Modeling and analysis of bubble mobility in fibrous porous media
Date
2014
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Publisher
University of Delaware
Abstract
In composites manufacturing, thermoset resin is introduced into a fibrous preform to cover the empty spaces in between fibers. It is important to extract voids or bubbles, formed due to trapped air or volatiles, before the resin solidifies. Voids in the cured composite reduce the mechanical properties and performance. This dissertation presents modeling and analysis that investigates the flow dynamics of resin and bubbles through fibrous porous media. The aim is to improve the physical understanding of the flow processes for improved process design and part quality. To remove bubbles, one needs unblocked pathways connected to a vacuum. First, a system-level process model is presented that describes the compaction and saturation of composite fibers partially impregnated with resin (i.e. prepregs). The model yields the degree of resin saturation due to the application of an external roller or vacuum pressure. The results estimate the degree of dry fiber region within the system, which correlates to the available unblocked pathways for later bubble removal. Next, a computational model of a prepreg flow channel to describe resin and bubble flow is presented. The channel boundaries consist of rectilinear porous media geometry. A parameter called bubble mobility is defined as the ratio of average bubble velocity to the average resin velocity. The model explores the replacement of porous media domains with slip velocity boundary conditions for computational simplification. The influence of permeability and channel geometry on the two-phase flow via porous wall effects is investigated. Results suggest that parameters can be optimized to increase bubble mobility and ultimately bubble removal. Finally, a model experiment is presented where a flow cell comprised of two transparent parallel plates is injected with a simulated resin and air bubbles. The flow cell is designed to simulate prepreg flow channels. A flow visualization setup records the two-phase flow through the flow cell. A computational model, based on the experimental setup, is formulated to describe the two-phase flow through the flow cell and fibrous porous media. A parametric scaling analysis is conducted to identify and quantify how parameters, such as bubble size and capillary number, can achieve increased bubble mobility.