Experimental Investigation and Correlation Development of the Overall Heat Transfer Coefficient for a Multi-channel Evaporator Heated by Subcritical CO₂

<title>Abstract</title> <p> Subcritical CO₂ has considerable potential as a heat-release medium in high-temperature thermal energy storage and steam generation systems. However, the overall heat transfer behavior of subcritical CO₂ during heat release in evaporators remains insufficiently understood. In this study, an experimental investigation was conducted on a multi-channel evaporator heated by high-temperature subcritical CO₂. The CO₂ pressure was maintained at approximately 0.2 MPa, and the effects of CO₂ mass flow rate and Reynolds number on the overall heat transfer coefficient were analyzed. The reliability of the experimental data was verified using heat balance analysis and uncertainty evaluation. The heat balance ratio between the water-steam-side and the CO₂-side ranged from 0.904 to 0.964, with an average value of 0.922, and the uncertainty of the overall heat transfer coefficient was 5.62%. The results show that the overall heat transfer coefficient increased with increasing CO₂ mass flow rate and Reynolds number. When the Reynolds number increased from approximately 1.20 × 10 ⁵ to 2.05 × 10 ⁵ , the overall heat transfer coefficient increased from 132.52 to 151.49 W/(m²·K). Based on the experimental data, three empirical correlations were developed and compared. The recommended correlation, , achieved an <italic>R</italic> ² of 0.9682, a MAPE of 0.5713%, an RMSE of 1.0354 W/(m²·K), and a maximum relative deviation of 2.7072%. The proposed correlation provides a useful reference for the design and performance evaluation of subcritical CO₂ evaporators in heat-release and steam generation applications. </p>