Analysis of heat transfer and flow resistance of twisted oval tube in low Reynolds number flow

Heat transfer enhancement of twisted oval tube is mainly concentrated in the high Reynolds number region. In the present study, the low Reynolds k-ε model is employed to investigate heat transfer and flow characteristics of water flow inside twisted oval tube (TOT) for Reynolds number in the range of 50-2000. Three dimensional numerical study is conducted to study the effects of the geometric parameters on the performance of twisted oval tube for a uniform wall temperature case. The flattening of 1.2, 1.4, 1.63, 1.8 and 2.0, and the twisted pitch ratio of 0.17, 0.25, 0.33 and 0.5. Local distributions of Nusselt number and friction factor are presented. The filed synergy principle is applied to reveal heat transfer enhancement mechanism. The results show that the heat transfer performance of twisted oval tube has been enhanced while having an increasing of pressure drop. One of the key findings of this study is that laminar to turbulent flow transition point was identified and located at Re = 500. The fluid is in laminar states with Reynolds number range of 50-250, while the fluid is in turbulent flow when the Reynolds comes to 500-2000. It is also found that the twisted oval tube performs well compared with the smooth tube due to the effect of secondary flow. The maximum enhancements factor PEC of 1.7 is obtained with flattening of 2.0, twisted pitch of 0.33 and Reynolds number of 350. Moreover, PEC of twisted oval tube increased with the increasing of flattening, but decreased with the increasing of twisted pitch ratio. These results would assist people in a comprehensive understanding of heat transfer performance of twisted oval tube in low Reynolds number flow, but also help in the design and development of compact heat exchanger.