Mechanisms of particle migration in electrostatic precipitators
Electrostatic precipitators are high efficiency gas cleaning devices widely used in industry for removing particulates from process gases. A major factor affecting their performance is particle migration, which is governed by the complex interaction of electrical and hydrodynamic phenomena. A fuller understanding of these fundamental mechanisms is therefore essential to the development of realistic mathematical models. The work described in this thesis concentrates on the fluid-particle interactions in a wire-plate-system. A pilot-scale rig was built using actual components from an industrial precipitator, allowing realistic conditions to be simulated in the laboratory. Hot-wire anemometry and laser-Doppler photon correlation techniques were employed to study the time-averaged velocity field. Several designs of wall strengthener were considered, and in each case the effect on the surrounding flow field was investigated using helium bubble visualisation. The turbulent nature of the fluid was characterised by local dispersion coefficient values and fluctuating velocity components. Alumina test dust in the size range 1-10 pm was used in the precipitator under a variety of operating conditions, and a technique was established for extracting representative dust samples. The samples allowed simultaneous measurement of concentration and size distribution, from which concentration profile development and collection efficiency information was obtained. Two alternative numerical models of the precipitator were developed, both incorporating the results from the fluid flow field experimentation. The first approach was based on the finite difference solution of the convective-diffusion equation, using appropriate boundary conditions. In the second approach, the transport of dust down the precipitator duct was simulated by the step-wise progression of a series of vertical line-sources, whose motion was governed by electrical migration and lateral diffusive spread. The validity of the models was tested by comparison of the predicted concentration profiles with corresponding experimental results.