<div>In order to better understand and quantify the effect of instabilities in systems utilizing flow boiling heat transfer, the present study explores dynamic results for pressure drop, mass velocity, thermodynamic equilibrium quality, and heated wall temperature to ascertain and analyze the dominant modes in which they oscillate. Flow boiling experiments are conducted for a range of mass velocities with both subcooled and saturated inlet conditions in vertical upflow, vertical downflow, and horizontal flow orientations. High frequency pressure measurements are used to investigate the influence of individual flow loop components (flow boiling module, pump, pre-heater, condenser, etc.) on dynamic behavior of the fluid, with fast Fourier transforms of the same used to provide critical frequency domain information. Conclusions from this analysis are used to isolate instabilities present within the system due to physical interplay between thermodynamic and hydrodynamic effects. Parametric analysis is undertaken to better understand the conditions under which these instabilities form and their impact on system performance. Several prior stability maps are presented, with new stability maps provided to better address contextual trends discovered in the present study.</div><div>Further, this study utilizes experimental results for vertical upflow boiling of FC-72 in a rectangular channel with finite inlet quality to investigate Density Wave Oscillations (DWOs) and assess their potential impact on design of two-phase systems for future space missions. High-speed flow visualization image sequences are presented and used to directly relate the cyclical passage of High and Low Density Fronts (HDFs and LDFs) to dominant low-frequency oscillations present in transient pressure signals commonly attributed to DWOs. A methodology is presented to determine frequency and amplitude of DWO induced pressure oscillations, which are then plotted for a wide range of relevant operating conditions. Mass velocity (flow inertia) is seen to be the dominant parameter influencing frequency and amplitude of DWOs. Amplitude of pressure oscillations is at most 7% of the time-averaged pressure level for current operating conditions, meaning there is little risk to space missions. Reconstruction of experimental pressure signals using a waveform defined by frequency and amplitude of DWO induced pressure fluctuations is seen to have only moderate agreement with the original signal due to the oversimplifications of treating DWO induced fluctuations as perfectly sinusoidal in nature, assuming they occur at a constant frequency value, and neglecting other transient flow features. This approach is nonetheless determined to have potential value for use as a boundary condition to introduce DWOs in two-phase flow simulations should a model be capable of accurately predicting frequency and amplitude of oscillation.</div><div>Additionally, this study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling using conclusions drawn from analysis of flow visualization images and transient experimental results as a basis from which to begin modeling. Counter to many prior studies attributing DWOs to feedback effects between flow rate, pressure drop, and flow enthalpy causing oscillations in position of the bulk boiling boundary, the present instability mode stems primarily from body force acting on liquid and vapor phases in a separated flow regime leading to liquid accumulation in the near-inlet region of the test section, which eventually departs and moves along the channel, acting to re-wet liquid film along the channel walls and re-establish annular, co-current flow. This process was modeled by dividing the test section into three distinct control volumes and solving transient conservation equations for each, yielding predictions of frequencies at which this process occurs as well as amplitude of associated pressure oscillations. Values for these parameters were validated against an experimental database of 236 FC-72 points and show the model provides good predictive accuracy and capably captures the influence of parametric changes to operating conditions.</div><div>Also, this study shows analysis of pressure signals in condensing systems reveal the presence of relevant oscillatory phenomena during flow condensation as well, which may impact performance in applications concerned with precise system control. Towards this end, the present study presents results for oscillatory behavior observed in pressure measurements during flow condensation of FC-72 in a smooth circular tube in vertical upflow, vertical downflow, and horizontal flow orientations. Dynamic behavior observed within the test section is determined to be independent of other components within the flow loop, allowing it to be isolated and interpreted as resulting from physical aspects of two-phase flow with condensation. The presence of a peak oscillatory mode (one of significantly larger amplitude than any others present) is seen for 72% of</div><div>vertical upflow test cases, 61% of vertical downflow, and 54% of horizontal flow. Relative intensities of this peak oscillatory mode are evaluated through calculation of Q Factor for the corresponding frequency response peak. Frequency and amplitude of peak oscillatory modes are also evaluated. Overall, vertical upflow is seen to exhibit the most significant oscillatory behavior, although in its maximum case amplitude is only seen to be 7.9% of time-averaged module inlet pressure, indicating there is little safety risk posed by oscillations under current operating conditions. Flow visualization image sequences for each orientation are also presented and used to draw parallels between physical characteristics of condensate film behavior under different operating conditions and trends in oscillatory behavior detected in pressure signals</div><div>Further, the present work outlines a new methodology utilizing temperature and pressure measurements to identify condensation flow regimes. For vertical upflow condensation, amplitude of dynamic temperature and pressure oscillations are shown to clearly indicate transition from counter-current flow regimes (i.e., falling film, oscillating film, flooding) to annular, co-current flow (climbing film flow regime). In horizontal flow condensation, standard deviation between multiple thermocouple measurements distributed around the tube circumference was calculated at all axial (stream-wise) measurement locations. High values of standard deviation are present for stratified flow (stratified flow, wavy-stratified, plug flow), while axisymmetric flow regimes (i.e., slug flow, annular flow) yield significantly lower values. Successful development of this technique represents a valuable contribution to literature as it allows condensation flow regime to be identified without the often-costly restriction of designing a test section to allow optical access. Identified flow regimes in both vertical upflow and horizontal flow orientations are compared to regime maps commonly found in the literature in pursuit of optimum performing maps.</div><div>Finally, the present study aims to better analyze the influence of body force on flow condensation heat transfer by conducting tests at multiple orientations in Earth’s gravity. Dielectric FC-72 is condensed in a smooth stainless-steel tube with 7.12 mm diameter and 574.55 mm condensing length by counterflow of cooling water across the outer surface of the tube. Test conditions span FC-72 mass velocities of 50.3 – 360.3 kg/m2s, test section inlet pressures of 127.0 – 132.1 kPa, and test section inlet thermodynamic equilibrium qualities of 0.13 – 1.15. A subset of data gathered corresponding to axisymmetric, annular condensation heat transfer is identified and a detailed methodology for data reduction to calculate heat transfer coefficient presented. Uncertainty analysis is also presented and indicates channel average heat transfer coefficients are calculated within ±3.6% to ±26.7% (depending on operating conditions). Analysis of parametric trends for condensation heat transfer reveals the dominant influence of mass velocity (flow inertia), secondary influence of vapor mass fraction (thermodynamic equilibrium quality), and strong dependence on orientation (body force) at low mass velocities. At higher mass velocities results for all orientations investigated begin to converge, indicating body force independent annular condensation heat transfer is achieved. Separated Flow Model predictions of vertical downflow condensation heat transfer provide reasonable agreement with experimental results, evidence by a Mean Absolute Error (MAE) of 31.2%. Evaluation of condensation heat transfer correlations for horizontal flow reveal most correlations struggle for cases with high liquid content. Specific correlations are identified for superior accuracy in predicting the measured data.</div>

Neste trabalho, foi estudado o desempenho de um sistema constituído de torres de resfriamento e a sua integração em uma planta industrial de hidrogenação de butadieno. Caracterizou-se o desempenho das torres de resfriamento com base em um modelo fenomenológico, cujos parâmetros foram obtidos a partir da medição de variáveis operacionais reais. O processo de hidrogenação foi configurado em um simulador de processos, sendo o caso base estabelecido nas condições de projeto. Elaborou-se um módulo específico referente às torres de resfriamento, que foi integrado ao processo configurado no simulador. Em seguida, analisaram-se as interações das condições operacionais da torre de resfriamento no desempenho do processo industrial. / In the present work, the performance of a system composed of a cooling tower integrated in butadiene hydrogenation plant was studied. An experimental investigation was made to characterize the cooling towers based on a phenomenological model and in real process conditions. The hydrogenation process was configured on a process simulator and design specifications were considered as base case. A cooling tower module was developed and integrated to the process simulator. The interaction of the cooling tower system and the plant operation was investigated.

The study is focused on study of surface and interfacial properties of oil at high temperatures, to understand the mechanisms of heat transfer and oil absorption during frying

Com base em um modelo fenomenológico e a partir de dados experimentais obtidos numa planta piloto, foi obtida uma correlação entre o desempenho de uma torre de resfriamento em função das principais variáveis de processo: fluxos mássicos do gás e da água pela torre, e temperatura de entrada da água. Os resultados apresentaram boa consistência, comparados com os da literatura. A metodologia desenvolvida pode, com relativa facilidade, ser aplicada para torres de resfriamento industriais, pois se baseia em medidas de variáveis, factíveis em termos práticos. Efetuou-se também um estudo paralelo com base em modelagem e simulações matemáticas do comportamento de uma torre de resfriamento de água em condições severas, com temperatura da água de alimentação superior a 50°C. Constatou-se que o coeficiente de transporte de massa na torre de resfriamento aparentemente não é afetado. / Cooling towers are widely used in many industrial and utility plants and its thermal performance is of vital importance. In the present work, using a phenomenological model and by experiments carried on over a pilot installation, the mass transfer coefficient dependence of air and water flow rates and inlet cooling water temperature is determined. The approach proposed may be useful in addition for characterization of industrial cooling towers since it depends on temperature and flow rate measurement usually available in typical plants. A parallel study concerning high mass transfer rate theory is accomplished. Through mathematical modeling and simulations based on this study no influence is detected on the mass transfer coefficient in the cooling tower, operating under harsh conditions with inlet water temperature up to 90°C.

<p>The thesis covers the following topics:</p> <p>1. aggregation and size effect on metal-polymer nanocomposite thermal interface materials</p> <p>2. diffusion limited cluster aggregation lattice simulation on thermal conductivty</p> <p>3. thermal interfacial resistance reduction between metal and dielectric materials by inserting an intermediate metal layer</p> <p>4. absence of coupled thermal interfaces in al2o3/ni/al2o3 sandwich structure</p> <p>5. ultra-efficient low-cost radiative cooling paints</p>

Disponibilidade dos recursos energéticos junto com o despertar da consciência ambiental criaram um interesse por uma condição climática sensível compatível com os recursos disponíveis. Dentro desse cenário o trabalho se volta à necessidade de se criarem e manterem ambientes industriais termicamente adequados aos processos de produção para minimizar as interferências que as condições ambientais exercem sobre os custos dos processos de produtivos e sobre o consumo energético. Os sistemas de resfriamento evaporativo, por sua vez, têm sido a ferramenta de maior potencial de aplicação na criação de ambientes termicamente adequados aos processos. Este modelo revisto de conforto térmico nos coloca um passo à frente para o aumento eficiência energética na construção de projeto de climatização vinculados a temperaturas interiores que atendam conjuntamente tanto aos ocupantes como às atividades que desenvolvem no interior da área climatizada. Apesar de esse sistema apresentar vantagens operacionais, quando comparado a outros sistemas convencionais, existem certas limitações no seu desempenho. Uma das maiores dificuldades das instalações destes sistemas reside na existência de incertezas em qualquer resultado. Possibilitar um controle dos parâmetros, minimizando os erros de aplicação, evitando criar no ambiente um desconforto de tal grau que inviabilize sua aplicação, é o fundamento deste trabalho. A busca passa a ser pela garantia da aceitabilidade dos resultados do sistema projetado e seus limites de aplicabilidade. O estudo das variáveis que interferem no processo do resfriamento por micro aspersão permitiu desenvolver um processo que alterara esses parâmetros durante o funcionamento do sistema, interferindo, conforme a necessidade no seu desempenho, garantindo a completa evaporação da água micro aspergida. / Energy resources along with an environmental conscience awakening has created an interest in sensitive climate together with a more understanding regarding the use of available resources. Inside this scenario our work focus on the needs of creating and maintaining industrial environments thermally suited to these production processes that seeking to minimize interference that environmental conditions have on the costs of production processes and energy consumption. Evaporative cooling systems, in turn, has been a interesting tool to be used in the creation of thermally suitable environments to these processes. This new revised thermal comfort model puts us a step forward to increase energy efficiency in elaborating air treatment projects linked to indoor temperatures that meet both the occupants and the activities that develop inside the controlled area. Although this system has operational advantages when compared to other conventional systems, there are some limitations in their performance. A major difficulty of the installation envolving these systems is about the existence of uncertainty in any results. To allow the control of these parameters in order to minimize the errors in this kind of application and to avoid creating environmental discomfort to such a degree that prevent the implementation, it is the foundation of this work. The search is to ensure the acceptability of the results of the system designed and their limits of applicability. The study of the variables that affect the cooling process by misting allowed us to develop a process that altered these parameters during operation of the system, interfering, as required in its performance, ensuring complete evaporation of water applied by misting in the area.

Le transfert de masse et de chaleur dans les vêtements est un phénomène faisant appel àdifférents mécanismes physiques : les échanges de chaleurs sèches et les transferts de vapeur etde liquide. Ces mécanismes sont fortement influencés par les facteurs liés à la construction, laforme du vêtement par rapport à celle du corps et l’utilisation du vêtement. Ces facteurs peuventêtre optimisés en changeant la taille et la forme des différentes couches d’air emprisonnées entrela peau et les vêtements. La plupart des modèles mathématiques de vêtements font l’hypothèse que l’épaisseur d’air entrela peau et l’étoffe est uniforme, ou alors ils l’ignorent. La non-uniformité et de la non-linéaritédes transferts de chaleur et d’eau ne sont alors pas prises en compte. En effet, le processus detranspiration dépend non seulement de l’aire de contact et de l’épaisseur d’air emprisonnée entrela peau et le vêtement mais également de la région du corps. Nous proposons une méthode permettant de déterminer, avec une plus grande précision que lestechniques existantes, l’épaisseur d’air et l’aire de contact entre le corps et un vêtement à l’aided’une analyse avancée de scans 3D d’un mannequin homme nu et habillé. L’effet du tauxd’humidité sur l’aire de contact et l’épaisseur du film d’air a été étudié en fonction de la zone ducorps et ceci pour différentes tailles, structures de l’étoffe et fibres. Cette méthode contribue àévaluer de façon plus réaliste les échanges de masse et de chaleur au travers de plusieurs couchesde vêtements et ainsi de fournir des données d’entrée précises aux modèles pour la conception devêtements avec prise en compte du confort et de l’ergonomie. / The heat and mass transfer within the clothing system is a composition of a number of physicalprocesses, such as: dry heat and vapour and liquid water transfer. Factors associated with theconstruction and use of the garment, such as body posture and movement, and clothing fitinfluence these processes significantly. This is achieved mainly by changing the size and theshape of the different layers of air trapped between the skin and clothing. Most existing mathematical clothing models assume uniform air gap between the body and fabric layers or ignore it. However, this approach disregards the non-uniform and non-linear heat,vapour and liquid water transfer, which depend on presence of contact between surfaces and onthe shape of the air layers trapped within clothing and the body regions which are not equivalentin terms of sweating process. In this study, we propose a method to accurately determine the air gap thickness and the contactarea between clothing and the human body through an advanced analysis of 3D body scans of thenude and dressed body of a male manikin. This method allowed more accurate measurement ofthe air gap thickness and the contact area than other existing methods. Additionally, in two casestudies the effect of garment design and moisture gain in fabric combined with effects of bodypart, garment type and its overall and regional fit, fabric structure and fibre type were determined.Consequently, this method will contribute to a more realistic evaluation of heat and massexchange rates through clothing systems and provide more accurate input for ergonomic andcomfort design of clothing.

De nos jours, l'humidité dans les bâtiments est une problématique majeure car elle impacte simultanément la consommation énergétique, le confort des occupants, ainsi que les risques de moisissures dans l'envelope du bâtiment. Les matériaux de construction ont la capacité d'absorber et de libérer de grandes quantités de vapeur d'eau et peuvent par conséquent amortir les variations d'humidité relative intérieure. Cette proporiété est appelée tampon hygrique. Elle est liée aux échanges de vapeur d'eau entre l'air intérieur et les parois, à la ventilation, et aux sources de vapeur. Cet effet a été précédemment mis en évidence à l'échelle du matériau et de la paroi par des expériences de laboratoire et des modèles numériques. Cependant, peu de modèles décrivent de manière réaliste les sources intérieures dues à la présence et aux activités des occupants. Dans ce travail, un modèle hygrothermique à l'échelle de la pièce a été développé en Python afin d'investiguer l'influence du scénario d'occupation, et l'impact du tampon hygrique des parois sur l'air intérieur. Ce modèle regroupe les transferts couplés d'énergie et de masse dans les parois, ainsi que les sources intérieures décrites par un système de conditionnement d'air et par la présence et les activités des occupants. Ce dernier étant modélisé de manière stochastique par un modèle implémenté dans une plateforme nommée No-MASS. Des indicateurs de performance sur les besoins énergétiques, le confort hygrique intérieur, et les risques liés aux moisissures dans les parois ont été définis pour quantifier la performance hygrothermique d'une pièce. La sensibilité des indicateurs de performance vis-à-vis du scénario d'occupation a été évaluée en simulant des scénarios stochastique, déterministe et constant. Les résultats ont montré une influence marginale du scénario à l'échelle de l'année. [...} / Humidity in buildings has nowadays become a major concern as it impacts simultaneously the energy consumption, the occupants' comfort and the moisture related risks in the buildings envelope. Buildings materials have the ability to absorb and release large amounts of moisture and therefore they may dampen the indoor relative humidity variations. This is called the moisture buffering capacity. It depends on the vapour exchanges between the air and the walls, the ventilation and the indoor moisture sources. This property was previously highlighted at material and wall scales through laboratory experiments and numerical models. However, few models describe the indoor sources due to the occupants' presence and activities in a realistic way. In this work, a hygrothermal room model was developed in Python to investigate the influence of the occupancy scenario and of the impact of the walls moisture buffering on indoor air balance. This model regroups the coupled heat and mass transfer in the walls, as well as the indoor sources depicted by the air-conditioning system and the occupants' presence and activities. The numerical modelling of the latter relies on a stochastic occupancy model implemented in a platform called No-MASS. Performance indicators on the energy demand, the indoor hygric comfort and the moisture related risks in the walls were defined to quantify the hygothermal performance of a room. The sensitivity of the performance indicators towards the occupancy scenario was assessed by simulating a stochastic occupancy scenario, a deterministic one and a constant one. Results showed a marginal influence of the scenario at year scale. However, at smaller time scales (seasonal or monthly), their impact on the indoor relative humidity dampening was not negligible, mainly due to the consideration of a seasonal effect for the stochastic scenario. [...]

An investigation of binary fluid heat and mass transfer in ammonia-water absorption was conducted. Experiments were conducted on a horizontal-tube falling-film absorber consisting of four columns of six 9.5 mm (3/8 in) nominal OD, 0.292 m (11.5 in) long tubes, installed in an absorption heat pump. Measurements were recorded at both system and local levels within the absorber for a wide range of operating conditions (nominally, desorber solution outlet concentrations of 5 - 40% for three nominal absorber pressures of 150, 345 and 500 kPa, for solution flow rates of 0.019 - 0.034 kg/s.). Local measurements were supplemented by high-speed, high-resolution visualization of the flow over the tube banks. Using the measurements and observations from videos, heat and mass transfer rates, heat and vapor mass transfer coefficients for each test condition were determined at the component and local levels. For the range of experiments conducted, the overall film heat transfer coefficient varied from 923 to 2857 W/m²-K while the vapor and liquid mass transfer coefficients varied from 0.0026 to 0.25 m/s and from 5.51×10^-6 to 3.31×10^-5 m/s, respectively. Local measurements and insights from the video frames were used to obtain the contributions of falling-film and droplet modes to the total absorption rates. The local heat transfer coefficients varied from 78 to 6116 W/m²-K, while the local vapor and liquid mass transfer coefficients varied from -0.04 to 2.8 m/s and from -3.59×10^-5 (indicating local desorption in some cases) to 8.96×10^-5 m/s, respectively. The heat transfer coefficient was found to increase with solution Reynolds number, while the mass transfer coefficient was found to be primarily determined by the vapor and solution properties. Based on the observed trends, correlations were developed to predict heat and mass transfer coefficients valid for the range of experimental conditions tested. These correlations can be used to design horizontal tube falling-film absorbers for ammonia-water absorption systems.

This work is a multi-scale effort to confront the rapidly evolving threat of biological weapons attacks through improved bioaerosol surveillance, detection, and response capabilities. The effects of bioaerosol release characteristics, transport in the atmospheric surface layer, and implications for bioaerosol sampler design and real-time detection were studied to develop risk assessment and modeling tools to enhance our ability to respond to biological weapons attacks. A simple convection-diffusion-sedimentation model was formulated and used to simulate atmospheric bioaerosol dispersion. Model predictions suggest particles smaller than 60 micrometers in aerodynamic diameter (AD) are likely to be transported several kilometers from the source. A five fold increase in effective mass collection rate, a significant bioaerosol detection advantage, is projected for samplers designed to collect particles larger than the traditional limit of 10 micrometers AD when such particles are present in the source distribution. A family of dynamically scaled wetted-wall bioaerosol sampling cyclones (WWC) was studied to provide bioaerosol sampling capability under various threat scenarios. The effects of sampling environment, i.e. air conditions, and air flow rate on liquid recovery rate and response time were systematically studied. The discovery of a critical liquid input rate parameter enabled the description of all data with self-similar relationships. Empirical correlations were then integrated into system control algorithms to maintain microfluidic liquid output rates ideally suited for advanced biological detection technologies. Autonomous ambient air sampling with an output rate of 25 microliters per minute was achieved with open-loop control. This liquid output rate corresponds to a concentration rate on the order of 2,000,000, a substantial increase with respect to other commercially available bioaerosol samplers. Modeling of the WWC was performed to investigate the underlying physics of liquid recovery. The set of conservative equations governing multiphase heat and mass transfer within the WWC were formulated and solved numerically. Approximate solutions were derived for the special cases of adiabatic and isothermal conditions. The heat and mass transfer models were then used to supplement empirical correlations. The resulting semi-empirical models offer enhanced control over liquid concentration factor and further enable the WWC to be deployed as an autonomous bioaerosol sampler. / text