Theoretical and experimental studies for a transcritical CO2 heat pump with spirally fluted tube gas cooler

Transcritical CO2 system has been widely used in many fields to provide heating and cooling capabilities, and optimizing the structure of the gas cooler is an effective way to improve its performance. This paper presents a theoretical and experimental study of a spirally fluted tube-in-tube heat exchanger used as a gas cooler in a transcritical CO2 heat pump. The theoretical research uses numerical simulations to investigate the flow and heat transfer characteristics. The effect of the different CO2 mass flow rates, inlet pressure, and temperature on the performance of the spirally fluted gas cooler is evaluated and analyzed. The simulation results show that the spirally fluted structure can significantly improve the heat transfer performance of the CO2 side compared to the flat tube, which indicates that the spirally fluted gas cooler could be an effective way to enhance the operating performance of the transcritical CO2 system. Then experimental research is conducted in a transcritical CO2 heat pump prototype to validate the functional performance of the spirally fluted gas cooler. The effect of the different CO2 mass flow rates, inlet pressure and temperature, and the different water inlet and outlet temperatures on the performance of the spirally fluted gas cooler in the prototype are tested. The heat exchanger efficiency - number of transfer units (epsilon-NTU) method is employed to analyze the operating performance based on the experimental data. In addition, the exergy loss method is used to quantitatively evaluate the side-effect of the large pressure drop by the spirally fluted structure. Both the theoretical and experimental results show that the spirally fluted tube gas cooler can significantly improve the performance of the transcritical CO2 system. Besides, a heat transfer and friction coefficient correlation are established based on the experimental data for future research guidance.