Thermo-hydraulic performance of tube with decaying swirl flow generators

In this study, the influences of swirl generators fixed at the pipe inlet and having different swirl angles (0 degrees, 22.5 degrees, 41 degrees and 50 degrees) on the heat transfer and fluid flow characteristics in a pipe flow were experimentally and numerically studied under a constant heat flux condition in the range of Reynolds (Re) number 2400-23000. The axial type passive swirl generator aims to increase the turbulence effect and gives tangential acceleration while increasing the total flow length in the pipe. This study is novel with the aspects of; (i) the number of helical air passage channels employed in the investigated swirl generators and, (ii) 360 degrees rotation of each helical channel at different swirl angles, i.e., lengths of considered swirl generators are different. Experiments were carried out in a smooth aluminum pipe having an inner diameter of 15 mm and a length of 675 mm with and without swirl generators for turbulent airflows. Empirical correlations for the local and mean Nusselt (Nu) numbers, and friction factor (f) were developed as a function of Re number and swirl angle as well as smooth pipe flow. The obtained experimental results for the heat transfer and friction factor at different swirl angles were compared with the smooth pipe results. The findings showed that the maximum heat transfer rate is achieved at a swirl angle of theta = 50 degrees, and the heat transfer rate increases as the swirl angle and Re number are increased. The decaying swirl flow leads to around 1.1-1.41 times enhancement of Nu and 2.3-6.76 times increase of f compared to the smooth pipe. The maximum value of performance evaluation criteria (PEC), around 0.83, is achieved by the decaying swirl flow when the theta = 0 degrees and Re = 2400. The experimental and numerical Nu and f values for swirl flow are in good agreement within the mean deviation +/- 7.1% and +/- 8.4%.