The effect of multi-longitudinal vortex generation on turbulent convective heat transfer within alternating elliptical axis tubes with various alternative angles

This study strives for a numerical examination of the turbulent flow and the heat transfer of water fluid flow in the alternating elliptical axis tube under various alternative angles. The purpose of this study is to increase the heat transfer by generating multi-longitudinal vortices along the tube flow. By increasing the rotation angle between pitches, the number of the formed multi-longitudinal vortices caused by the secondary flow increase from four to eight. This increase in the number of the multi-longitudinal vortices causes the cold fluid of the tube centre to have interaction in more paths with the hot fluid close to the wall and be mixed as well. This results in an increase of the heat transfer in this type of tubes. In addition, the results show that an improvement in the mean of average Nusselt numbers of alternating elliptical axis tube with the rotation angles of 40°, 60°, 80°, and 90° compared to the circular tube are 7.77%, 14.6%, 16.93%, and 24.42%, respectively. Finally, the correlations of the friction factor and the Nusselt number with the Reynolds number are presented for four AEA tubes with different pitches degree (40°, 60°, 80°, and 90°).