Thermal-hydraulic performance of heat transfer tubes with different combinations of dimples and protrusions

Enhancing tubular heat exchanger efficiency to improve dryer cylinder exhaust waste heat recovery is critical for reducing paper mill energy consumption. Given heat transfer tube surface area constraints, this work proposes boosting heat transfer via combined dimples and protrusions. Steady-state numerical simulations at Reynolds numbers ( Re ) ranging from 5,000 to 20,000 are performed to analyze thermal-hydraulic performance differences among three tubes: dimple-only tube (SDT), dimple-protrusion tube (DPT), and dimple-dimple tube (DDT). The results indicate that the combination of dimples and protrusions can effectively disturb the flow and reduce the thickness of the boundary layer. Among them, DDT exhibits superior performance: compared with SDT and DPT, its maximum heat transfer performance is improved by 19% and 9%, respectively, while its maximum thermal-hydraulic performance is enhanced by 11.2% and 6%, respectively. Additionally, for DDT with varying connection widths ( W1 =2-4 mm) and depths ( H1 =0.5-1.5 mm), a smaller W1 better enhances thermal-hydraulic performance at high Re , while a smaller H1 works better at low Re . This study confirms the feasibility of DDT in enhancing thermal-hydraulic performance further and identifies the optimization directions for its structural parameters.