Turbulent effects of vortex generators on the separation of fine particles

Abstract

The separation of fine particles has always been challenging in flotation. Strong turbulence is considered to be beneficial. In our previous study, vortex generator (VG), commonly used to improve heat transfer by enhancing turbulence, was proved to be effective for fine particle flotation. However, the turbulent effect of VG on different sized fine particles is still poorly understood, which hinders the industrial application. Flotation tests and flow field simulations were conducted on a pipe flow unit with VGs and without VGs under different Reynolds numbers (Re). Results showed that, VGs improved the flotation of particles with a diameter less than 37 μm and especially for particles less than 15 μm. Nevertheless, VGs were detrimental to the flotation of particles in the size range of 37–74 μm when Re exceeds 10,000. These results are attributed to the high turbulent energy dissipation (ε) caused by counter-rotation vortex pairs induced by VGs. The increased ε enhances the collision efficiency while reduces the stability efficiency for increased particle size, which indicates there is a correlation between particle size and turbulence at which the optimal recovery can be achieved. In the experimental range of this study, the optimal flotation rate was observed when the ratio of turbulent Kolmogorov eddy scale and particle diameter is 0.3.