Traditionally data centers were designed with reliability, functionality, and up front cost as the primary design drivers, with secondary consideration given to cost of operation and sustainability. In the past decade, data center design has shifted giving more weight to operational energy use and sustainability. This thesis outlines two work streams aimed at allowing data center designers and operators to design more efficient, cost effective, sustainable data centers. The first track deals with evaluating data center sustainability quickly and easily. The metric chosen to quantify data center sustainability is life-cycle exergy consumption. Other sustainability indicators are investigated, and reasons for choosing this metric for data centers are provided. Two case studies are provided to illustrate the sustainability ramifications of different design and operation decisions. A Life-Cycle Assessment (LCA), perhaps the most widely accepted industry practice of assessing the environmental ramifications of a product or process, is also completed for comparison purposes. A MATLAB based graphical user interface (GUI) developed to perform a life-cycle exergy consumption analysis (LCEA) on data centers was designed and its usefulness is demonstrated.

The second track of this thesis deals with expeditious velocity and temperature field prediction in data centers, developed to be an ultrafast alternative to conventional computational fluid dynamics (CFD) code. This expeditious predictive scheme is based on a potential flow velocity prediction method with a Rankine vortex superposition correction scheme to correct for physical processes neglected by the relatively simpler but much more computationally efficient potential flow equations. Energy considerations are accomplished through convective energy transport equations. It is shown that without the Rankine vortex superposition correction, potential flow is inadequate for accurately predicting velocity and temperature fields in data centers. It is also shown that with the Rankine vortex superposition, in areas of interest within data centers, temperature predictions are as accurate or are more accurate than a commercially available, conventional CFD software package that uses the full Navier-Stokes and energy equations to solve for velocity and temperature fields.

Three case studies are carried out based on experiments that were conducted in a data center

at HP Labs in Palo Alto, California. Measurements taken in that data center are used for comparison with CFD predictions from ANSYS FLUENT and a MATLAB based, potential flow with Rankine vortex code named COMPACT (Compact Model of Potential Flow and Convective Transport) . FLUENT's temperature field predictions are also compared with COMPACT's predictions for both computational speed and accuracy.

The ultimate goal of these two streams, when combined, is to give a data center designer or operator an ultrafast, comprehensive tool to make data center sustainability decisions while constrained by operational temperature limit considerations. It is shown in this work that such a goal is attainable.

The film rupture process that dictates merging of adjacent bubbles is particularly important in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. To understand the mechanisms of bubble merging in nano-structured boiling surfaces and in nanotubes, it is useful to explore film stability and onset of rupture at the molecular level. This dissertation reports the results of such an investigation combining three strategies that includes a new formulation of capillarity theory for free liquid films, molecular dynamics (MD) simulations using similar interaction potentials and bubble merging experiments. Two forms of our molecular film capillarity theory are developed here: one for non-polar fluids based on a Lennard-Jones interaction potential, and a second specifically for water using a modified treatment of the SPC/E interaction potential that accounts for water dipole interactions. The capillarity theory provides theoretical relationships among parameters that govern film structure and thermophysical behavior, while the companion MD simulations allow more detailed molecular level exploration of the film thermophysics. Results obtained with theoretical models and MD simulation studies indicate that the wave instability and the lack of thermodynamics intrinsic stability can lead to rupture of the liquid film, as its thickness decreases below a critical value. It is further predicted that wave instability predominates as an onset of rupture mechanism for liquid films of macroscopic extent, but for free liquid films with nanoscale lateral extent (in, for example, nanostructured boiling surfaces), lack of core stability is more likely to be the mechanism. For electrolyte aqueous solutions, theoretical models and MD simulation studies suggest that dissolved salts tend to alter the surface tension at liquid vapor interfaces and affect the stability of the free liquid film between adjacent bubbles. Bubble merging experiments are designed and carried out for various electrolyte aqueous solutions. The interaction of pairs of bubbles injected into solution with different dissolved salt concentrations is studied experimentally to determine the probability of merging from statistics for ensembles of bubble pairs. The results of these experiments indicate that for some types of salts, very low dissolved salt concentrations can strongly reduce the tendency of adjacent bubbles to merge, implying that the presence of the dissolved salt in such cases strongly enhances the stability of the free liquid film between adjacent bubbles. The trends are compared to predictions of free liquid film stability by wave instability theory and MD simulations.

Thermal management of systems under high heat fluxes on the order of hundreds of W/cm^2 is important for the safety, performance and lifetime of devices, with innovative cooling technologies leading to improved performance of electronics or concentrating solar photovoltaics. A novel, spiraling radial inflow microchannel heat sink for high flux cooling applications, using a single phase or vaporizing coolant, has demonstrated enhanced heat transfer capabilities. The design of the heat sink provides an inward swirl flow between parallel, coaxial disks that form a microchannel of 1 cm radius and 300 micron channel height with a single inlet and a single outlet. The channel is heated on one side through a conducting copper surface, and is essentially adiabatic on the opposite side to simulate a heat sink scenario for electronics or concentrated photovoltaics cooling. Experimental results on the heat transfer and pressure drop characteristics in the heat sink, using single phase water as a working fluid, revealed heat transfer enhancements due to flow acceleration and induced secondary flows when compared to unidirectional laminar fully developed flow between parallel plates. Additionally, thermal gradients on the surface are small relative to the bulk fluid temperature gain, a beneficial feature for high heat flux cooling applications. Heat flux levels of 113 W/cm^2 at a surface temperature of 77 deg C were reached with a ratio of pumping power to heat rate of 0.03%. Analytical models on single phase flow are used to explore the parametric trends of the flow rate and passage geometry on the streamlines and pressure drop through the device.

Flow boiling heat transfer and pressure drop characteristics were obtained for this heat sink using water at near atmospheric pressure as the working fluid for inlet subcooling levels ranging from 20 to 80 deg C and mean mass flux levels ranging from 184-716 kg/m^2s. Flow enhancements similar to single phase flow were expected, as well as enhancements due to increased buoyant forces on vapor bubbles resulting from centripetal acceleration in the flow which will tend to draw the vapor towards the outlet. This can also aid in the reduction of vapor obstruction of the flow. The flow was identified as transitioning through three regimes as the heat rate was increased: partial subcooled flow boiling, oscillating boiling and fully developed flow boiling. During partial subcooled flow boiling, both forced convective and nucleate boiling effects are important. During oscillating boiling, the system fluctuated between partial subcooled flow boiling and fully developed nucleate boiling. Temperature and pressure oscillations were significant in this regime and are likely due to bubble constriction of flow in the microchannel. This regime of boiling is generally undesirable due to the large oscillations in temperatures and pressure and design constraints should be established to avoid large oscillations from occurring. During fully developed flow boiling, water vapor rapidly leaves the surface and the flow does not sustain large oscillations. Reducing inlet subcooling levels was found to reduce the magnitude of oscillations in the oscillating boiling regime. Additionally, reduced inlet subcooling levels reduced the average surface temperature at the highest heat flux levels tested when heat transfer was dominated by nucleate boiling, yet increased the average surface temperatures at low heat flux levels when heat transfer was dominated by forced convection. Experiments demonstrated heat fluxes up to 301 W/cm^2 at an average surface temperature of 134 deg C under partial subcooled flow boiling conditions. At this peak heat flux, the system required a pumping power to heat rate ratio of 0.01%. This heat flux is 2.4 times the typical values for critical heat flux in pool boiling under similar conditions.

The first part of the dissertation presents a study that implements micro and nano scale engineered surfaces for enhancement of evaporation and boiling phase change heat transfer in both capillary wick structures and pool boiling systems. Capillary wicking surfaces are integral components of heat pipes and vapor chamber thermal spreaders often used for thermal management of microelectronic devices. In addition, pool boiling systems can be encountered in immersion cooling systems which are becoming more commonly investigated for thermal management applications of microelectronic devices and even data centers. The latent heat associated with the change of state from liquid to vapor, and the small temperature differences required to drive this process yield great heat transfer characteristics. Additionally, since no external energy is required to drive the phase change process, these systems are great for portable devices and favorable for reduction of cost and energy consumption over alternate thermal management technologies.

Most state of the art capillary wicks used in these devices are typically constructed from sintered copper media. These porous structures yield high surface areas of thin liquid film where evaporation occurs, thus promoting phase change heat transfer. However, thermal interfaces at particle point contacts formed during the sintering process and complex liquid/vapor flow within these wick structures yield high thermal and liquid flow resistances and limit the maximum heat flux they can dissipate. In capillary wicks the maximum heat flux is typically governed by the capillary or boiling limits and engineering surfaces that delay these limitations and yield structures with large surface areas of thin liquid film where phase change heat transfer is promoted is highly desired. In this study, biporous media consisting of microscale pin fins separated by microchannels are examined as candidate structures for the evaporator wick of a vapor chamber heat pipe. Smaller pores are used to generate high capillary suction, while larger microchannels are used to alleviate flow resistance. The heat transfer coefficient is found to depend on the area coverage of a liquid film with thickness on the order of a few microns near the meniscus of the triple phase contact line. We manipulate the area coverage and film thickness by varying the surface area-to-volume ratio through the use of microstructuring. In some samples, a transition from evaporative heat transfer to nucleate boiling is observed. While it is difficult to identify when the transition occurs, one can identify regimes where evaporation dominates over nucleate boiling and vice versa. Heat fluxes of 277.0 (± 9.7) W/cm2 can be dissipated by wicks with heaters of area 1 cm2, while heat fluxes up to 733.1 (± 103.4) W/cm2 can be dissipated by wicks with smaller heaters intended to simulate local hot-spots.

In pool boiling systems that are encountered in immersion cooling applications, the heat transfer coefficient (HTC) is governed by the bubble nucleation site density and the agitation in the liquid/vapor flow these bubbles produce when they detach from the surface. The nucleation site density and release rate is usually determined by the surface morphology. Another important parameter in pool boiling systems is the maximum heat flux (CHF) that can safely be dissipated. In practice, this quantity is about two orders of magnitude smaller than limitations suggested by kinetic theory. For essentially infinite, smooth, well wetted surfaces, hydrodynamic instability theories capturing liquid/vapor interactions away from the heated surface have been successful in predicting CHF. On finite micro and nano structured surfaces where applying the hydrodynamic theory formulation is not easily justified, other effects may contribute to phase change heat transfer characteristics. Here, we also present a pool boiling study on biporous microstructured surfaces used in capillary wick experiments. Structures are manipulated by reduction of pore size to determine if increased capillary pressure can enhance rewetting from heater edges and delay CHF. A comparative study between the two experimental systems indicates that while the capillary limitation is significant in capillary wick experiments, for these well wetted microstructured surfaces used in pool boiling systems the hydrodynamic limitation defined based on heater size causes the occurrence of CHF. Other hierarchical nanowire surfaces containing periodic microscale cavities are investigated as well and are seen to yield a ~2.4 fold increase in heat transfer coefficient characteristics while not compromising CHF compared to surfaces where cavities are not present. These studies indicate pathways for enhancement of heat transfer coefficient via implementing hierarchical structures, while no clear method in increasing CHF is determined for finite size surfaces of various morphologies.

In the second part of this dissertation, solar energy storage is sought in `phase change' of photochromic molecular systems: the storage of solar energy in the chemical bonds of photosensitive molecules (a photochemical reaction) and subsequent recovery of the energy in a back reaction in the form of heat, reversibly. These molecular systems are interesting alternatives to photovoltaic and solar thermal technologies which cannot satisfy the needs of load leveling, or for portable municipal heating applications. Typically made of organic compounds, these molecules have become known for rapid decomposition, short energy storage time scales and poor energy storing efficiencies. Thus, they have been abandoned as practical solar energy storage systems in the past several decades. On the other hand, organometallic molecular systems have not been extensively probed for these applications. Recent research has indicated that organometallic (fulvalene)diruthenium FvRu2 has demonstrated excellent energy storage characteristic and durability. Here, we report on a full cycle molecular solar thermal (MOST) microfluidic system based on a bis(1,1-dimethyltridecyl) substituted derivative of FvRu2 that allows for long term solar energy storage (110 J/g), and "on demand" energy release upon exposure to a catalyst. The microfluidic systems developed here are excellent for photoconversion characterization and scrutinizing potential catalysts and can be extended to studying many other molecular systems. The objective of the work presented here is to demonstrate that "on demand" solar energy storage and release in MOST systems is viable and motivate future research on other photochromic organometallic systems.

In low-income countries, 93% of waste is burned or dumped while only 2% of waste in high incomecountries is improperly disposed of in this manner. This research focuses on the plastic waste problem in Uganda—a country where 600 tonnes of plastic waste is generated daily and only 6% is collected. Polyethylene terephthalate (PET) is the most prevalent type of plastic waste in Uganda, but it cannot be recycled anywhere in East Africa. Plastic waste in developing countries is a complex, multifaceted problem, and little comprehensive research exists on the subject. Through a technical, social, and business approach, I attempt to present a holistic analysis.

An extended exergy analysis was conducted to quantify the resource use and environmentalimpact of seven different disposal and recycling options for plastic waste that are feasible with Uganda’s limited infrastructure. The results revealed that recycling into a new product is the most resource efficient end-of-life option for PET plastic in Uganda and that transportation can be a major exergy cost. To reduce transportation and keep economic benefits within a community rather than exporting the majority of the profits, a local, circular economy recycling solution was proposed. Additionally, a fundamental phase change heat transfer analysis was used to model the solar drying process many Ugandan recyclers use to dry their plastic after washing.

To demonstrate the feasibility of this proposed distributed recycling model, I co-foundeda social enterprise in Gulu, Uganda. We developed a process and built local machines to transform PET waste into durable, beautiful wall tiles making us the only recyclers of PET in Uganda. Using a community-based approach, emphasizing product-market fit, and pushing the bounds of recommended polymer processing practices, we developed and proved the viability of a solution that did not before exist. This dissertation presents a framework and an example of how to create sustainable solutions for complex technical and social problems that are technically viable, financially sustainable, and embraced by community members.

This dissertation summarizes results of an experimental exploration of heat transfer during vaporization of a water droplet deposited on a nanostructured surface at a temperature approaching and exceeding the Leidenfrost point for the surface and at lower surface temperatures 10-40 degrees C above the saturated temperature of the water droplet at approximately 101 kPa. The results of these experiments were compared to those performed on bare smooth copper and aluminum surfaces in this and other studies.

The nanostructured surfaces were composed of a vast array of zinc oxide (ZnO) nanocrystals grown by hydrothermal synthesis on a smooth copper substrate having an average surface roughness of approximately 0.06 micrometer. Various nanostructured surface array geometries were produced on the copper substrate by performing the hydrothermal synthesis for 4, 10 and 24 hours. The individual nanostructures were randomly-oriented and, depending on hydrothermal synthesis time, had a mean diameter of about 500-700 nm, a mean length of 1.7-3.3 micrometers,and porosities of approximately 0.04-0.58.

Surface wetting was characterized by macroscopic measurements of contact angle based on the droplet profile and calculations based on measurements of liquid film spread area. Scanning electron microscope imaging was used to document the nanoscale features of the surface before and after the experiments. The nanostructured surfaces grown by hydrothermal synthesis for 4 and 24 hours exhibited contact angles of approximately 10, whereas the surfaces grown for 10 hours were superhydrophilic, exhibiting contact angles typically less than 3 degrees.

In single droplet deposition experiments at 101 kPa, a high-speed video camera was

used to document the droplet-surface interaction. Distilled and degassed water droplets

ranging in size from 2.5-4.0 mm were deposited onto the surface from heights ranging from approximately 0.2-8.1 cm, such that Weber numbers spanned a range of approximately 0-99. Heat transfer coefficients were determined from thermal measurements in the test apparatus. All experiments were conducted inside an ISO Class 5 clean room enclosure.

It was observed that when a liquid water droplet impinged upon the ZnO nanostructured

at surface temperatures less than 140 degrees C, the nominally spherical droplet spread into a thin film over the surface. The film thickness depended on many parameters but in general it measured approximately 100-400 micrometers. As a result, it was found that the droplet evaporated by film evaporation without initiating nucleate boiling. At wall superheat levels of 10-20 degrees C, it was found in some cases that the heat transfer coefficients were nearly 4 times greater than for those of nucleate boiling at the same superheat level. For these conditions, no bubble nucleation was observed visually, and, nevertheless, extremely high heat transfer coefficients resulting from rapid evaporation of the thin liquid film formed by the spreading droplet were observed.

At high wall superheat levels, the vaporization process exhibited Leidenfrost droplet vaporization. The extreme wetting of the nanostructured surfaces resulted in high Leidenfrost transition temperatures in the range of 310-376 degrees C, among the highest in the literature, exceeding those exhibited by bare metal surfaces by 100 degrees C or more. The Leidenfrost transition was detected from a recording of the acoustic signal generated from each experiment during the deposition and subsequent evaporation process. It was defined as the first point for which there is no disturbance to the acoustical signal in the form of a sizzling sound beyond the initial violent popping generated during the droplet deposition. The results document a trend of increasing Leidenfrost temperature with decreasing contact angle, which is consistent with earlier studies. The results of this study are compared with earlier work in this area and the implications for applications are discussed.

Within the types of condensation that can form on a surface, dropwise condensation has been previously shown to produce condensation heat transfer coefficients up to an order of magnitude greater than film condensation. Among dropwise condensation investigations, it has also been shown that smaller droplets result in higher heat transfer coefficients. An area that is currently under investigation within condensation advancements is creating superhydrophobic surfaces that can sustain smaller droplets during condensation. However, as droplet diameters are reduced to sizes comparable to the flow's mean free path, various mechanisms are expected to affect transport as the flow transitions from a continuum to free molecular flow: non-continuum transport effects, curvature effects on surface tension and on saturation conditions, and interactions with nearby droplets.

In this dissertation, we investigate the limits of heat transfer performance on surfaces that strive to sustain dropwise condensation for smaller droplets. We explore and compare the limitations of dropwise condensation as mean droplet sizes are reduced to micro and nanoscales using three different models: one that uses an approximation for micro and nanoscale transport on an array of droplets, one that uses the DSMC method to simulate transport on a single droplet, and a third model that uses the DSMC method to simulate transport on an array of droplets.

We found the three different models to show similar trends; dropwise condensation heat transfer coefficients increased as droplet sizes were reduced, but only up to a certain point where non-continuum transport and curvature effects became significant. For pure steam condensing on a cold wall at standard atmospheric condition with 3 degrees Celsius of subcooling, drop- wise condensation heat transfer coefficients were found to peak when droplets approached diameters near 200 nm. The effects of varying contact angle, thermal accommodation, pres- sure, amount of subcooling, spacing between droplets, and introduction of noncondensible particles into the system are also explored and discussed in detail.

Novel energy technologies have the potential to address climate change by efficiently using natural resources and reducing greenhouse gas emissions into our shared global atmosphere. In preparation for a future society powered by renewables, engineering solutions must include environmentally conscious and cost-effective methods to store, transport, and convert energy.

I have numerically investigated the use of latent thermal energy storage (TES) technology, with solid to liquid phase change material (PCM), to shift cooling loads to off-peak hours. I first non-dimensionalized differential equations describing sensible and latent heat transfer in the PCM, and their finite difference counterparts, in order to facilitate scaling for the wide array of asynchronous cooling applications that could benefit from this technology. Next, I compared closed form analytical solutions and experimental testing of a TES prototype with the numerical prediction of its melting and freezing processes. Both the analytical solutions and the experimental tests matched the predicted results within 10% agreement, validating the computational model’s capacity to capture the physics governing the transient behavior of the device with high precision and accuracy. As no adjustable parameters were tuned to maximize agreement, the numerical model can be effectively employed to determine the performance of different designs without the need to fabricate, charge, and test them.

Using this model, I explored potential improvements to power and refrigeration cycles for power plants and commercial buildings integrated with thermal storage. I accomplished this task by taking simple representations of these systems in MATLAB and transforming them into declared relationships between complex components using the dynamic programming language of Modelica. Simulations in integrated development environments of both languages demonstrate improvements of up to a 1.4% increase in power plant energy output and a 2.4% decrease in building chiller energy consumption with thermal storage. With thoughtful selection of the phase change material and better charging and discharging control strategies for the thermal energy storage, further performance enhancement of such systems can be achieved.