Thesis (M.Eng.)--Chulalongkorn University, 2000

To design a high-performance air filter with longer service life, it is important to study how the morphology of particles accumulating on an electret fiber changes and affects the collection efficiency of the filter at dust-loaded condition. An electret filter is composed of permanently charged electret fibers and is capable of collecting fine particles at a high efficiency. In this study, a three-dimensional stochastic model is utilized to simulate collection and agglomeration of particles on the cylindrical electret fiber by two different electrical deposition mechanisms, namely, induced forces (for uncharged particles) and coulombic forces (for charged particles) mechanisms. Moreover, the effect of diffusional mechanism is included. The morphology of particle Agglomerates obtained in the simulated results is found to agree very well with experimental observations obtained by Kanaoka et al. for both uncharged and charged particles. The distributionof agglomerates on the fiber surface and the change in the fiber collection efficiency can be used to explain the effects of peclet number, interception parameter, electrical parameters and twist angle of the fiber on the murphology of captured particles, the angular distribution of particles on the fiber, number distribution of dendrites as a function of time, average dendrite size by age and the collection efficiency raising factor. In addition, the ratio of collection efficiency at any time instant, n, to the initial collection efficiency, n0, can be represented as linear function in the case of low electrical parameters. However, when the electrical parameters are large, the normalized collection efficiency has to be represented by two linear correlations, i.e., at low dust load and high dust load. Furthermore, the results of this stochastic model agreed well with the experimental results better than that of Kanaoka's model.