Concentric tube heat exchanger installed by twisted tapes using various wings with alternate axes

The effects of twisted tapes using various wings with alternate axes on heat transfer, flow friction and thermal performance characteristics in a concentric tube heat exchanger are experimentally investigated. The influence of wing shape including triangle, rectangle and trapezoid are also studied. The experiments are performed using twisted tape with the constant wing-chord ratio (d/W) of 0.31, constant wing-span ratio (b/W) of 0.23 and constant twist ratio (y/W) of 3.8, over a Reynolds number range of 5800–18,500. The alternate axes are made by arranging each plane of twisted tape to 60-degree difference about the adjacent plane. The tests using the plain tube and tube with typical twisted tape were also conducted for a comparison. The results show that both heat transfer rate and friction factor associated with all twisted tape are consistently higher than those without twisted tape. Under the similar operating conditions, Nusselt number, friction factor as well as thermal performance factor given by the tape with alternate-axes and trapezoidal wings are higher than those given by the others. The maximum thermal performance factor of 1.44 is found with the use of twisted tape with alternate-axes and trapezoidal wings, where the heat transfer rate and friction factor increase to 1.91 and 5.2 times of those in the plain tube, respectively.