Building a clean and low-carbon new power system is a key vehicle for achieving the strategic goals of carbon peaking and carbon neutrality. Developing clean, low-carbon, high-efficient, and flexible new thermal power generation technologies has become a major strategic requirement for building a new energy system. The semi-closed supercritical carbon dioxide (S-CO₂) Brayton cycle directly heats the composite working fluid through the combustion of the fuel and the pure oxygen. Not only can it enhance the power generation efficiency of the system, but it also enables carbon capture at the same time. This study aims to investigate the unclear heat transfer and mass transfer characteristics of the CO₂/H₂O composite working fluid during the cooling and condensation processes in the heat exchanger of the semi-closed S-CO₂ Brayton cycle.

A three-dimensional numerical simulation model for the cooling, condensation and flow heat transfer of the CO₂/H₂O composite working fluid was established. This study systematically investigated the influence pattern of the mass flow rate (2×10^–4~4×10^–4 kg/s), the heat flux (–9~–14 kW/m²), and the mole fraction of the inlet water vapor (3.3%~20.0%) on the distribution of the liquid film of the condensate, the surface heat transfer coefficient, and the mass transfer rate.

The results indicate firstly that the average surface heat transfer coefficient increases with increasing mass flow rate. However, at different mass flow rates, the variation pattern of the average surface heat transfer coefficient differs as the heat flux increases. Moreover, the axial mass transfer rate exhibits a trend of increasing first and then decreasing along the flow direction of the composite working fluid. Furthermore, under low mass flow rate and high heat flux conditions, the condensate accumulates at the bottom of the circular pipe, while under high mass flow rate and low heat flux conditions, the condensate forms a ring-shaped distribution along the inner wall surface of the circular pipe. Additionally, when the mole fraction of the inlet water vapor increases from 3.3% to 20%, the average surface heat transfer coefficient increases by 20.22%. Besides, the peak value of the mass transfer rate shifts toward the inlet direction.

The results can provide theoretical support for the design of the heat exchangers in the semi-closed S-CO₂ Brayton cycle, and then contribute to improving the efficiency of the system and the performance of the carbon capture.