Flow and heat transfer investigation of supercritical carbon dioxide in a novel biomimetic honeycomb fractal gas cooler of transcritical CO₂ heat pumps

Flow and heat transfer characteristics of gas coolers, one of the most significant components in transcritical CO₂ heat pumps, significantly influence the overall performance of the heat pumps. To ensure high heat transfer efficiency and increased structural stability of gas coolers, a novel biomimetic honeycomb fractal heat exchanger is proposed in this study. The conjugate heat transfer between supercritical CO₂ and water in this novel heat exchanger is then investigated by numerical methods under different operating and geometric parameters. The results indicated that the heat transfer coefficients of CO₂ were quite different in different flow channels and different locations in each flow channel, especially near the supercritical region, which was significantly influenced by the buoyancy effect. Based on the thermophysical parameters, buoyancy, and hydraulic diameter, a new correlation for supercritical CO₂ was developed. The relative errors between the data calculated by the correlation and the numerical results were mostly in the range of ±15 % and the root mean square deviation of the ratio of the calculated data to the numerical results for the Nusselt number was 5.4 %.