Thermal characterization of turbulent tube flows over diamond-shaped elements in tandem

Experiments have been conducted to investigate the heat transfer and friction factor characteristics of the fully developed turbulent airflow through a uniform heat flux tube fitted with diamond-shaped turbulators in tandem arrangements. In the experiments, strong turbulence and recirculation flow is expected by using tandem diamond-shaped turbulators (D-shape turbulator) connected to each other by a small rod and placed inside the test tube. The parameters for this study are consisted of Reynolds number (Re) from 3500 to 16,500, the included cone angle (theta = 15 degrees, 30 degrees and 45 degrees), and the tail length ratio (TR = l(t)/l(h), = 1.0, 1.5 and 2.0) defined as the ratio of the tail length (l(t)) to the head length of turbulator (l(h)). The variation of Nusselt number and friction factor with Reynolds number under the effect of those parameters are determined and presented. The experimental result reveals that the heat transfer rate increases with increasing Reynolds number and the included cone angle (theta) but decreases with the rise of the tail length ratio (TR). This is because of the mixing of the fluid in the boundary layer thereby enhancing the convective heat transfer and increasing pressure loss. For the tube with the turbulator of theta = 45 degrees, the heat transfer enhancement is found to be 67%, 57% and 46% for tail length ratio, TR = 1.0, 1.5 and 2.0, respectively. Correlations of the Nusselt number (Nu) and friction factor (f) are developed for the evaluation of interactive effects of using the turbulators on the heat transfer and pressure loss. The good agreement between the experimental and the correlated results is obtained within 5-7% deviation. In addition, the heat transfer enhancement efficiency determined under constant pumping power is also provided. (C) 2009 Elsevier Masson SAS. All rights reserved.