Conjugate Heat Conduction In High Conductive Composite Materials Filled With Artificially Designed Filler Shapes

Heat exchangers have been widely used in various industries. Conventional heatexchangers are mainly made of monolithic metals and metal alloys, which are corrosive inacid environment. To solve this problem, polymers instead of metals have been used as thematerials to make heat exchangers. However the thermal conductivity of these polymers israther low, which makes the polymers heat exchangers poor in performance. To increase thethermal conductivity of polymers, adding fillers of high conductivity to the base materials hasbeen practiced as an efficient method to make new materials for heat exchangers. In this way,the thermal conductivity of the polymers is increased while their virtue to resist corrosion canbe largely kept.To reflect the effects of filler, various correlations and mathematical models have beenproposed to estimate these effects. Regretfully, none of them is a general correlation that canpredict the effective thermal conductivity accurately, for different fillers and contents. Thesestudies believe that composite is a simple mixing of fillers and polymer base, fillers contentand its thermal conductivity are the two dominating factors influencing the overall effectivethermal conductivity of polymer composites. However the effective thermal conductivity ofthe composite materials is not simple mixing between the fillers and polymer base.Interactions between the fillers and polymers, the shapes and distributions of fillers, andinteractions between fillers also have great impacts on the final thermal conductivity of filledcomposites materials. These impact factors have been investigated by the perspective ofconjugate heat conductivity in heterogeneous system. Aitifically designed filler shapes areused. The main works are summarized as following:(1) The conjugate heat conduction models are bulit. From the points of view of heattransfer, the effective thermal conductivity is a conjugate heat transfer problem between thefillers and the base materials. It is not a simple mixing of two materials. The accuracy ofnumerical results is determined by heat conduction models. The model is small, but largeenough to represent composites. The size of model must be large enough to contain theessential number of fillers. The accuracy of the results calculated and the time consumed arecontrolled by mesh refinement.(2) An inverse problem approach with nonlinear optimization programming technique isproposed to search for the optimized filler shapes for filled composite materials. Theoptimization process can be regarded as a growth process which comprises of three stages: the handicapped stage, the infant stage and the adult stage. Correspondingly, the optimized fillershapes are vertical dumbbells, less-developed I shapes and full developed I shapes,respectively; The tendency in filler shape optimization is to increase the filler height to itspermitted limits first, then its width; Of the six shapes of fillers tested, the I shaped fillers arethe best. At the same filler content of0.2, the thermal conductivity of materials with I fillers is1.5time higher than that with spherical fillers.(3) The interactions between the fillers and the base material have been considered bysolving the heterogeneous two-dimensional conjugate heat conduction problem in compositematerials. It is found that the filler shapes and their orientation angles have great impacts onthe effective thermal conductivity of the composite materials. Three-like or constructal shapesare the best choice for the intensification of heat conductivity. The slices fillers and fibers areeffective only when they are distributed parallel to heat flow directions.(4) The conjugate heat conduction characteristics of composites with high filler contentsare studied. A conjugate heat conduction model is built and applied to predict the thermalconductivity of composite with wide range filler contents. This model considers the contactsbetween fillers. The comparisons between modeling results and verification experimentsindicate that this model predicts the thermal conductivity of composites well at a wide volumefraction.(5) The conjugate heat conduction models of composites with fillers randon distributionare built. Then it is used to reflect the effects of fillers. Best shapes filler should have longheat conduction distances and large contact areas even when they are placed unfavorably. Theeffects of distributions are different for various shapes fillers. It is little use to increase theconductivity of the composite by solely increasing the conductivity of the fillers, if the fillerconductivity has already been1000times higher than the base materials.(6) Expanded graphite (EG) sheets have been recognized as an economic and efficientfiller material to make composites of high thermal conductivity. Random models aresatisfactory in predicting the heat conductivity augmentation in polymers by EG sheets. Bestsheet fillers should have larger height to thickness ratios. EG sheets are good candidate forfillers because they can be easily processed. The layered structure makes them promising inproducing ultra thin sheets, which are beneficial for heat conduction.