Diffusion is an important filtration
mechanism of fibrous filters
for removing fine particles from gas streams. An analytical model
is proposed in this work, based on fractal theory, to quantify the
effective diffusion coefficient (EDC) across micro/nanofibrous filters
with a layered structure. To show the influence of macroscopic parameters
of fibrous filters on EDC, the present models are expressed in terms
of fiber diameter. Polyacrylonitrile (PAN) nanofibers were prepared
by electrospinning on melt-blown polypropylene (PP) microfiber filter
materials to form micro/nanofibrous filters. To validate this model,
a three-dimensional (3D) fiber model with physical parameters of the
samples was reconstructed using the GeoDict code, and the Brownian
movement of nanoparticles was simulated to calculate the EDC. Two
kinds of pore size distributions for the layer-structured fibrous
filters have been identified, which also shows the internal heterogeneity
of the filter media. With decreasing fiber diameter, the tortuosity
of the pore channel increases, while the EDC decreases. Compared with
the numerical simulation and the experimental data reported in the
literature, the current model gives a better theoretical prediction.
In addition, the diffusion mechanism in classical filtration theory
was modified to reduce the deviation between theoretical prediction
and experimental results.