Effects of upwind area of tube inserts on heat transfer and flow resistance characteristics of turbulent flow

Effects of reducing the upwind area of conical-strip tube inserts on heat transfer and friction factor characteristics of turbulent flow are experimentally investigated. The conical-strip insert resembles one quarter of the conical-ring insert in terms of its upwind area. Results show that the Nusselt number for the case with conical-strip inserts is only 53–56% of that for the case with conical-ring inserts, the friction factor for the former is merely 4–6% of that for the latter, and thus the thermo-hydraulic performance factor can be enhanced by 36–61% if replacing conical-ring inserts with conical-strip inserts for turbulent flow within Re range 5000–25,000. In addition, a comparative study of the short conical strip and the T-type conical strip, which can be regarded as boundary-cutting and core-cutting to the normal conical strip respectively, is performed. Results indicate that the flow resistance can be reduced by both methods. However, a weakened heat transfer and a worsened thermo-hydraulic performance are obtained for the case with boundary-cutting strip inserts. In contrast, the case with T-type core-cutting conical-strip inserts, if a suitable core-cutting size is taken, shows a better thermo-hydraulic performance as compared with the case with normal conical-strip inserts when the Reynolds number is relatively large. Effects of the pitch between neighboring strips have also been examined. It is found that smaller pitch leads to higher heat transfer rate whereas the flow resistance is increased. A moderate pitch between conical strips is beneficial to the overall thermo-hydraulic performance.