Effect of arc-shaped vortex generator on coherent structures and heat transfer enhancement in turbulence

The vortex generator is a common passively enhanced heat transfer element, which is widely used due to its high cost performance. In this paper, for the square pipe with an arc-shaped vortex generator, the Time-Resolved Particle Image Velocimetry (TRPIV) and the Large Eddy Simulation (LES) were used to analyze the characteristics of the induced coherent structure by the vortex generator in the turbulent flow and its effect on heat transfer enhancement. The statistical information of the turbulent characteristic quantities obtained by experimental studies shows the imprints of the hairpin vortex legs and low-speed streaks. The hairpin vortices and the Counter-rotating quasi-streamwise Vortices Pair (CVP) were extracted by the vortex identification criteria. It is found that these two different types of vortex structures coexist and interact with each other in the flow field induced by the arc vortex generator. With the evolution and development of the structures, their spanwise scales increase continuously. In order to study the relationship between turbulent coherent structures and heat transfer enhancement, the spatial distribution of the coherent structure and temperature gradient is analyzed by numerical simulation. It is noted that the distribution of the coherent structure and temperature gradient has good regional consistency, which reflects the influence and control of the turbulent coherent structure on heat transfer enhancement. By comparing the average Nusselt number before and after the installation of the arc-shaped vortex generator, it is found that the average Nusselt number increases significantly, up to about 20%. And the trend of the average Nusselt number’s change along the streamwise direction is consistent with the development and evolution of the coherent structure in turbulence.