Computational modelling of particle tribocharging in small, high-speed cyclones

Abstract

Small, high-speed cyclones are used in domestic vacuum cleaners to separate sub-micron sized particulates from an air stream using no moving parts. The complex fluid flow within the cyclone is turbulent and highly-swirling, with small sub-micron particles encountering and responding to fluid structures of a wide spatial and temporal range. The process of particle separation in these cyclonic devices is not well-known and accurate numerical prediction of separation efficiency remains a computationally expensive task. In addition to this, particles are known to exchange charge through collisions, in a process known as tribocharging, which can cause smaller particles to agglomerate into larger structures. This thesis aims to develop a computationally inexpensive particle model able to predict the transport of particle charge which is valid in this highly-swirling flow for small particles. Models of particle tribocharging in the literature have only been validated for particles many orders of magnitude larger than those of interest here. Novel experiments were performed where sub-micron particles were aerodynamically focused to impact a metal plate using an aerosol beam. The results validated the physics of charge transfer at these small scales, with an equilibrium level for the voltage on the plate reached which charged analogous to a capacitor charging circuit. These measurements provided estimates of tune-able constants for a simple capacitor-like tribocharging model from the literature. Capturing collisions between particles in a Lagrangian reference frame is prohibitively expensive computationally, therefore an Eulerian particle model was chosen which can deal with collisions efficiently. The Equilibrium Eulerian method for computing the Eulerian particle velocity field for low Stokes number particles was validated in an analytical Taylor-Green flow. This flow field features fluid gradients and strong streamline curvature, with the length and velocity scales matched with the smallest turbulent scales in a representative cyclone. Strengths and weaknesses of the model were assessed in this challenging flow field, with an Eulerian sub-model for the transport of particle charge implemented. A large eddy simulation of a small, high-speed cyclone was performed with high wall resolution. Flow rates and pressure drop were matched to experimental evidence and partially resolved turbulent structures were observed along the cyclone walls. Lagrangian particle tracking provided a separation efficiency which matched experiment except for the smallest 0.1µm particles. Particles were observed travelling in distinct streaky bands by the walls which matched video evidence obtained experimentally. The Eulerian particle model provided an excellent match to the Lagrangian particle result for separation efficiency, with banding of large 1µm particles by the walls able to be captured, matching both the Lagrangian particles and experiment. The transport of charge and resulting electric field in the cyclone showed significant tribocharging occurring at the cone tip of the cyclone and by the walls which is novel. This work is expected to inform future cyclone design to improve the separation efficiency of small particles.

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