetic Algorithm-II (NSGA-II) is employed to carry out the geometry optimization with Performance Evaluation Criterion (PEC) and heat transfer area (HTA) as the objective functions. The results indicates that the coupled heat transfer performance is primarily affected by the mass flow of s-CO2, the mass flow of lead and the inlet temperature of lead. When the mass flow of s-CO2 increases by 52%, the overall heat transfer coefficient (HTC) of helically coiled tubes increases by 21%. To improve the comprehensive performance, three structural parameters of HCHE that should be preferred in the process of optimization design are tube-outer diameter, the number of layers and the number of tubes on the first layer.