<p>Evaporative cooling is a highly energy efficient alternative to conventional vapor compression cooling system. The sensible cooling effect of evaporative cooling systems is well documented in the literature. Direct evaporative cooling however increases the relative humidity of the air as it cools it. This has made it unsuitable for data centers and other applications where humidity control is important. Desiccant-based dehumidifiers (liquid, solid or composites) absorb moisture from the cooled air to control humidity and is regenerated using waste heat from the data center. This work is an experimental and theoretical investigation of the use of desiccant assisted evaporative cooling for data center cooling according to ASHRAE thermal guidelines, TC 9.9. The thickness (depth) of the cooling pad was varied to study its effect on sensible heat loss and latent heat gain. The velocity of air through the pad was measured to determine its effect on sensible cooling. The flow rate of water over the pad was also varied to find the optimal flow for rate for dry bulb depression. The configuration was such that the rotary desiccant wheel (impregnated with silica gel) comes after the direct evaporative cooler. The rotary desiccant wheel was split in a 1:1 ratio for cooling and reactivation at lower temperatures. The dehumidification effectiveness of a fixed bed desiccant dehumidifier was compared with that of a rotary desiccant wheel and a thermoelectric dehumidifier. A novel condensate recovery system using the Peltier effect was proposed to recover moisture from the return air stream, (by cooling the return air stream below its dew point temperature) thereby optimizing the water consumption of evaporative cooling technology and providing suitable air quality for data center cooling. The moisture recovery unit was found to reduce the mass of water lost through evaporation by an average of fifty percent irrespective of the pad depth.</p> <p> </p>

<div> <p>The distribution of local water vapor in the surrounding air has been shown to be the driving mechanism for several phase change phenomena during dropwise condensation and condensation frosting. This thesis uses reduced-order modeling approaches, which account for the effects of the vapor distribution to predict the droplet growth dynamics during dropwise condensation in systems of many droplets. High-fidelity modeling techniques are used to further probe and quantify the heat and mass transport mechanisms that govern the local interactions between a freezing droplet and its surrounding ambient, including neighboring droplets. The relative significance of these transport mechanisms in the propagation of frost are investigated. A reduced-order analytical method is first developed to calculate the condensation rate of each individual droplet within a group of droplets on a surface by resolving the vapor concentration field in the surrounding air. A point sink superposition method is used to account for the interaction between all droplets without requiring solution of the diffusion equation for a full three-dimensional domain. For a simplified scenario containing two neighboring condensing droplets, the rates of growth are studied as a function of the inter-droplet distance and the relative droplet size. Interactions between the pair of droplets are discussed in terms of changes in the vapor concentration field in the air domain around the droplets. For representative systems of condensing droplets on a surface, the total condensation rates predicted by the reduced-order model match numerical simulations to within 15%. The results show that assuming droplets grow as an equivalent film or in a completely isolated manner can severely overpredict condensation rates.</p> <p>The point superposition model is then used to predict the condensation rates measured during condensation experiments. The results indicate that it is critical to consider a large number of interacting droplets to accurately predict the condensation behavior. Even though the intensity of the interaction between droplets decreases sharply with their separation distance, droplets located relatively far away from a given droplet must be considered to accurately predict the condensation rate, due to the large aggregate effect of all such far away droplets. By considering an appropriate number of interacting droplets in a system, the point sink superposition method is able to predict experimental condensation rates to within 5%. The model was also capable of predicting the time-varying condensation rates of individual droplets tracked over time. These results confirm that diffusion-based models that neglect the interactions of droplets located far away, or approximate droplet growth as an equivalent film, overpredict condensation rates.</p> <p>In dropwise condensation from humid air, a full description of the interactions between droplets can be determined by solving the vapor concentration field while neglecting heat transfer across the droplets. In contrast, the latent heat released during condensation freezing processes cause droplet-to-ambient as well as droplet-to-droplet interactions via coupled heat and mas transfer processes that are not well understood, and their relative significance has not been quantified. As a first step in understanding these mechanisms, high-fidelity modeling of the solidification process, along with high-resolution infrared (IR) thermography measurements of the surface of a freezing droplet, are used to quantify the pathways for latent heat dissipation to the ambient surroundings of a droplet. The IR measurements are used to show that the crystallization dynamics are related to the size of the droplet, as the freezing front moves slower in larger droplets. Numerical simulations of the solidification process are performed using the IR temperature data at the contact line of the droplet as a boundary condition. These simulations, which have good agreement with experimentally measured freezing times, reveal that the heat transferred to the substrate through the base contact area of the droplet is best described by a time-dependent temperature boundary condition, contrary to the constant values of base temperature and rates of heat transfer assumed in previous numerical simulations reported in the literature. In further contrast to the highly simplified descriptions of the interaction between a droplet and its surrounding used in previous models, the model developed in the current work accounts for heat conduction, convection, and evaporative cooling at the droplet-air interface. The simulation results indicate that only a small fraction of heat is lost through the droplet-air interface via conduction and evaporative cooling. The heat transfer rate to the substrate of the droplet is shown to be at least one order of magnitude greater than the heat transferred to the ambient air.</p> <p>Subsequently, the droplet-to-droplet interactions via heat and mass exchange between a freezing droplet and a neighboring droplet, for which asymmetries are observed in the final shape of the frozen droplet, are investigated. Side-view infrared (IR) thermography measurements of the surface temperature for a pair of freezing droplets, along with three-dimensional numerical simulations of the solidification process, are used to quantify the intensity and nature of these interactions. Two droplet-to-droplet interaction mechanisms causing asymmetric freezing are identified: (1) non-uniform evaporative cooling on the surface of the freezing droplet caused by vapor starvation in the air between the droplets; and (2) a non-uniform thermal resistance at the contact area of the freezing droplet caused by the heat conduction within the neighboring droplet. The combined experimental and numerical results show that the size of the freezing droplet relative to its neighbor can significantly impact the intensity of the interaction between the droplets and, therefore, the degree of asymmetry. A small droplet freezing in the presence of a large droplet, which blocks vapor from freely diffusing to the surface of the small droplet, causes substantial asymmetry in the solidification process. The droplet-to-droplet interactions investigated in thesis provide insights into the role of heat dissipation in the evaporation of neighboring droplets and ice bridging, and open new avenues for extending this understanding to a system-level description for the propagation of frost.</p> </div> <br>

No description available.

This master's thesis explores the role of bespoke air handling units in enhancing energy efficiency in existing buildings. The context for the study is set against the backdrop of global initiatives, including the United Nations' Sustainable Development Goals, specifically Goal 7, which emphasizes the need to improve energy efficiency to combat climate change. The significance of enhancing energy efficiency is well-established, evident both at the EU level and in national policies and regulations. Buildings represent a significant portion of the energy utilization puzzle, with substantial potential for enhancing energy efficiency, although it is often underutilized. One of the contributing factors to inefficiency is outdated ventilation systems, which lead to high thermal losses. This challenge can be addressed by retrofitting these systems with modern, efficient air handling units, thus contributing to energy conservation and cost savings. This study focuses on the adoption of bespoke air handling units adjusted to the site and capable of accommodating constraints related to factors such as space limitations in machine rooms, existing ductwork layouts, and the location of shafts. The primary goal is to empower engineers to move beyond conventional approaches, enabling them to optimize technology choices based on local conditions, specific system performance requirements, and the economic viability of each project. The aim of this study is twofold: first, to develop a methodology for designing bespoke air handling units; and second, to demonstrate the practical application of this methodology in the context of two distinct renovation projects. In line with the aim of the thesis, a design methodology for site-tailored units equipped with a two stage flat crossflow heat exchanger and an indirect evaporative cooling system was developed. The methodology delves into different aspects of data analysis, 3D modeling, and the conduct of performance calculations.The established methodology was applied in two reconstruction projects in central Stockholm, where bespoke air handling units were designed in compliance with provided technical specifications. In both scenarios, a viable option emerged for accommodating a tailored unit within the technical room situated on the first floor. For both units, the energy performance metrics signify a notable achievement in terms of heat recovery efficiency, coupled with relatively modest requirements for heating and cooling power capacity from the combined heating and cooling aircoil. However, the calculated maximum specific fan power for a single unit with heat recovery exceeded the stipulated value specified in the technical specifications, which was accepted by the client. The results of the study included air handling unit product drawings, ventilation blueprints of the technical room with the integrated air handling unit, component specifications, unit flowcharts, performance calculations, and control operating pictures. The results of this work indicate that the improvement of the building's energy efficiency is rendered feasible through the installation of bespoke air handling units in the studied reconstruction projects.

Les insectes sont soumis aux fluctuations thermiques de leur environnement mais disposent d’un panel varié de réponses comportementales, physiologiques et biochimiques pour en minimiser les effets délétères et maintenir leur intégrité physiologique. Ainsi certaines espèces régulent activement leur température interne indépendamment de la température de l’environnement. Si ces insectes peuvent s’affranchir des contraintes thermiques imposées par leur environnement, ceux qui se nourrissent du sang chaud d’hôtes vertébrés endothermes n’ont pas d’autres choix que de se confronter à une situation de stress thermique à chaque prise alimentaire. Le principal objectif de ce travail de thèse est de comprendre comment des insectes hématophages, employant des stratégies alimentaires différentes, gèrent le stress thermique associé au flux massif de chaleur engendré par l’ingestion du repas de sang. Nos résultats montrent que ces insectes ont su s’adapter en développant différentes stratégies de thermorégulation. / Insects are submitted to thermal fluctuations of their environment and have developed a wide ranged panel of behavioral, physiological and biochemical responses, to minimize the subsequent deleterious effects and maintain their physiological integrity. Some species actively regulate their internal temperature independently of the temperature of the environment. If these insects can overcome the constraints imposed by their thermal environment, those that feed on warm-blooded vertebrate hosts have no choice but to confront a situation of thermal stress at each feeding event. The main objective of this work is to understand how bloodsucking insects manage heat stress associated with the massive flow of heat generated by the ingestion of the blood meal. Our results show these insects have developed different strategies of thermoregulation to protect themselves from overheating.

Since Feynman's realization of using quantum systems to investigate quantum dynamics, interest in creating controllable quantum systems to simulate condensed matter phenomenon has been high. With the realization of BECs in 1995, the realization of a relatively clean testbed for simulating some of these phenomenon became a reality. My PhD research has been an exploration of the production and use of Bose-Einstein Condensates for the study of superfluidity. The first 3 years have been spent in the actual building of a Na BEC apparatus. During this time, we’ve implemented a distinct technique to trap ultra cold Na atoms, i.e. the Optically Plugged Trap. In the process, we have shown how atoms in a linear trap can show spin metastability and thus maintain a nonequilibrium state for long periods of time. In studying the interaction of ultra-cold atoms with light, we have developed a technique to measure the velocity distribution of atoms using a standing optical wave (Bragg Spectroscopy). Alongside this, we have also created optical traps for atoms in which we can change to shape of the trap itself to probe different condensed matter systems. The eventual goal being the investigation of condensed matter physics, specifically superfluidity, using ultra-cold atoms.

A novel cooling mechanism based on evaporation of thin liquid films is presented for thermal management of confined heat sources, such as microprocessor hotspots. The underlying idea involves utilization of thin nanoporous membranes for maintaining microscopically thin liquid films by capillary action, while providing a pathway for the vapor generated due to evaporation at the liquid-vapor interface. The vapor generated by evaporation is continuously removed by using a dry sweeping gas keeping the membrane outlet dry. This thesis presents a detailed theoretical, computational and experimental investigation of the heat and mass transfer mechanisms that result in dissipating heat. Performance analysis of this cooling mechanism demonstrates heat fluxes over 600W/cm2 for sufficiently thin membrane and film thicknesses (~1-5µm) and by using air jet impingement for advection of vapor from the membrane surface. Based on the results from this performance analysis, a monolithic micro-fluidic device is designed and fabricated incorporating micro and nanoscale features. This MEMS/NEMS device serves multiple functionalities of hotspot simulation, temperature sensing, and evaporative cooling. Subsequent experimental investigations using this microfluidic device demonstrate heat fluxes in excess of 600W/cm2 at 90 C using water as the evaporating coolant. In order to further enhance the device performance, a comprehensive theoretical and computational analysis of heat and mass transfer at micro and nanoscales is carried out. Since the coolant is confined using a nanoporous membrane, a detailed study of evaporation inside a nanoscale cylindrical pore is performed. The continuum analysis of water confined within a cylindrical nanopore determines the effect of electrostatic interaction and Van der Waals forces in addition to capillarity on the interfacial transport characteristics during evaporation. The detailed analysis demonstrates that the effective thermal resistance offered by the interface is negligible in comparison to the thermal resistance due to the thin film and vapor advection. In order to determine the factors limiting the performance of the MEMS device on a micro-scale, a device-level detailed computational analysis of heat and mass transfer is carried out, which is supported by experimental investigation. Identifying the contribution of various simultaneously occurring cooling mechanisms at different operating conditions, this analysis proposes utilization of hydrophilic membranes for maintaining very thin liquid films and further enhancement in vapor advection at the membrane outlet to achieve higher heat fluxes.

Die Dissertation beschäftigt sich mit der gekoppelten Simulation von Klimaanlage, Regelung und Gebäude mit Hilfe des Simulationssystems TRNSYS. Während für das Gebäude ein vorhandenes TRNSYS Modul verwendet wird, wurden für Klimaanlage und Regelung neue Simulationsmodule entwickelt. Der Klimaanlagensimulator ist seinerseits modular aufgebaut enthält sowohl geometrisch physikalische und empirische als auch kombinierte Modelle für die Simulation von Ventilatoren, Lufterhitzern, Feuchtluftkühlern, Befeuchtern, Regeneratoren, Plattenwärmeüberträgern, Kreislaufwärmerückgewinnern, Ventilen, hydraulischen Schaltungen usw., die für die Simulation sehr variabel miteinander verschaltet werden können. Es wurden optimierte Regelstrategien für konventionelle und DEC-Anlagen entwickelt und entsprechende TRNSYS-Module zur Umsetzung in die Simulation programmiert. Für die Sequenzregelung mehrerer Größen (z.B. Temperatur, Feuchte) wurde ein frei programmierbarer Mehrsequenzregler entwickelt, der den scheinbaren Reglerstillstand über Verknüpfungen blockierter Stellglieder einer Regelsequenz verhindert. Die Qualität der Regelsequenzen wurde mit Hilfe eines über das Rosenbrockverfahren und der dynamischen Optimierung ermittelten optimalen Vergleichsprozesses bewertet.

Made available in DSpace on 2016-06-02T20:09:24Z (GMT). No. of bitstreams: 1 DissGRN.pdf: 2609082 bytes, checksum: 09147cf07533f0a088d8e1734fc80ee4 (MD5) Previous issue date: 2005-12-09 / Financiadora de Estudos e Projetos / The cooling effect of evaporation is used in dry climates buildings since ancient times. Water evaporation can remove heat of roofs, cooling the interior of buildings indirectly. This study presents the measurement results of a test bed which received two identical tiles, one of them receiving water application and the other kept dry. Its objective was to investigate the effects that the evaporation provokes on internal surface temperature of ceramic tiles and fibrocement tiles. Using regression analyses, strong correlations were found among the falls observed in tiles internal surface temperatures and the climatic conditions. The evaporation provoked reduction of until 18,7 oC in the ceramic roof and until 17ºC reduction in the fibrocement one. / O resfriamento evaporativo de ambientes é conhecido pelos habitantes de regiões de clima seco desde a antiguidade. A evaporação de água sobre coberturas retira calor das mesmas, refrigerando indiretamente o interior da edificação. Este trabalho apresenta resultados de um estudo em que foram medidos os efeitos que a evaporação provoca sobre as temperaturas superficiais internas de telhas de barro e de fibrocimento, sujeitas às variações climáticas como o vento e radiação solar. Para tanto, montou-se uma bancada de testes onde foram monitoradas telhas idênticas, uma com gotejamento de água e outra mantida seca, em situações de inverno em laboratório e em situações de verão em campo. Por meio de análise de regressão, identificou-se fortes correlações entre as quedas observadas nas temperaturas superficiais internas das telhas e as condições climáticas. A evaporação provocou redução de até 18,7 ºC na temperatura superficial interna da telha de barro e de até 17ºC na de telha de fibrocimento. Os resultados indicaram que o gotejamento de água sobre superfícies externas de telhas de barro e de fibrocimento reduz a temperatura superficial interna das mesmas por meio da refrigeração evaporativa, sendo um potencial método de resfriamento passivo de telhados de barro e de fibrocimento na região de São Carlos-SP.

abstract: Passive cooling techniques, specifically passive downdraft cooling (PDC), have proven to be a solution that can address issues associated with air conditioning (AC). Globally, over 100 buildings have integrated PDC in its different forms, most of which use direct evaporative cooling. Even though all surveyed buildings were energy efficient and cost-effective and most surveyed buildings were thermally comfortable, application of PDC remains limited. This study aims to advance performance of the single stage passive downdraft evaporative cooling tower (PDECT), and expand its applicability beyond the hot dry conditions where it is typically used, by designing and testing a multi-stage passive and hybrid downdraft cooling tower (PHDCT). Experimental evaluation on half-scale prototypes of these towers was conducted in Tempe, Arizona, during the hot dry and hot humid days of Summer, 2017. Ambient air dry-bulb temperatures ranged between 73.0°F with 82.9 percent coincident relative humidity, and 123.4°F with 7.8 percent coincident relative humidity. Cooling systems in both towers were operated simultaneously to evaluate performance under identical conditions. Results indicated that the hybrid tower outperformed the single stage tower under all ambient conditions and that towers site water consumption was at least 2 times lower than source water required by electric powered AC. Under hot dry conditions, the single stage tower produced average temperature drops of 35°F (5°F higher than what was reported in the literature), average air velocities of 200 fpm, and average cooling capacities of 4 tons. Furthermore, the hybrid tower produced average temperature drops of 45°F (50°F in certain operation modes), average air velocities of 160 fpm, and average cooling capacities exceeding 4 tons. Under hot humid conditions, temperature drops from the single stage tower were limited to the ambient air wet-bulb temperatures whereas drops continued beyond the wet-bulb in the hybrid tower, resulting in 60 percent decline in the former’s cooling capacity while maintaining the capacity of the latter. The outcomes from this study will act as an incentive for designers to consider incorporating PDC into their designs as a viable replacement/supplement to AC; thus, reducing the impact of the built environment on the natural environment. / Dissertation/Thesis / Doctoral Dissertation Architecture 2017