Improving supercritical CO2 cooling using conical tubes equipped with non-uniform twisted inserts

Comprehensive understanding of how flow path geometry influences heat transfer and flow characteristics is essential for designing effective heat exchange devices in supercritical Brayton cycles. This study focuses on the numerical investigation of the horizontal flow of supercritical CO2 in conical horizontal tubes (CHTs) equipped with non-uniform structures of twisted inserts (TIs). A meticulously validated numerical model, implemented using the k-omega turbulence model in ANSYS Fluent, is employed to investigate the impact of gravity and operating conditions on the performance of studied models. It is found that in areas characterized by a small tube diameter and a large twist length, the temperature disparity between the top and bottom walls is negligible, indicating a very weak buoyancy effect. Utilizing TIs in diverging CHTs can effectively enhance the heat transfer coefficient and reduce the pressure drop in comparison to uniform and converging cases. Additionally, the combination of diverging CHTs with a TI featuring high-to-low twist lengths results in a maximum percentage enhancement of 43.1% in heat transfer coefficient. Simultaneously, there is an almost 36.7% reduction in pressure drop. This model exhibits the maximum overall performance index value, with approximately a 25% enhancement compared to the reference case.