So far the characteristics and mechanisms of particle adhesion have not been clearly understood and this hinders the development of methods for reducing particle adhesion. This thesis aims to understand the characteristics and mechanisms of particle adhesion and to develop some simple and passive methods for reducing particle adhesion in cyclones. Firstly, particle adhesion patterns in a cyclone are examined in a single cyclone experiment. They include the large scale spiral patterns (SPs), small scale wave patterns (WPs) and the thick adhesion layer (TAL) near the cyclone tip. The formation and development of these patterns are investigated with the aid of CFD simulations and the Barchan sand dune mechanism. It is found that the SPs are the footprint of the spiral band of particle trajectories in the cyclone. The backward creeping motion of the WPs is caused by the build-up of particles on their windward side, which is comparable to the movement of the Barchan sand dune. The precessional bent vortex end (PBVE) near the cyclone tip is supposed to be responsible for formation of the TAL. CFD simulations have been undertaken to gain an insight of the cause of particle adhesion. However, CFD simulations tend to overestimate particle velocities in the wall boundary layer, because sizes of mesh cells in the near-wall region are much larger than the particle size. In order to obtain a more accurate prediction of particle dynamic motion in the wall boundary layer and hence particle adhesion, a 2D mesh-free simulation of particle motions within the cross section of the cyclone is developed and validated with the experimental data. The results of the 2D mesh-free simulation show that the centrifugal force has little impact on particle adhesion, because the flow velocity near the wall is close to zero as the result of the non-slip condition. Instead, the radius of cyclone, particle charge, inlet velocity, coefficient of restitution (CoR) and the coefficient of friction (CoF) are found to affect particle adhesion considerable. During the experiment on a Dyson vacuum cleaner with a multi-cyclone configuration, a massive reduction of particle adhesion (about 94% reduction) is achieved by adding a small wedge in the inlet of the cylindrical chamber of the Dyson vacuum cleaner. A series of experiments were then carried out to identify the reasons for this reduction. It is found that the particle inlet location is significantly changed by the wedge. The author believed that the particle inlet position would affect the aerodynamic force on the particle in the cyclone. In this way, the particle adhesion is reduced by the wedge. The effect of particle inlet position is examined further in a single cyclone to investigate the detailed mechanisms on particle adhesion. The results show that there is a relation between particle adhesion and pressure drop of the cyclone. A high pressure drop is found to be associated with a small amount of particle adhesion. This finding confirms that particle inlet position affects the aerodynamic force on the particle. Combining the experimental results with CFD simulation results, a hypothesis is hence proposed to explain the mechanism behind the effect of particle inlet position.
|Date of Award||1 Aug 2017|
- The University of Manchester
|Supervisor||Lin Li (Supervisor) & Shan Zhong (Supervisor)|