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
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/161515 |
| Date | January 2012 |
| Creators | Bai, Chao, 柏超 |
| Contributors | Wang, L |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Source | http://hub.hku.hk/bib/B47869562 |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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