Experimental study on convective heat transfer characteristics of supercritical carbon dioxide in a vertical tube with low mass flux

Abstract

A thorough understanding of the heat transfer behaviour of supercritical CO2 (sCO2) flowing through channels is crucial for designing safe and effective heat exchangers in sCO2 power cycles. Existing findings on convective heat transfer of sCO2 in tubes with low mass flux have not reached a consistent conclusion, further research on the complex mechanism of convective heat transfer of sCO2 is still needed. This study experimentally investigates the convective heat transfer of sCO2 as it flows upward and downward in a 10 mm diameter tube with pressures of 7.4–10 MPa, mass fluxes of 100–350 kg/(m2 s), and heat fluxes of 10–130 kW/m2. The effects of heat flux, mass flux, pressure and flow orientation on heat transfer have been examined. The results show that a rising heat flux impairs heat transfer, and a wall temperature valley appears in the heat transfer enhancement (HTE) region at high heat flux for both upward and downward flow. An increase in mass flux enhances heat transfer, while an increase in pressure harms heat transfer. Downward flow exhibits better heat transfer performance compared to upward flow under the same conditions. The mechanism of the wall temperature valley in the HTE region was analyzed based on the velocity, turbulent kinetic energy and thermophysical property distribution of sCO2 in the tube, attributing to the effects of buoyancy and specific heat. In addition, new heat transfer correlations were proposed for the upward and downward sCO2 flow with similar heat transfer behaviour at low mass flux, with over 90 % of the predicted heat transfer coefficients falling within a relative error range of ±20 %.