Constructal structures for best system performance of nanofluids

Autor:	Bai, Chao, 柏超
Jazyk:	angličtina
Rok vydání:	2012
Předmět:	Nanofluids - Mechanical properties.
Druh dokumentu:	PG_Thesis
DOI:	10.5353/th_b4786956
Popis:	Nanofluids are two-phase mixtures of base fluids and nanoparticles. They possess unique thermal, magnetic, electronic, optical and wetting properties, and thus have tremendous applications in many fields. For practical applications of nanofluids in heat-transfer systems, we often try to achieve a global aim such as optimization of system highest temperature and optimization of system overall thermal resistance. To improve energy efficiency, attention should focus on designing nanofluids for the best global performance. As indicated by constructal theory, flow structures emerge from the evolutionary tendency to generate faster flow access in time and easier flow access in configurations that are free to morph. Constructal theory can not only predict natural flow architectures but also guide design of flow systems. In this thesis, constructal design is applied to study nanofluid heat conduction such that the system (global) performance can be constantly improved. An examination of the variation of preferred heat-transfer modes for different matter states concludes that the preferred heat-transfer modes for solid, liquid and gas are conduction, convection and radiation, respectively. After an analogy analysis of plasma heat conduction and nanofluid heat conduction, it is proposed that forming continuous particle structures inside base fluids may enhance the heat conduction in nanofluids. Staring from the conventional nanofluids with particles dispersed in base fluids (dispersed configuration of nanofluids), we first perform a constructal design of particle volume fraction distribution of four types of nanofluids used for heat conduction in eight systems. The constructal volume fraction distributions are obtained to minimize system overall temperature differences and overall thermal resistances. The constructal overall thermal resistance is found to be an overall property fixed only by the system global geometry and the average thermal conductivity of nanofluids. The constructal nanofluids that maximize the system performance under dispersed configuration are the ones with higher particle volume fraction in region of higher heat flux density. Based on the proposal of forming continuous particle structures inside base fluids, blade configurations of nanofluids are analyzed analytically and numerically for both heat-transferring systems and heat-insulating systems. Comparisons are made with dispersed configurations of nanofluids with constructal particle volume fraction distributions or thermal conductivities of upper or lower bounds. The superiority of blade configuration is always very obvious even with rather simple particle structures. As the blade structures are more sophisticatedly designed, system performance of blade configuration will become even better. To improve the particle structure design, efforts are put on optimizing crosssectional shape of particle blade to achieve better system performance. The triangular-prism-shaped blade is shown to perform the best. Since heat conduction and fluid flow inside trees follow the same linear transport mechanism, the prevalent leaf structures in nature are expected to provide some guidelines for the design of blade-configured heat-conduction system. Analytical and numerical studies are thus done on the quasi-rhombus-shaped and quasi-sector-shaped systems up to the one branching level. More sophisticated blade shapes are verified to lead to better system performance. The advantage of quasi-rhombusshaped system compared to quasi-sector-shaped system is also shown. published_or_final_version Mechanical Engineering Doctoral Doctor of Philosophy
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