CFD investigation of enhanced heat transfer using periodically spherical-expanded tube

Passive heat transfer enhancement can improve heat exchanger performance without additional energy consumption. This study numerically investigates the heat transfer, flow characteristics, and thermal-hydraulic performance of a periodically spherical-expanded tube under turbulent flow conditions. Three-dimensional CFD simulations were performed using the realizable k–ε turbulence model with enhanced wall treatment, with air as the working fluid and a uniform wall heat flux of 1000 W m −2 . Reynolds numbers of 10000–20000, sphere diameters of 25–35 mm, and pitches of 50–80 mm were investigated. The spherical expansions promote flow recirculation and thermal boundary-layer redevelopment, thereby enhancing convective heat transfer. The average Nusselt number increases from approximately 44–47 at Reynolds number of 10000 to 67–73 at Reynolds number of 20000. Increasing the sphere diameter from 25 to 35 mm enhances the average Nusselt number by approximately 5–8%, while decreasing the pitch from 80 to 50 mm provides an enhancement of approximately 6–10%. The thermal-hydraulic performance factor remains greater than unity for all investigated configurations. A sphere diameter of 30 mm provides the highest performance among the investigated diameters, with a maximum factor of approximately 1.33, while a pitch of 50 mm provides the highest performance among the investigated pitches. The main limitations are the assumption of constant fluid properties and the lack of direct experimental validation of the proposed geometry. Overall, the results demonstrate the potential of periodically spherical-expanded tubes for passive heat transfer enhancement.