Numerical investigation on the performance characteristics of a novel biomimetic honeycomb fractal gas cooler of transcritical CO₂ heat pump

In order to improve the heat transfer performance of the gas cooler in transcritical CO₂ heat pump system, a novel biomimetic honeycomb fractal heat exchanger, with the advantages of great heat transfer efficiency and high compactness, is proposed for the transcritical CO₂ heat pump in this study. Also, the conjugate heat transfer performance between supercritical CO₂ and water in the novel gas cooler is investigated by the computational fluid dynamics method. The results firstly indicate that the difference of peak heat transfer coefficient in various flow channels reaches to 20.4% when the inlet flow flux of CO₂ is 30 kg/(m²·s), significantly influenced by the uneven distribution of density and the secondary flow caused by buoyancy force. Secondly, when the flow flux of CO₂ is 30 kg/(m²·s), compared with the horizontal flow, the heat transfer coefficients decrease by 11.4% and 40.7%, respectively, as CO₂ flows vertically upward and downward. Finally, the heat transfer coefficient and comprehensive performance of the biomimetic honeycomb fractal heat exchanger are 144.6% and 20.4% higher than those of the printed circuit heat exchanger. The comprehensive performance of the 2^nd and 3^rd level biomimetic honeycomb fractal heat exchangers are 59.7% and 92.4% higher than that of the 1^st level biomimetic honeycomb fractal heat exchanger.