Large-Eddy Simulation of a Hydrocyclone with an Air Core Using Two-Fluid and Volume-of-Fluid Models

Abstract

Large-eddy simulations have been conducted for two-phase flow (water and air) in a hydrocyclone using Two-Fluid (Euler–Euler) and Volume-of-Fluid (VOF) models. Subgrid stresses are modeled using a dynamic eddy–viscosity model, and results are compared to those using the Smagorinsky model. The effects of grid resolutions on the mean flow and turbulence statistics have been thoroughly investigated. Five block-structured grids of 0.72, 1.47, 2.4, 3.81, and 7.38 million elements have been used for the simulations of Hsieh’s 75 mm hydrocyclone Mean velocity profiles and normal Reynolds stresses have been compared with experimental data. Results of the two-fluid model are in good agreement with those of the VOF model. A fine mesh in the axial and radial directions is necessary for capturing the turbulent vortical structure. Turbulence structures in the hydrocyclone are dominated by helical vortices around the air core. Energy spectra are analyzed at different points in the hydrocyclone, and regions of low turbulent kinetic energy are identified and attributed to stabilizing effects of the swirling velocity component.

Two-Fluid Large-Eddy Simulation of Two-Phase Flow in Air-Sparged Hydrocyclone

“…The LES two-phase flow model used in the present study was validated by Fayed et al [10] for a conventional hydrocyclone. They presented grid-independent LES results for the dynamic SGS model, and obtained good agreement with the experimental data for mean flow velocity and turbulence statistics by Hsieh [31].…”

“…Herring et al [36] also conducted direct numerical simulations of decaying two-dimensional turbulence and found a law of k −4 . Fayed et al [10] also predicted the f −4 power law for the near-wall turbulence of a conventional hydrocyclone. For points on the air core boundary, a short inertial subrange of slope −5/3 is found, followed by the viscous dissipation of the small scales with a slope of −7 or −8.…”

“…However, these experimental velocity measurements in ASH are not suitable for a direct comparison with the CFD results of the present simulations which include air injection. Fayed et al [10] provided validation for the same computational model for a conventional hydrocyclone. Hsieh and Rajamani [31] and Hsieh [32] conducted experimental measurements in a conventional hydrocyclone using particle image velocimetry (PIV).…”

“…The profiles of the mean axial and tangential velocity components were reported at different axial locations. Fayed et al [10] conducted two-phase LES for Hsieh's hydrocyclone. They studied the effects of grid refinement on the mean velocity profiles and turbulence statistics and showed a good comparison with the experiments.…”

“…Low static pressure in the center due to the high tangential (swirling) velocity allows air to be pulled inward from the bottom opening, thereby developing an air core. The diameter of the air core depends on the swirling velocity, back pressure, and the underflow opening, all of which impact the cut-size of a hydrocyclone and its separation efficiency [8][9][10][11]. The separation efficiency of a conventional hydrocyclone will rapidly deteriorate if the water flow exiting from the bottom suffers from either the atomization or the roping phenomenon [12,13].…”