Hydrocyclones can be used to concentrate the entrained sands in sewage and alleviate the clogging and erosion of the drainage network, but in practical application, there are problems such as low concentrations of underflow and a high content of fine particles, which cause a significant load on the subsequent sand dewatering and recycling. This paper designs five spigot structures of hydrocyclones and investigates the separation performance by numerical simulation, aiming to improve the applicability of hydrocyclones in the sewage treatment process by optimizing the spigot structure. The research results show that a large cone spigot delays the external downward swirling flow and reduces fine particle content in the underflow, but its effective separation space is reduced, and the turbulence in the cone section area is more intensive, which influences the separation accuracy. An elongated spigot has a reduced underflow water distribution; fine particles are more enriched in the internal swirling flow, and the underflow recoveries of 1 μm and 5 μm particles drop by 2.34% and 2.31%. The spigot structure affects the downward fluid and air intake states; complicated spigot structures contribute to increasing the resistance of particle discharge through underflow, alleviating fine particle misplacement.

The solid particle suspension inside a flotation machine is significantly dependent on the flow field, particularly the flow hydrodynamics characteristics near the bottom of the flotation machine. In this study, a laser Doppler anemometer (LDA) was utilized to investigate the influence of the impeller diameter and the impeller off-bottom clearance of a flotation machine on the flow velocity distribution characteristics near its bottom. The results showed that centripetal, centrifugal, and transitional spiral ascending vortexes were generated for different cases of the impeller variables. The impeller diameter and the off-bottom clearance were found to have a significant and interactive influence on the flow pattern, radial and axial velocities, velocity vector distribution, and axial fluctuating root mean square (RMS) velocity characteristics. When the centripetal flow was generated with a large impeller diameter and a small off-bottom clearance, the vortex stability was improved, the low-velocity distribution area was reduced near the bottom center, and the high axial RMS velocity distribution area was extended and became more consistent. The latter provided an advantageous condition for the momentum transfer between the liquid flow and the solid particles, as well as the airflow. However, the axial RMS velocity in the centrifugal flow formed in other cases of the impeller variables was less than that in the centripetal flow. Although the increase in the impeller off-bottom clearance contributed to increasing the velocity magnitude, this is certainly disadvantageous to the service life of the impeller blades, as expected from the high-velocity area extension. These results may provide a reference for the impeller design and optimization of a KYF (Kuang Yuan Flotation) flotation machine, as well as a basis for further investigation on the behavior of the dispersed phases inside a flow field.

The particle motion behavior in hydrocyclones has received increasing attention, but the particle circulation flow has received relatively limited attention. In this paper, the particle circulation flow is regulated by changing the secondary-cylindrical section diameter to optimize the separation effect. The effects of secondary-cylindrical section diameters on flow field characteristics and separation performance are explored using the two-fluid model (TFM). The findings demonstrate that particle circulation flows are ubiquitous in the secondary-cylindrical hydrocyclone and are induced by the axial velocity wave zone. The increase in the secondary-cylindrical section diameter intensifies the coarse particle circulation and aggrandizes the coarse particle’s aggregation degree and aggregation region, leading to an increment in cut size. The circulation flow component can be regulated by adjusting the secondary-cylindrical section, thus improving the classification effect. An appropriate diameter of the secondary-cylindrical section facilitates improved particle circulation, strengthening the separation sharpness.

Two cylindrical section hydrocyclones can suppress particle misplacement by regulating the circulation flow, but few researchers have investigated the effect of the cylindrical height ratio. In this paper, numerical simulations and physical tests were conducted to investigate the effect of height ratio on the particle motion behavior and separation performance of the two cylindrical section hydrocyclone. According to the numerical simulation results, with increasing height ratio, the separation cut size decreased, the separation accuracy and recovery rate of medium and coarse particles in the underflow increased, the coarse particle misplacement in overflow decreased significantly, and the proportion of medium particle circulation flow gradually increased. According to the test results, the number of misplaced fine particles in underflow could be effectively reduced when H₁/H₀ = 0.30. With increasing height ratio, the number of misplaced coarse particles in the overflow decreased and the classification efficiency of fine particles increased. The maximum separation efficiencies of medium and coarse particles could be obtained at H₁/H₀ values of 0.47 and 0.17, respectively. Therefore, increasing the height ratio could inhibit coarse particle misplacement in overflow and improve the separation performance of two cylindrical section hydrocyclones.

The cylindrical hydrocyclone can be regarded as a special-shaped hydrocyclone comprising entirely cylindrical sections without conical sections, featuring a unique flat-bottom design combined with central discharge, which promotes substantial particle circulation flow in the separation chamber, directly affecting separation performance. A validated TFM model is employed to investigate the flow field and particle motion behavior in the cylindrical hydrocyclone. The results indicate that the distributions of tangential velocity, radial velocity, pressure, and pressure gradient in the cylindrical hydrocyclone are consistent with patterns observed in the conventional hydrocyclone. The flat-bottom design combined with the central discharge configuration of the cylindrical hydrocyclone results in two distinct axial velocity transitions in the bottom region, forming downward axial velocity flow around the air core. Accordingly, particles moving toward the spigot must pass through the internal swirling flow region, facilitating the fine particles entrained by the coarse particles to enter the internal swirling flow, reducing the misplacement of fine particles in the underflow. Simultaneously, coarse particles entrained by the internal swirling flow return to the external swirling flow region under centrifugal force, forming a substantial coarse particle circulation flow. As a result, a mass of coarse particles accumulates in the separation chamber, hindering the centrifugal settling of medium particles and resulting in an enlarged cut size and severe coarse particle misplacement.

The outlet diameter of hydrocyclones is a critical structural parameter that impacts product distribution and separation performance, drawing significant attention. In this paper, the separation efficiency and particle motion behavior in the cylindrical hydrocyclone with varying spigot diameters and vortex finder diameters are systematically analyzed using a TFM model. The numerical results indicate that a larger spigot diameter and a smaller vortex finder diameter reduce the axial velocity and expand the external swirling flow region, while a smaller spigot diameter and a larger vortex finder diameter enhance the particle circulation flow ratio and the coarse particle circulation flow proportion, thereby increasing the cut size. Slightly reducing the spigot diameter and increasing the vortex finder diameter enhances the separation accuracy. Nevertheless, for D_u = 0.075 D and D_o ≥ 0.4 D, the recovery rate in the underflow remains below 50% for all particle sizes, exhibiting severe particle misplacement and loss of separation efficiency. For D_u = 0.125 D, the reduction in coarse particle misplacement in the overflow is attributed to the abrupt changes in the coarse particle circulation flow proportion and medium particle circulation flow proportion. Generally, an appropriate coarse particle circulation flow proportion in the cylindrical hydrocyclone is beneficial for alleviating particle misplacement and improving separation accuracy.