In Liquid Composite Molding processes (LCM) such as Resin Transfer Molding (RTM) and Vacuum Assisted Resin Transfer Molding (VARTM), a liquid resin is driven through a porous medium, which is usually a network of bundles (tows), made up of thousands of micron-thick cylindrical fibers. After the filling stage, the resin cross-links (cures) into a solid composite part. The structural properties of such part are dependent on the ability of the resin to fill the spaces between and within fiber tows, which at the same time displaces air and volatiles out of the mold cavity.
The present work addresses micro flow in fibrous porous media using analytic analysis, numerical simulation and experiments. Experiments were conducted on a model fiber tow to illustrate that (i) fiber tows immersed in a liquid can be impregnated mainly due to capillarity in the absence of an imposed pressure gradient and that (ii) entrapped air impedes the impregnation. An analytical model was formulated to model transverse flow across an array of cylindrical fibers representing a fiber tow. The model accounts for both capillarity by means of an average capillary pressure, and the effects of air/gas entrapment and dissolution. Experimental validations of the analytical model using both custom-made electrical sensors embedded in porous samples, and Magnetic Resonance Imaging (MRI) confirm that when the entrapped air within the tow is provided with an easy route to escape, it does not slow down the inward flow of liquid, whereas when air is entrapped it significantly slows down or stalls the impregnation. In addition continuous monitoring of the micro flow front by MRI allowed for estimation of variation of air pressure inside.
A numerical simulation of capillary flow across fiber tows is developed using the Lattice Boltzmann Method (LBM). The qualitative validation of the code that describes capillary flow through fiber tows is presented. The results from the Lattice Boltzmann approach are then projected at the mold level and compared with an original experimental technique designed to monitor the micro-saturation and micro-void content. This application underlines the potential of the LBM tool in simulating the flow inside fiber tows, and identifies the limitations as well.
The collective contribution of the analytical, numerical and experimental approaches proposed in this work should prove helpful in improving the understanding of micro-flow in dual-scale porous media, with special emphasis on the role of capillary forces and of the entrapped air during the filling stage of Liquid Composite Molding processes.
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|Advisor:||Advani, Suresh G.|
|Commitee:||Gillespie, John W., Roy, Valery, Wang, Lian Ping|
|School:||University of Delaware|
|Department:||Department of Mechanical Engineering|
|School Location:||United States -- Delaware|
|Source:||DAI-B 71/04, Dissertation Abstracts International|
|Keywords:||Capillary effects, Flow through porous media, Lattice Boltzmann method, Liquid composite molding, Microflow, Porous media|
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