Cooling Heat Transfer Attributions of Supercritical CO2 in a Spiral Groove Tube Casing Heat Exchanger: A Numerical Investigation

Abstract To improve the performance of the CO2 heat pump water heater, the spiral groove tube casing heat exchanger is used as a gas cooler. At present, the flow mode of supercritical CO2 (SCO2) flowing between inner and outer tube channels is mainly adopted. However, the efficiency of the gas cooler is studied rarely when the SCO2 flows in the inner tube channel (ITC). So, the heat transfer of SCO2 in the two flow channels are studied and compared in this paper. A physical model of the cooling heat transfer of SCO2 is established for the spiral groove tube casing. The impact of SCO2 pressure, the mass flow ratios of SCO2, and water on the heat transfer attributions of SCO2 in the tube are analyzed using numerical simulation. The conclusions designate that the flow channel in the exchanger can affect the heat transfer attributions of SCO2. When the mass flow ratio of SCO2 becomes lower, the average heat transfer coefficient (h) of SCO2 flowing between inner and outer tube channels is higher, with about 2.09%. As the mass flow ratio of SCO2 rises, the average h of SCO2 flowing in the ITC is higher, with about 3.90%. Moreover, both the safety performance of the system operation and the heat transfer attributions of the working medium should be considered; the flow mode of SCO2 flowing in the ITC is recommended.