Heat transfer and flow structure around a heated cylinder by upstream installation of a grooved cylinder

The aim of this study was to investigate the effects of installing the grooved and smooth control cylinder upstream of a circular bare cylinder with a focus on the heat transfer and the flow structure behind the bare cylinder. The Reynolds number based on the bare cylinder diameter (D = 50 mm) is Re = 5 000. Control cylinder has a constant diameter as d = 25 mm. Flow characteristics and heat transfer were determined the varying gap space L (center-to-center distance between the bare cylinder and the control cylinder) and surface geometries. The experimental study was conducted in a water channel to collect the temperature and velocity data. Heat transfer coefficient and Nusselt number were determined by measuring the surface temperature distributions. A digital particle image velocimetry (PIV) system was also used to quantify the flow characteristics in order to assess the effectiveness of the flow control with different grooved surface (smooth, rectangular, triangular and circular grooved), in comparison with a bare cylinder case. As to the temperature measurement results, the optimum gap space was defined as L/D = 2.0. It was found that when the control cylinder is introduced, the separation point shifted downstream of the bare cylinder. The cylinders with groove geometry, however, proved to improve heat transfer in comparison to a single main cylinder. The velocity profiles in the time-averaged streamwise direction caught the peak of the single cylinder at L/D = 2.0, where the time-averaged velocity profiles of the control cylinders were minimum. The vorticity components showed that grooved cylinders were more effective in flow control, especially at L/D = 2.0, compared to a single cylinder.