A quantitative study on the interaction between curvature and buoyancy effects in helically coiled heat exchangers of supercritical CO2 Rankine cycles

The utilization of helical coils in supercritical CO2 Rankine cycles faces some challenges, among which an urgent one is the quantification of the interaction between curvature and buoyancy effects. In this work flow and heat transfer of supercritical CO2 in helically coiled tubes was investigated over a wide range of fluid temperature, mass flux, heat flux and coil geometries. A dimensionless buoyancy parameter, Rig,CT, was introduced to quantify those two effects on average heat transfer. When Rig, CT > 8 the curvature effect was overwhelmed by gravitational buoyancy effect and helically coiled tubes present similar performance with straight tubes. On the other hand, when Rig, CT < 5 the curvature effect dominates and helically coiled tubes show superior heat transfer characteristics than straight tubes. This superiority can be estimated by the Moni-Nakayama correlation with relative errors less than 10%. Local heat transfer was also characterized by a dimensionless number Ψ which considers both the first-order and second-order effects. Providing Ψ ≈ 1, gravitational buoyancy and curvature effects were almost the same; when Ψ < 0.1, gravitational buoyancy dominates local heat transfer; while as Ψ > 10, buoyancy has little impact and centrifugal force dominates.