A review on single-phase convective heat transfer enhancement based on multi-longitudinal vortices in heat exchanger tubes

In this work, a review on single-phase convective heat transfer enhancement based on multi-longitudinal vortices is carried out. Theoretical investigations on convective heat transfer optimization from different principles such as entropy generation minimization principle, field synergy principle, entransy dissipation extremum principle, power consumption minimization principle, and exergy destruction minimization principle for the better trade-off between heat transfer augmentation and flow resistance reduction are firstly evaluated. It is found that the optimal flow fields are mainly characterized by multi-longitudinal vortices, implying that heat transfer enhancement techniques which can generate the flow patterns similar to the optimal flow fields may also enjoy the satisfactory balance between heat transfer enhancement and flow resistance reduction. Then, various techniques such as artificial roughness, special-shaped tubes, multiple swirl devices, and longitudinal vortex generators that can construct the flow pattern of multi-longitudinal vortices are summarized. Results indicate that most of the techniques show excellent thermal-hydraulic performance, but some techniques still suffer from high flow resistance. Based on the discussion, some new perspectives on the existing research gaps, challenging, and future research directions have been provided for the development of enhanced heat transfer techniques by generating multi-longitudinal vortices in heat exchanger tubes.