| The diesel engines have been used under numerous circumstances because of their excellent power performance, high durability, and low fuel consumption. However, diesel emitted pollutants have posed severe environment problems and thus restrict its development. These pollution problems, especially those caused by particulate matter, have called serious concern from the public and the governments. To meet the regulation of the Hong Kong government on light-duty diesel vehicles, the Hong Kong Polytechnic University developed a type of particle filter made of stainless steel fibers. The type of filter is easy to retrofit, low in cost and high in filtration efficiency. The present thesis made a theoretical and experimental study of the type of filter. The study analyzed the characteristics of the diesel particulate matter and discussed the critical factors that influence the filtration efficiency and pressure drop across the filter. On the basis of these, the study put forward the operation characteristics and the optimization guidelines of the filter design.The work had been carried out can be summarized as follows:1. On the assumption of the "staggered array of the fibers", the "fully developed periodic flow" model was established. The computing code of the iterative algorithm to solve the fluid flow equations was developed. The computing code was also developed to solve the particle trajectories in order that the filtration efficiency of a single fiber can be calculated.2. A model filter was made according to the parameters set by the model computation. The sampling system, the dilution system and the particle analyzing system were designed.3. The sample was diluted by a 2-phase ejector-typed dilutor and anaylzed by the SMPS and the APS (particle size range 0.015-0.67 m and 0.5-20 m respectively). The particle size distribution of number concentration was measured under various engine operating conditions (on test-bench). The experiment also recorded the exhaust temperature, the pressure drop through the filter and those parameters necessary as the model computation input.4. The computation model predicted the pressure drop through the filter. The results were compared to the experimental data and on the basis the influential factors that affected the pressure drop were analyzed.5. The research of the present study proves that one of the most critical factors that influence the filtration efficiency is the particle size distribution. As engine speed and load affect the particle size distribution, which in turn affect the particlecollection mechanisms that apply. Particles within the size range close to the geometric mean diameter of the distribution affect the filtration efficiency most. Therefore, the filter design is required to achieve desired filtration efficiency at regions of the geometric mean diameter at board engine operating conditions.Compared to previous studies, the present research isnnovative in:1. The compuational model simultaneously considered all the capturing mechanisms in the simulation of particle collection process, instead of treating them separately. This makes the model fit a wide range of particle size.2. A compact, easy-to-use testing system was developed. The characteristics of the particle size distribution were analyzed mathematically.3. A more reasonable calculation method of filter collection efficiency was derived to represent the feature of particle size distribution. The particle size distribution is thus embodied in the evaluation of the filter collection efficiency.4. The mechanism of particle depositon on the fiber surface was simulated to evaluate this effect on filtration efficiency enhancement. Simulation of particle depositon will be critical problem in the further study of filter capability and period between regeneration cycles.It can be concluded that the particle size distribution is a critical influenial factor on the filter efficiency. The variation of the particle size distribution decides the strength of the Brownian diffusion and the inertial impac...
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