|Title:||Gas and vapor transport through nano- and micro- fibrous materials|
Hong Kong Polytechnic University -- Dissertations
|Pages:||xxiv, 198 leaves : ill. (some col.) ; 30 cm.|
|Abstract:||Fibrous materials have a variety of applications, such as filtration, fuel cell, textile fabric, fiber reinforced composite, and tissue scaffold. Recently, of particular interest are fibrous preforms composed of nanofibers and microfibers, which are tailored to meet a range of advanced requirements. For many applications, permeability of gas flow and diffusivity of vapor diffusion are two important mass transport behaviors observed. However, the characterization of the transport phenomena in realistic fibrous structures is challenging and still not fully understood, especially in nano- and micro-scale regimes. Therefore, the current work is aimed at systematically bridging the microstructures to the transport properties of nano- and micro-fibrous materials, by analytically solving transport equations in equivalent fibrous structures based on deterministic and statistical methods. Fibrous structures can be broadly classified into two types: single-scale and dual-scale. The absence of yarns makes the mean pore radius in single-scale mats of the same magnitude, and hence they can be characterized by a single permeability. However, after finer fibers or filaments are bundled into yarns, they are woven or stitched into different structures, which contain pore sizes in two distinctly different magnitudes and are therefore called dual-scale fibrous media. The single-scale fibrous medium is commonly referred to as a nonwoven web, which can be one-dimensional (1D: all fibers parallel with each other), two-dimensional (2D: all fibers parallel with the same plane), and three-dimensional (3D: all fibers distributed in different orientations in a cubic space) arrangements. Dual-scale fibrous media can be knitted or woven structures, which are always constructed by interlacing threads. The first part of this study was aimed at studying gas flow through single-scale fibrous materials. For highly porous fibrous media, gas permeabilities from 1D ordered structure to randomly located 2D or 3D fiber assembles were determined by Voronoi Tessellation Method and mixing laws. The slip flow on the fiber surface of nanofibers was particularly considered. For densely packed fibers, a modified scale estimate approach was utilized to predict the gas permeability. In the second part, a permeability model throughout the range of porosities was obtained for single-scale fibrous layers, whose pore size distribution was found to statistically follow the fractal power law. The third part investigated gas flow in dual-scale fibrous media, where complexities are introduced as the inter-yarn flow is coupled with the intra-yarn flow. A "slip" boundary at the interface between yarns and open channels was used to account for the coupled effect. A semi-analytical model was also provided for rapid predictions of permeabilities from unidirectionally aligned yarns to 3D woven fabrics.|
The fourth part presented an analytical model of vapor diffusivities for 1D, 2D, and 3D randomly distributed fibers. The model was established by extending the 1D regular model to 1D random array through Voronoi Tessellation Method, and to 2D and 3D structures by mixing rules. In the fifth part, a diffusivity model of nanofiber webs was derived as a function of porosity, fiber radius, and fractal dimensions, which statistically characterize the pore size distribution and tortuosity of fibrous media. To verify the proposed model, experimental measurements of water vapor diffusivities for electrospun nanofiber mats were conducted by inverted-cup test method. All the models established in this study were well validated by the results collected from experiments in present study or related literature, numerical simulations, and past theoretical models. Moreover, the effects of structural parameters were extensively analyzed, and the following conclusions can be made: 1. Gas permeability of microfibers scales with the square of fiber radius, while vapor diffusivity of microfibers is independent of fiber/pore size. 2. In nanofiber mats, gas permeability is enhanced by slip effect, but vapor diffusivity is decreased due to Knudsen effect. Electrospun nanofibers are found to be good candidates of breathable materials experimentally and theoretically. 3. Both gas permeability and vapor diffusivity are not sensitive with in-plane fiber orientation, but increase with increasing through-plane fiber orientation. 4. In terms of fiber distribution, more random, more permeable, but less diffusive when porosity is high. 5. Comparing with squarely packed fibers, fibers with hexagonal configuration are more transversely permeable but less axially permeable in low porosity range. 6. Elliptical fibers with major axis parallel with flow direction are more transversely permeable than circular fibers, and they have similar permeability in the high porosity range. 7. The intra-yarn permeability increases the overall permeability of dual-scale fibrous materials when the ratio between them is more than 0.01. Based on above models and findings, future work may be directed towards employing the models to specific uses, developing relevant software, improving models of coupled heat and moisture transfer, and designing optimized fibrous structures with controllable transport properties.
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