Experimental study on heat transfer characteristics of subcritical carbon dioxide in a vertical tube

Abstract

Research is actively being conducted globally to investigate the supercritical carbon dioxide (S-CO2) Brayton cycle which is considered a highly promising energy conversion system. During startup, load change, or shutdown processes, the system experiences subcritical pressure conditions where the phase change of carbon dioxide (CO2) may occur. Understanding these heat transfer characteristics becomes particularly crucial as the system transitions through subcritical regions, where abrupt property variations and phase change effects could significantly impact operational stability and safety. To address these challenges, the heat transfer properties of subcritical CO2 in a vertically arranged tube with a heated length of 750 mm and a diameter of 4 mm were investigated in this work. The effects of heat flux, pressure, and mass flux on the heat transfer characteristics were analyzed. Results indicated that with increased heat flux, heat transfer deterioration (HTD) occurs, leading to a peaky wall temperature. Increasing pressure reduces the surface tension of CO2 leading to earlier HTD. Additionally, higher mass flux slightly improves heat transfer coefficient under saturated boiling. Four correlations were selected to estimate the accuracy of predicting the heat transfer coefficient by the test result. The Fang correlation (Fang, 2013) was shown to exhibit the most accurateness than other correlations considered with average deviation and standard deviation of 2.42 % and 43.98 %, respectively. Improved predictions in the heat transfer coefficient were achieved by fitting a new coefficient based on the Fang correlation The critical heat flux to trigger HTD was obtained, and an empirical correlation was proposed.