The microscale simulation of ultrafine particle transport and deposition in fibrous filtration media was achieved with a novel particle tracking model using a 3D Monte Carlo model. The particle deposition process is governed by the convection–diffusion field. Simulations were performed by considering the fibrous filtration media as an array of identical parallel fibers, in which the flow field was accurately described by an analytical solution. The model of particle movement was described by the random probability distribution characterized by a dimensionless factor, the Peclet number (Pe), based on a convection–diffusive equation of particle transport in fluid. The influence of the particle Peclet number (Pe) on the particle deposition process and the resulting deposition morphology was investigated. The results were analyzed in terms of dust layer growth, particles’ trajectories and dust layer porosity for a vast range of Peclet numbers. The development of distinct deposition morphologies was found by varying the Peclet number (Pe). With a small Peclet number, diffusion dominated deposition and led to the formation of a more open and looser dust layer structure, while with larger Peclet numbers, convection dominated deposition and was found to form compact deposits. According to the change in the location of the packing densities along the dust layer height direction, the dust layer structure could be divided into three typical parts: the substructure, main profile and surface layer. In addition, the deposit morphologies observed for a high Pe were in good agreement with the experimental results found in the literature.

A three-dimensional Monte Carlo model was developed to simulate the deposition of aerosol particles onto horizontal solid surfaces. The random walk method was employed to solve the particle transport equation, which allowed obtaining the trajectory of particle motion by a combined mechanism of Brownian diffusion and gravity sedimentation. The particle transport mechanism was described in terms of a Peclet number (Pe). The local structures of the dust layer, the relationship between the structure of the dust layer and particle transport mechanisms, and the number of the particles attached to the solid surface were investigated. The results showed that for a small Pe, when Brownian diffusion was a controlling mechanism for aerosol transport, the dust layer might exhibit a more open and looser structure, while for a large Pe, the dust layer was dense and tight. The differences of deposition morphologies under different transport mechanisms were caused by the different random intensities of particle motion. There was an upper limit of the maximum number of particles attached to the surface, and it strongly depended on particle transport mechanisms and size distributions. Additionally, the deposit morphologies obtained with the 3D Monte Carlo model were in good agreement with the experimental results found in the literature.

A three-dimensional stochastic simulation was developed to simulate growing process of particles deposit on a single fiber in Kuwarbara cell due to the collection mechanisms from inertial impaction and interception. The distributions of size of particle dendrites were calculated to quantificationally describe the complex deposit morphologies. The results indicate that the deposition morphologies strongly depend on Stokes number and interception parameter. A small stokes number tends to give a fewer but larger particle dendrites with a more open and loose structure, whereas a greater stokes number leads to a compact structure. The interception parameter shows different influencing degree to deposition morphology at different Stokes numbers. In the case of low Stokes number, there is no significant difference in heights of particle dendrites between various interception parameters, whereas for high Stokes number, the interception parameter plays a significant role in the morphologies transition from compact structures to dendritic. In addition, a larger interception parameter may weaken the effect of particle inertia on formation and growth of particle dendrites.

An off-lattice Monte Carlo model has been developed to simulate growth of dust cakes from ultrafine particles in a model filter composed of an array of identical parallel fibers placed transverse to the flow. Unlike previous simulations on the dust cakes, this study considerd the effect of the flow feature in the vicinity of fibers on the morphologies of dust cakes. The flow was described by the modified Kuwabara flow field with consideration of the slip effect at fibers surface. The probability of a particle moving in four different directions was derived from the two-dimensional particle convection-diffusion-migration equation. The resulting microstructure of dust cake formed on fibrous medium was characterized by its density and fractal dimension. The dependences of the porous properties of dust cakes (density, density distribution and fractal dimension) upon the particle transport mechanisms (measured by Pelect number, Pe in this work) were also investigated. The results indicate that the dust cakes structures strongly depend on particle transport mechanism, a smaller Pe tends to give a more open and loose structure with a lower density and fractal dimension. For the case of Pe = 1, the fractal dimension for the cake structure approaches to 1.710, a value for the diffusion-limited aggregation (DLA) model. When Pe > 100, the deposition behavior is very close to the pure ballistic aggregation model (LA), and the resulting structure is on average uniform at all length scale. Furthermore, a dust layer near filtration medium with lower porosity can be observed for a higher Peclet number (Pe ≥10), unlike previous results on particles deposition on a simply two dimensional threadiness substrate.