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A novel airfoil fin (AFF) Printed Circuit Heat Exchanger (PCHE) with a 100 kW class heat transfer capacity was experimentally tested as a cooler in a supercritical CO2 system. Numerical analysis was also conducted to have an insight into the flow and heat transfer mechanism in the AFF channels. The results showed that the novel AFF structure has superior hydraulic performance as the pressure drop in the AFF PCHE is only about 1/6 of that in the zigzag channel PCHE with a comparative heat transfer rate. In comparison with the mass flow rate and operating pressure, the inlet temperature has relatively insignificant effect on the heat transfer rate and pressure drop in the AFF heat exchanger. The broadening of the fin has no obvious improvement in heat transfer coefficient but increases the friction factor massively, and thereby resulting in worse comprehensive performance in the fin channel. At smaller temperature and mass flow rate, the periodic fluctuations of the heat transfer coefficient and comprehensive performance criterion could be weakened significantly. In addition, the determinant effect of the effective thermal conductivity in the viscous sublayer and buffer layer on local heat transfer coefficient is also validated in AFF channels.

Supercritical CO2 (SCO2) Brayton cycle is one of the best approaches for concentrated solar power tower, with tubular solar receiver being one of the most attractive options to directly heat SCO2. The superposition of drastically variable properties of SCO2 and uneven distribution of solar irradiance flux poses a great challenge to the design and optimisation of solar receiver, thus the studies on thermal-hydraulic characteristics of SCO2 in solar receiver become crucial to deal with the challenge. In this study, the thermal-hydraulic characteristics of SCO2 in a vertical receiver tube under axially and circumferentially non-uniform heat-flux conditions are numerically studied with average heat flux ranging from 150 kW m−2 to 210 kW m−2 and mass flux ranging from 400 kg m−2 s−1 to 700 kg m−2 s−1, and the influences of buoyancy on the flow and heat transfer characteristics are evaluated. The non-uniform distribution of the axial heat flux leads to 1.2%–20.5% increase in the overall heat transfer coefficient, while more severe local heat transfer deterioration is observed. The buoyancy effect changes the flow structure, resulting in suppression of turbulence intensity and formation of thick momentum and thermal boundary layers, which is closely related to the severe heat transfer deterioration of SCO2 under non-uniform heat-flux conditions. Bo* could well predict the buoyancy effect in a vertical tube under non-uniform heat-flux conditions. Compared with upward flow, downward flow could effectively alleviate heat transfer deterioration and reduce wall temperature gradient especially for axially non-uniform heat-flux condition. The present work could provide guideline for the design and optimisation of tubular solar receiver and solar power system with SCO2 as heat transfer fluid.

The thermohydraulic characteristics of supercritical CO2 flows in a vertical tube at cooling conditions are numerically investigated, and the influence of the heat-flux condition and of the flow direction are evaluated. Constant (i.e., uniform), linearly increasing and linearly decreasing heat-flux conditions are considered as three typical heat-flux distributions over the pipe length. The simulation results show that there exists a maximum heat transfer coefficient at all heat-flux conditions when the fluid bulk temperature is slightly higher than the pseudo-critical temperature, but also that the heat-flux condition has little effect on the peak value of the heat transfer coefficient. From the viewpoint of the second law of thermodynamics, the influence of the heat-flux condition on the local entropy generation can be attributed to the distributed matching between the heat flux and the difference between the wall temperature and the fluid bulk temperature, as a better matching is associated with a higher uniformity of the local entropy generation and reduced overall irreversibilities. Upward and downward flows are considered, along with flows without gravity as a baseline case for comparison purposes, with the field synergy principle employed to explain the different phenomena in these flows. The buoyancy effect laminarises the downward flows and raises the temperature gradient; hence, the heat transfer deteriorates and the irreversibility increases. In the upward flows, the buoyancy effect augments the turbulence and alleviates the variations in temperature and velocity in the core region, consequently reducing the irreversible loss and enhancing heat transfer. The present study provides insights into the mechanisms of supercritical CO2 heat transfer characteristics as well as practical guidance on the design and optimisation of relevant components.

A printed circuit heat exchanger (PCHE) is one of the most significant components in the supercritical CO2 (SCO2) Brayton cycle system based on concentrated solar power, whose performance significantly affects the efficiency and compactness of the system. To improve the performance of PCHE with semicircular-straight channels, rib structures placed on the top flat wall of the semicircular channel are proposed, which are achieved by the double sides etched heat transfer plates, and the effects of the distributions of rib structures on the performance of channels are also investigated. The thermal-hydraulic performance of the conventional semicircular channel and the semicircular channel with different rib structures are compared. The results indicate that rib structures lead to an apparent increase of turbulence kinetic energy in channels, and improve the synergy between velocity and temperature gradient fields, leading to the heat transfer enhancement. Among the channels with different rib structures, the comprehensive performance of the semicircular channel with the spacing distribution of short rib structures (channel DR3) is the best, which is relatively 19.3–19.8% higher than that of the conventional semicircular channel. When the channel DR3 is adopted in PCHE, the effectiveness of PCHE could reach 98.4–98.7%, the efficiency of the SCO2 Brayton cycle system based on solar power could be increased by 15.3%, and the compactness of the system could be improved by 3.8%. This present work is of great significance for deepening the understanding of PCHE heat transfer mechanism and performance optimisation, as well as improving the overall performance and compactness of large-scale SCO2-based power systems.