A COMPUTATIONAL APPROACH FOR THE MODELING OF SHORT-FIBER ORIENTATION IN A VISCOUS FLOW DURING THE MOLD-FILLING PROCESS USING DIFFERENT CLOSURE APPROXIMATIONS

• Main Author: SINNADURAI, SUREN LIM
• Format: Thesis
• Language: English
• Published: 2011
• Online Access: http://utpedia.utp.edu.my/3040/1/Suren_LS_-_thesis_body_-_A_Computational_Approach_for_the_Modeling_of_Short_Fiber_Orientation_in_a_Viscous_Flow_during_th~1.pdf; http://utpedia.utp.edu.my/3040/

The last half century has seen an increase in the use of fiber composites in many areas. One such class of composites is Short Fiber Reinforced Thermoplastics (SFRT), well known for their versatility in various applications. However, the physical properties of the finished molding are highly dependent upon the fiber orientation, which in turn is heavily influenced by the processing conditions. It is thus of interest to model the mold-filling process and compute the resulting fiber orientation due to the flow field that develops within the mold. This was done in this research in three stages. In the first stage, the mold-filling flow was modeled as a non-isothermal, incompressible, non-Newtonian fluid in a three-dimensional flow. The flow equations were solved numerically using the commercial Computational Fluid Dynamics (CFD) code, FLUENT 6.3. The simulation setup was validated by means of comparison with a numerical test case from literature. In the second stage, the fiber orientation evolution equation was discretized and numerically solved in Matlab, utilizing coefficients obtained from data imported from FLUENT. The third stage included a comparison of the performances of three closure models; linear, quadratic and hybrid, used to complete the fiber orientation evolution equation. The experimental data set was obtained from literature and it consisted of fiber orientation measurements from an injection molded, film-gated rectangular strip. For all three closure models, the dominant orientation component was directed along the flow direction – the a11 orientation tensor. The numerically computed a11 orientation tensor produced results which agreed well with the experimental data where the typical deviation range observed was about 45%. For the non-dominant components: a22, a33 and a13 the simulation results demonstrated better agreement with the experimental data set, however there was a broader range of deviation among the three closures than was observed with the a11 component. From an analysis of the deviation trends for all three closure models, it was concluded that for the film-gated mold geometry simulated here, the linear closure model performed best. A qualitative comparison of the numerical and experimental data trends showed that the hybrid closure model demonstrated over-prediction of the a11 orientation in regions of high shear rate in the flow. In regions of low shear rate, near the midplane of the flow, all three models demonstrated significant under-prediction of the a11 orientation. The highest degree of agreement between the numerically obtained a11 orientation and experimental data occurred in regions of high shear for all three closures. From the analyses performed it is clear that the simulation results were in qualitative agreement with the experimental data. Nevertheless the observed deviations between simulation and experiment highlight the importance of coupling effects between the fluid momentum and fiber orientation as well as the necessity of accurate closure models.