Novel designs and performance appraisal of non-uniformly heated annuli for heat transfer augmentation with supercritical carbon dioxide

Abstract

Deterioration of heat transfer and possible appearance of high temperatures are common concerns in supercritical flow channels, inspiring several innovating modifications in traditional tubes. Despite the relevance of annular geometry and rod bundles in high-power applications, similar efforts are quite rare in configurations beyond the conventional channels. Present study aims at addressing this research gap by proposing and analyzing two novel designs of supercritical flow channel with variable flow areas. Annular channels with continually increasing or decreasing heater diameter within a shell, accordingly allowing axial variation in heat flux based on the local thermal conditions, facilitate converging or diverging flow paths. Five different taper angles are contemplated for either of the geometries, and numerical simulations are performed to envisage their relative performances during energy addition with supercritical carbon dioxide as the working medium. Both the designs are able to demonstrate marked improvements in overall thermalhydraulic response, with the converging duct being identified as the more preferable choice. Local heat transfer coefficient at the channel exit can be as much as 126% greater for a converging channel compared to a plain annular one, without any significant rise in pressure losses. A converging channel is also able to maintain lower temperature levels throughout the flow path, and the effects are more prominent with greater tapering. Diverging channel also records considerable gain in heat transfer, but may experience higher local temperatures and augmented pressure losses. Buoyancy effect is found to be dominant within short entrance region, while flow acceleration governs the extent of interactions in later segments. Converging duct is able to sustain temperature close to pseudocritical value within the buffer region, which can be a primary reason for its superiority. The disparity between both configurations are less significant at lower flow rates and higher powers, and the use of diverging channel can be recommended only for large power-to-flow-rate ratios, which can limit temperature ranges better than converging channel only for such specific cases.