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Constructal structures for best system performance of nanofluidsBai, Chao, 柏超 January 2012 (has links)
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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Heat transport in nanofluids and biological tissuesFan, Jing, 范菁 January 2012 (has links)
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
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