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

The present work contains two parts: nanofluids and bioheat transport, both involving multiscales and sharing some common features. The former centers on addressing the three key issues of nanofluids research: (i) what is the macroscale manifestation of microscale physics, (ii) how to optimize microscale physics for the optimal system performance, and (iii) how to effectively manipulate at microscale. The latter develops an analytical theory of bioheat transport that includes: (i) identification and contrast of the two approaches for developing macroscale bioheat models: the mixture-theory (scaling-down) and porous-media (scaling-up) approaches, (ii) rigorous development of first-principle bioheat model with the porous-media approach, (iii) solution-structure theorems of dual-phase-lagging (DPL) bioheat equations, (iv) practical case studies of bioheat transport in skin tissues and during magnetic hyperthermia, and (v) rich effects of interfacial convective heat transfer, blood velocity, blood perfusion and metabolic reaction on blood and tissue macroscale temperature fields. Nanofluids, fluid suspensions of nanostructures, find applications in various fields due to their unique thermal, electronic, magnetic, wetting and optical properties that can be obtained via engineering nanostructures. The present numerical simulation of structure-property correlation for fourteen types of two/three-dimensional nanofluids signifies the importance of nanostructure’s morphology in determining nanofluids’ thermal conductivity. The success of developing high-conductive nanofluids thus depends very much on our understanding and manipulation of the morphology. Nanofluids with conductivity of upper Hashin-Shtrikman bounds can be obtained by manipulating structures into an interconnected configuration that disperses the base fluid and thus significantly enhancing the particle-fluid interfacial energy transport. The numerical simulation also identifies the particle’s radius of gyration and non-dimensional particle-fluid interfacial area as two characteristic parameters for the effect of particles’ geometrical structures on the effective thermal conductivity. Predictive models are developed as well for the thermal conductivity of typical nanofluids. A constructal approach is developed to find the constructal microscopic physics of nanofluids for the optimal system performance. The approach is applied to design nanofluids with any branching level of tree-shaped microstructures for cooling a circular disc with uniform heat generation and central heat sink. The constructal configuration and system thermal resistance have some elegant universal features for both cases of specified aspect ratio of the periphery sectors and given the total number of slabs in the periphery sectors. The numerical simulation on the bubble formation in T-junction microchannels shows: (i) the mixing enhancement inside liquid slugs between microfluidic bubbles, (ii) the preference of T-junctions with small channel width ratio for either producing smaller microfluidic bubbles at a faster speed or enhancing mixing within the liquid phase, and (iii) the existence of a critical value of nondimensional gas pressure for bubble generation. Such a precise understanding of two-phase flow in microchannels is necessary and useful for delivering the promise of microfluidic technology in producing high-quality and microstructure-controllable nanofluids. Both blood and tissue macroscale temperatures satisfy the DPL bioheat equation with an elegant solution structure. Effectiveness and features of the developed solution structure theorems are demonstrated via examining bioheat transport in skin tissues and during magnetic hyperthermia. / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy