Investigation on the performance of fluted tube-in-tube gas cooler in transcritical CO₂ heat pump water heater

The utilization of fluted inner tube in tube-in-tube heat exchangers is an effective way to improve the heat transfer performance. Based on the segmented model, the performance of a fluted tube-in-tube gas cooler (FTGC) in the transcritical CO₂ heat pump water heater was studied in this paper. The simulation model was validated and adopted to numerically investigate the effects of different parameters on the heat transfer performance of the gas cooler, which was compared to the smooth tube-in-tube gas cooler (STGC) with the same geometric parameters. The results showed that the FTGC had higher heat transfer capacity in most conditions. The heat transfer rate first increased and then decreased as the CO₂ inlet pressure increased. The temperature difference at the pinch point in the FTGC was lower than that in the STGC. In the FTGC, the pinch point position was mainly influenced by the CO₂ inlet pressure and water inlet and outlet temperatures. Regarding the system COP, the improvement of maximum COP could be up to 29.41% by using the FTGC compared to STGC when the water inlet temperature was 10 °C and the water outlet temperature was 80 °C. However, when the water inlet temperature was 40 °C, the improvement of COP could be impaired to 12.98%. Through changing the designs of the FTGC with the fixed total heat transfer area, when the water inlet and outlet temperatures were 10 °C and 60 °C, the highest maximum COP could be obtained with the design used in the experiments.