Thermal characteristic of a laminar flow through a circular tube fitted with wire-net cylinder inserts

A novel type of porous media inserts, wire-net cylinder inserts, was developed to improve convection heat transfer in the present work. Heat transfer performance and pressure drop was numerically studied. Wire-net cylinder inserts with different spacer length (S/D = 2.1, S/D = 4.2, S/D = 8.3) and different open area rate (OAR = 0.33, 0.5 and 0.67) were investigated at the Reynolds number range of 290-2024. Liquid water was used as the working fluid. The results show that wire-net cylinder inserts have an excellent heat transfer performance in the laminar regime. This was mainly due to the increase in the velocity at the inlet and the high intensity of the fluid mixing between annular region and the core region. The open area rate has a great impact on the heat transfer rate. The best heat transfer rate resulting from inserts with P = 0.2 mm (OAR = 0.5) is about 3.45 times that of smooth tube. The friction factor increases with the decrease in open area rate. Although inserts with P = 0.4 mm (OAR = 0.67) have the lowest heat transfer rate, its friction factor is also the lowest. As a result, inserts with P = 0.4 mm achieve the best PEC values (performance evaluation criterion). Heat transfer performance of wire-net cylinder inserts is also affected by the spacer length. The heat transfer rate increases with the decrease in the spacer length. Although a short spacer length can result in a high friction factor, the PEC values are still very high. It is still valuable to adopt a short spacer length in the design of heat exchanger. Compared with other inserts, wire-net cylinder inserts are simple and can be fabricated easily at a low cost.