Heat transfer characteristics of supercritical carbon dioxide flowing in vertical mini square channels

In the common square channels of printed circuit heat exchangers, the unique thermophysical properties of supercritical carbon dioxide (sCO2) lead to the poor prediction effect of the existing heat transfer correlations for circular channels, and different flow directions also affect the heat transfer coefficient. To address these gaps, this work experimentally investigates heated sCO2 flow in a 1.6 × 1.6 mm2 vertical square channel. The operating ranges are system pressure p = 7.4–8.4 MPa, mass flux G = 1000–2000 kg/(m2 s), and heat flux q = 100–200 kW/m2. Based on the distribution characteristics of the heat transfer curve, the sCO2 flow heat transfer was divided into three regions: liquid-like, two-phase-like, and vapor-like. The effects of thermal–hydraulic parameters, as well as buoyancy and thermal acceleration effects, on convective heat transfer are systematically analyzed. The results indicate that the heat transfer coefficient for upward flow is higher than for downward flow, while both buoyancy and thermal acceleration have only minor influences. Existing heat transfer correlations are further evaluated, and a new heat transfer predictive correlation for vertical mini square channels on sCO2 was established. The deviation between the predicted value and experimental data is within −20% to +30%.