Heat transfer enhancement of tubes in various shapes potentially applied to CO2 heat exchangers in refrigeration systems: Review and assessment

To make carbon dioxide (CO2) heat exchangers in refrigeration systems more efficient and compact, passive methods for heat transfer enhancement produced by tubes in various shapes were surveyed and assessed. The tubes were classified into eight types by proposed classification method based on their geometrical features. The overall thermal-hydraulic performance of each type and the thermal-hydraulic performance of individual tubes in each type were elucidated using friction factor ratio, Nusselt number ratio, performance evaluation criteria (PEC) and enhancement efficiency in laminar and turbulent flow regimes. A critical friction factor ratio of 5, i.e. the friction factor of the tube with heat transfer enhancement is five times the factor of the plain tube, was introduced to assess these methods. The mechanism of heat transfer enhancement, entropy generation rate analysis, second law analysis and field synergy analysis were discussed. The twisted polygon tube, periodical convergent-divergent tube, helically corrugated tube and wavy tube suffer from high pressure drop and poor PEC. The twisted elliptical tube and eccentrical helical tube are the most suitable passive methods for enhancing heat transfer in CO2 heat exchangers due to lower pressure drop and better PEC. Vortex formation, development and thermal boundary layer disruption are associated with the mechanism of heat transfer enhancement in the twisted oval tube, twisted polygon tube, periodical convergent-divergent tube, helically corrugated tube, wavy tube and eccentrical helical tube. Entropy production rate analysis and field synergy analysis are applied in the periodical convergent-divergent tube and twisted oval tube. Heat transfer enhancement inside and outside the twisted oval tube and eccentrical helical tube under CO2 flow conditions is worth being studied in the future.