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Single bubble theoretical models were developed to simulate heterogeneous pool boiling (HPB) of pure liquids on a single site in a natural surface. / A dynamic model was developed for bubble nucleation in HPB for predicting the waiting time of a bubble for given boiling conditions. The variation of the surface temperature was incorporated through an additional model proposed for the same. Its application to equilibrium pool boiling of pure components and binary mixtures predicted waiting times in agreement with the experimental values obtained here. / A dynamic single bubble departure model considering all the forces acting on a bubble was developed to predict the departure diameter and the wall superheat required for continuously forming bubbles (CFB). The model was developed for 3 temperature distributions (TD) in the thermal layer. It was also applied to binary mixtures and the predictions were found to be in agreement with previously available data. For Ja(,m) (LESSTHEQ) 15, bubble departure is controlled by the surface tension and buoyancy forces while for Ja(,m) > 15, liquid inertia also becomes important. / The static single bubble nucleation and departure models of Howell and Siegel were extended. The equations were developed for 3 TD in the thermal layer. The former predicted the wall superheat required for nucleating a bubble on the site, while the latter predicted the departure diameter and the wall superheat for the CFB.

Thesis--University of Wisconsin--Madison. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 206-212).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1983. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 254-259).

Thesis (M.S.)--University of Wisconsin--Madison, 1980. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 76-77).

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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)

Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Boiling heat transfer is a critical process in large-scale industrial applications such as steam engines and heat exchangers in power plants, and in microscopic heat transfer devices such as heat pipes and microchannels for cooling electronic chips. Enhancing boiling heat transfer thus has great significance on lots of energy transportation and utilization systems. Recent studies has suggested that micro/nano structured surfaces can produce considerably different boiling heat transfer curves than normal plain surfaces, resulting in different values of the critical heat flux (CHF) and heat transfer coefficient (HTC). In this thesis, pool boiling on several new micro/nano structured surfaces was experimentally investigated to further understand the mechanism of boiling heat transfer and increase boiling performance. We first evaluated enhanced boiling heat transfer on three kinds of micro/nano structured super-hydrophilic surfaces: 1) nanowire coated super-hydrophilic surfaces, 2) hybrid microscale cavity and nanowire structured surfaces and 3) hybrid microscale pillar and nanowire structured surfaces. All three surfaces showed significant enhancement of CHF and HTC compared to plain silicon surfaces. Combined micro/nano structured surfaces presented better performance than nanowire coated surfaces suggesting that both active nucleation density and surface roughness significantly affect boiling heating transfer. Experimental investigations indicate an optimum design both in size (~ 20μ𝑚) and density (between 0 and 10000=cm^2) of cavities for microscale cavity/nanowire structured surfaces. The highest CHF and peak HTC values were obtained on microscale pillar/nanowire structured surfaces. Among the test surfaces, the largest enhancements of CHF and peak HTC were 228% and 298%, respectively, compared to plain silicon surfaces. For a better understanding of the boiling phenomena, pool boiling on super-hydrophobic surfaces was also studied. We found that, for super-hydrophobic surfaces, the major heat transfer mechanism at the initial boiling regime is natural convection of liquid water. In conclusion, micro/nano structured surfaces can greatly influence nucleate boiling heat transfer. The various physical attributes employed with the structured surfaces further revealed the profound influence of surface topography on enhancing boiling heat transfer. / 2031-01-01

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