Modelling of flow in a micro-hydrocyclone
(Zhu, Liow, Neely)
Micro-hydrocyclones are miniature-scale hydrocyclones with applications in micro-devices. As hydrocyclones do not have any moving parts, they are easier to control in micro-devices and have the potential to be more reliable. The flow in a 5 mm diameter micro-hydrocyclone was modelled in FLUENT to investigate the fluid flow and particle separation ability. Direct numerical simulation (DNS) results have shown that the flow transition and subsequent unsteady state behaviour occurred in the micro-hydrocyclone at a low Reynolds number (Rein=300) because of the onset of centrifugal instability. The centrifugal instability results in flow transition from laminar to the development of turbulent flow in the hydrocyclone. This flow transition has not been studied in previous modelling work of hydrocyclones as they normally operate in the highly turbulent region in industry. The centrifugal instability in the micro-hydrocyclone begins as Görtler vortices developing in the boundary layer which subsequently affect the flow field. Particle motion tracing showed that improved separation with finer cut size, d50, and steeper separation sharpness were obtained as the inlet velocity was increased. This improvement is enhanced by the change in the flow characteristics when the flow transits to turbulent flow.
Contours of vorticity in a vertical plane (0 and 180° azimuthal positions) of the micro-hydrocyclone for (A) 0.1, (B) 0.2 and (C) 0.4 m/s inlet velocities (A, B & C1 - one time-step results after the flow reached a statistically steady state and C2 - time-averaged results). The increased vorticity at the wall with higher inlet velocities are a consequence of the formation of Görtler vortices.
