CFD simulation of hydrocyclone-separation performance influenced by reflux device and different vortex-finder lengths

Abstract

Hydrocyclone has a history of approximately 129 years, and its separation performance is closely related to its vortex-finder length. To date, however, most studies have been limited to the range of from 0 to cylindrical-section length, and almost all researchers argue that the optimum vortex finder is shorter than the cylindrical-section length. To comprehensively study the effect of vortex-finder length and understand its mechanism, a hydrocyclone with reflux device and vortex-finder length of from 0 to twice cylindrical-section length was studied with FLUENT. Reynolds Stress Model and Discrete Phase Model were used. Results showed that, when the vortex-finder length was not longer than the cylindrical-section length, both the separation efficiency and split ratio were proportional to the vortex-finder length in general. This was consistent with the sandy-water experimental results obtained by Ni et al. in 2017. When it was just a little longer than the cylindrical-section length, they first decreased and then increased slightly with increase of the vortex-finder length due to effect of the cylindrical-conical interface. When the vortex-finder length was long enough, the separation efficiency was always close to 100%, whereas the split ratio was inversely proportional to the vortex-finder length. Because at this time, the axial distance between two minimum-pressure positions in the centric-axial direction was inversely proportional to the vortex-finder length, and it was so small that the fluids on the two positions could attract each other. Even at the vortex-finder length of 475–535 mm, they were combined into one. Consequently, some “circulation eddies” appeared near bottom of the vortex finders with length of 415–535 mm and near top of the underflow pipe at the vortex-finder length of 535 mm. Besides, the high separation efficiency (99.87–100%) and low split ratio (29.13–32.54%) were simultaneously obtained when the vortex-finder length was 1.68–2.02 of cylindrical-section length.