Numerical investigation on the unsteady flow and heat transfer characteristics of supercritical carbon dioxide within a vertical tube under offshore conditions

Abstract

The S-CO₂ Brayton cycle is regarded as one of the most promising energy conversion technology for ship power and other floating power plants due to its high efficiency, light weight and compact structure. Being different from land-based systems, movements of floating platforms at sea are complicated, which not only change the device position but also induces additional inertial forces. Extensive research has been conducted on the thermal–hydraulic characteristics of supercritical carbon dioxide (S-CO₂) under land-based steady-state conditions. However, despite the promising application prospects of S-CO₂ power cycles in marine vessels and floating platforms, investigations into the flow and heat transfer behavior of S-CO₂ under ocean motion conditions remain notably scarce. Therefore, a systematically comparative study on the unsteady flow and heat transfer characteristics of S-CO₂ under steady state and offshore conditions is conducted. The results indicated that under the rolling motion, the periodically changing inertial forces parallel to axial direction contribute to weakening the buoyance effect, and the periodically changing inertial forces perpendicular to axial direction induce the secondary flow which intensifies the mixing between the mainstream and near-wall fluid. These contribute to alleviating and even avoiding the heat transfer deterioration, causing a better heat transfer performance of S-CO₂. Moreover, the variation in aerodynamic and thermal parameters of S-CO₂ has no effect on the rolling motion-induced periodically changing accelerations, but it changes the density magnitude and distribution of S-CO₂, and thus the inertia force effect on the flow structure varies, causing different heat transfer and resistance characteristics of S-CO₂.