Constructal structures for best system performance of nanofluids

Abstract

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.