published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Thesis (Ph. D.) -- California Institute of Technology, 2010. / Title from home page (viewed 03/22/2010). Advisor and committee chair names found in the thesis' metadata record in the digital repository. Includes bibliographical references.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

An experimental investigation was carried out to study the effect of heater surface preparation on pool boiling of nanofluids. The boiling surface was prepared using different methods: (1) using a diamond turning machine, (2) a conventional lathe machine, and (3) polished using emery sandpaper. The average surface roughness of the diamond turning machined surface was 6 nm and 470 nm for the surfaces prepared using the lathe and sandpaper. The boiling surfaces considered in this study are flat copper surfaces with a diameter of 25.4 mm. Al2O3-Water nanofluids prepared using nanoparticles with an initial size of 10 nm and concentration of 0.05%wt were used throughout the present study. In order to improve the nanofluids stability Sodium dodecylbenzenesulfonate (SDBS) was added to the base fluid (water) with a concentration of 0.1%wt. In order to understand the effect of the nanofluids and the surfactant separately and together, pool boiling experiments using distilled water only, nanofluids, distilled water plus the SDBS surfactant, and nanofluids mixed with SDBS (nanosuspensions) were carried out on clean surfaces. The nanofluids and nanosuspensions boiling experiments were followed by distilled water boiling experiments in order to assess the change of the surface characteristics due to any nanoparticles deposition. The same set of boiling experiments was carried out on each of the three prepared surfaces. The experimental results indicated that for the smooth and rough machined surfaces, the heat transfer coefficient was increased for the nanofluids and the nanosuspensions with respect to distilled water. Distilled water boiling experiments on the unclean (used) surfaces showed that the heat transfer behavior is almost similar to the distilled water on the clean surface, which indicates that the deposition on the smooth and rough machined surfaces was minimal and hence the enhancement in the heat transfer was due to the change in the thermo-physical properties of the nanofluids and not due to the change in the heater surface condition. A similar trend was observed in the case of the polished surface in which case nanofluids and nanosuspensions resulted in an enhancement in the rate of heat transfer. However, distilled water boiled on unclean surfaces showed that the boiling curve has shifted to the left compared with the curve of the distilled water on the clean surface. Boiling of distilled water on unclean surfaces showed that the boiling curve was enhanced, which can be attributed to the change in the surface condition and the change in the thermo-physical properties of nanofluids. Photographs of the boiling surfaces and surface measurements taken before and after the nanofluids and nanosuspensions experiments showed that the machined surfaces had less nanoparticles deposition than the sandpaper polished surface. These results indicate that the method of surface preparation has a significant effect on nanoparticles deposition and consequently on the pool boiling heat transfer in which the polished surface tends to have higher number of the active nucleation sites. / Thesis / Master of Applied Science (MASc)

Colloidal solvents, containing dispersed nanometer (~1-100 nm) sized particles, are categorized as nanofluids. With the growing heat loads in engineering systems that exceed the current technological limits, nanofluids are considered as an attractive option for more efficient heat removal for thermal management applications. Recent results reported in the literature show that the thermo-physical properties of coolants are enhanced considerably when seeded with very minute concentrations of nanoparticles. Hence, nanofluids research has provoked interest in thermal management applications. The convective heat transfer characteristics of nanofluids are reported in this study. Exfoliated graphite nanoparticles were dispersed in poly-alpha-olefin (PAO) at concentrations of 0.3% and 0.6% (by weight). The heat flux into a convective cooling apparatus was monitored and the results for nanofluid and the base fluid are presented. Thermo-physical properties of the nanofluid were measured and compared with the base fluid. The thermo-physical properties of the fluid are observed to increase with the addition of the nanoparticles. The specific heat of nanofluid was increased by ~50% compared to PAO. The thermal diffusivity was enhanced by ~400% compared to PAO. The viscosity of the nanofluid was enhanced by 10-1000 times compared to PAO. The viscosity of the nanofluid was observed to increase with temperature while the viscosity of PAO decreases with temperature. The convective heat flux was enhanced by the nanofluids by up to ~8 % for experiments performed at different heat inputs. The experimental results show that the convective heat transfer enhancement potentially results from the precipitation of nanoparticles on the heated surface and results in enhanced heat transfer surfaces (“nano-fins”).

Colloidal solvents, containing dispersed nanometer (~1-100 nm) sized particles, are categorized as nanofluids. With the growing heat loads in engineering systems that exceed the current technological limits, nanofluids are considered as an attractive option for more efficient heat removal for thermal management applications. Recent results reported in the literature show that the thermo-physical properties of coolants are enhanced considerably when seeded with very minute concentrations of nanoparticles. Hence, nanofluids research has provoked interest in thermal management applications. The convective heat transfer characteristics of nanofluids are reported in this study. Exfoliated graphite nanoparticles were dispersed in poly-alpha-olefin (PAO) at concentrations of 0.3% and 0.6% (by weight). The heat flux into a convective cooling apparatus was monitored and the results for nanofluid and the base fluid are presented. Thermo-physical properties of the nanofluid were measured and compared with the base fluid. The thermo-physical properties of the fluid are observed to increase with the addition of the nanoparticles. The specific heat of nanofluid was increased by ~50% compared to PAO. The thermal diffusivity was enhanced by ~400% compared to PAO. The viscosity of the nanofluid was enhanced by 10-1000 times compared to PAO. The viscosity of the nanofluid was observed to increase with temperature while the viscosity of PAO decreases with temperature. The convective heat flux was enhanced by the nanofluids by up to ~8 % for experiments performed at different heat inputs. The experimental results show that the convective heat transfer enhancement potentially results from the precipitation of nanoparticles on the heated surface and results in enhanced heat transfer surfaces (“nano-fins”).

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

Thesis (M.S.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on January 4, 2008) Includes bibliographical references.

Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 111-117). Also available on the World Wide Web.