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Highlights • Three new types of arc-winglet vortex generator (VG) are proposed. • The two types of planar arc-winglet VGs improve the secondary flow behind the tubes. • The curved arc VG can effectively guide more fluid into the region behind the tubes. • The comprehensive performance for curved arc VG is better than that of planar arc VG. Abstract Vortex generator can effectively enhance the heat transfer performance on the fin surface by introducing the secondary flow and intensifying the mixing of fluid. In this study, four new types of arc-winglet vortex generator (VG) are proposed and the three-dimensional CFD numerical simulation was employed to investigate the fluid flow and heat transfer characteristics. The results indicate that compared with the conventional rectangular-winglet VG, the two types of plane arc VGs provide a significant improvement on the size of secondary flow behind the tubes and all the wake region size of the fins with the four types of arc-winglet VGs decreased to some extent, especially for the two types of curved arc VGs. The heat transfer enhancement of the fin with curved arc VGs is mainly completed by guiding more fluid to flow into the previous wake region and then scour the downstream tubes directly. In addition, compared with the rectangular-winglet VGs, the Nusselt number averagely increases by 11.5% for the equal-perimeter arc VGs fin, 19.9% for the curved equal-area arc VGs fin, and 35.9% for the curved equal-perimeter arc VGs fin. Although the equal-perimeter arc VG and the curved equal-perimeter arc VG have the same geometry size, the Nusselt number of the fin with the former one averagely increases by 22.2% while the pressure drop only increases by 13.5%. [ABSTRACT FROM AUTHOR]

Highlights • A helical coil with reversed loops show unique pressure drop and heat transfer characteristics. • The thermal effectiveness of a common helical coil can be improved by adding reversed loops. • Adding reversed loops may enhance turbulence mixing in helical coil heat exchanger. • New friction factor and heat transfer correlations for a helical coil with reversed loops have been postulated. Abstract In this experimental study, flow and heat transfer characteristics of water through a newly designed helical coil heat transfer test section were investigated. A structural modification of the traditional helical coil has been implemented to improve its thermal effectiveness. Specifically, a 360° plastic reversed loop was added after each 180° of the main helical coil loop to redistribute the flow’s velocity profile, which should have a direct effect on the thermal performance of the helical coil heat transfer test section. Results reveal that reversed loops do enhance heat transfer rates by more than 20%, while pressure drop penalty also increases but not at the same rate. The curvature of the reversed loops with respect to the main coil is a critical factor in terms of thermal performance. Furthermore, as the size of the reversed loop decreases, the heat transfer enhancement and pressure drop penalty increment for the modified helical coil test section increase as well. Accordingly, it is suggested that there may exist an optimum helical coil structural modification for which the best tradeoff between pressure drop penalty and heat transfer enhancement can be found. [ABSTRACT FROM AUTHOR]

Longitudinal vortices generated by winglet type vortex generators mounted on fin surface improve fluid mixing with disruption of boundary layer growth, which results in convective heat transfer enhancement. The present study reports the three-dimensional numerical investigation of airflow through heated fin-tube heat exchangers in presence of common flow down configured rectangular winglet pairs (RWPs) for staggered arrangements of circular tubes. It is well known that the thermal performance of a fin-tube heat exchanger is majorly influenced by various locations of winglets corresponding to tube centre. Henceforth, fluid flow and heat transfer characteristics of different possible RWP locations concerning each tube are examined. Further, for higher performance, a search for an effective angle of attack ranging from 15° to 60° is also performed for optimized locations. The effect of different inlet flow conditions is also investigated for Reynolds number ranges from 2000 to 10,000. Thermohydraulic performance is studied by considering Nusselt number, friction factor, secondary flow intensity and thermal performance factor. Irreversibilities due to RWPs are analysed by calculating the entropy generation rate due to viscous and heat transfer effects. Obtained results illustrate that the selection of promising RWP locations plays a vital role in improving thermal performance. The relationship between heat transfer, secondary flow and irreversibility parameters are also reported. • Performance of staggered tube arrangement in presence of winglets is analysed. • Promising and counter-productive locations are examined by using JF. • Longitudinal and horseshoe vortices promote heat transfer enhancement. • Irreversibilities due to thermal and viscous effects are analysed. • Upstream located RWPs show higher thermal-hydraulic performance. [ABSTRACT FROM AUTHOR]