Influences of twisted-tape with parallel rectangular-wing on thermal performance of a heat exchanger

The present paper reports heat transfer enhancement and friction factor characteristics in the tubes inserted with rectangular-winged twisted-tapes (TT-RWs). The wing-depth ratio (d/W) was varied from 0.1 to 0.3 while the tape-twist ratio was kept constant at y/W = 4.0. The experiments were conducted under uniform heat flux condition for Reynolds number between 5500 and 20,200 (based on the inner diameter, D), using water as the working fluid. The tubes with typical twisted tape (TT) and twisted tape with alternate-axis (TA) as well as a plain tube were also tested for assessment. The obtained results demonstrate that the utilization of the tubes with TT-RWs leads to the increases of both Nusselt number and friction compared to those from the uses of the tubes with TT, TA and the plain tube. In addition, Nusselt number increases as a wing-depth ratio of the twisted tape increases. The improvement of Nusselt number is directly related to the superior destruction of the boundary layer which is responsible by the stronger swirl flow and turbulence intensity. According to the results, the TT-RW with d/W = 0.3 yields the highest Nusselt number which is around 100% higher than that of the plain tube, corresponding to the thermal performance factor of 1.36 at constant pumping power.