Experiments and Stress-Blended Eddy Simulations of turbulent flow over edge-rounded dimples

Experiments and numerical simulations have been conducted to reveal the detailed turbulent flow and heat transfer characteristics over the surfaces with spherical dimples with different edge schemes, including the sharp edge, the fully rounded edge and the upstream rounded edge. The dimples have the same depth-to-diameter ratio of hd/d = 0.2. Steady-state heat transfer experiments were carried out for the turbulent flow over the three dimpled surfaces within the Reynolds number range of 10,000 ≤ ReDh ≤ 60,000, and the globally averaged heat transfer enhancements and friction factor ratios were obtained. Furthermore, transient liquid crystal thermography technique was used to obtain the local heat transfer characteristics on the dimpled surfaces. In addition, numerical simulations were carried out with a newly developed hybrid RANS-LES method of Stress-Blended Eddy Simulation (SBES) and the local heat transfer enhancement, detailed flow structure and turbulence statistics data were obtained and analyzed. The experimental results show that the best thermal performance is achieved by the dimples with upstream rounded edge, indicating an increase of about 11.4% in total heat transfer enhancement and an increase of about 5.2% in pressure loss when compared to the conventional dimples with sharp edge. The numerical results show that the flow over the dimpled surfaces can be divided into five zones: the mainstream zone, the recirculation zone, the Kelvin–Helmholtz vortex zone, the upwash zone and the rear separation zone, and each zone has its unique flow features. It is also found that the lowest pressure is achieved by the dimple with fully rounded edge, which is due to the absence of flow separation behind the rear edge of the dimple. The upstream-edge-rounded dimples show the reduced recirculation flow in the recirculation zone, the enhanced impingement and upwash flow in the upwash zone near the rear edge. These cause the stronger production of turbulent kinetic energy behind the rear edge, which are responsible for the higher thermal performance.