This aim of task deals with study of heat dissipation mechanisms and the description of physical phenomena, which is accompanied by non-stationary measurement of thermal characteristics by the method "hot-wire method". In particular, we observe the coefficient of thermal conductivity and its dependence on various variables such as the temperature of the measured sample, its moisture state, the volume of the sample and its porosity. The above mentioned findings are used for the invention of the measuring device of a nonstationary gauge, which is based on regular heating and is dedicated to measuring the thermal conductivity coefficient and the heat capacity by the "hot-wire method" method. In the last part of the thesis is verified functionality of the proposed measuring device, the suitability of the created algorithm for the processing of the measured data and the evaluation of the results was verified. The reproducibility of the measurements was verified and the measured results were compared with the measurement methods, which are commonly used. the influence of humidity on the coefficient of thermal conductivity.

The present work was written as a cumulative dissertation based on peer-reviewed papers and is completed by yet unpublished results. The overall objective was to get a deeper insight into the role of the advective fluxes of sensible heat and latent heat in relation to the energy balance and its imbalance at the earth’s surface (typically the sum of the turbulent fluxes sensible and latent heat does not match the available energy). Data from two advection experiments at four coniferous sites across Europe served as the basis for the analysis. One was the advection experiment MORE II which took place in Tharandt (Germany) and the other advection experiment ADVEX was conducted at three different sites (Ritten/Renon, Italy; Wetzstein, Germany; Norunda, Sweden). An inspection of the available energy (AE) that is redistributed to the atmosphere by the sensible heat flux (H) and latent heat flux (LE) showed that the uncertainty of the available energy itself cannot explain the lack of energy balance closure for these four sites. The mean absolute uncertainty of the available energy was largest during midday and ranged from 41 W m-2 to 52 W m-2 (approx. 12 % of AE). During nighttime, the mean absolute uncertainty was smaller (20 W m-2 – 30 W m-2) but the relative uncertainty was much larger as AE itself is small. Among the investigated storage terms the heat storage change of the biomass was most important. The energy balance closure was improved for all investigated sites when storage terms were included. In principle, storage terms should not be neglected in energy balance studies. An investigation of the budget of sensible heat, not only including the vertical advection and the horizontal advection but also the horizontal turbulent flux divergence, was undertaken for the coniferous site at Tharandt. Inclusion of these fluxes resulted in an enlarged mean daily amplitude and suggests an improvement of the energy balance closure, at least during nighttime. The commonly determined budget (vertical turbulent flux plus storage change) was reduced by about 30 % when advective fluxes were included. Results suggest that the horizontal turbulent flux divergence is of minor importance but further studies are needed for an overall evaluation. First results for the inclusion of the advective fluxes of both sensible heat and latent heat indicate that the lack of energy balance closure is partly reduced but the imbalance still exists. Advective fluxes of sensible heat were also compared to advective fluxes of CO2. It became apparent that the advective fluxes of sensible heat and CO2 are, on average, of opposite sign during nighttime and both share large scatter. Both budgets (sensible heat and CO2) were considerably changed (although differently for different sites) when advective fluxes were included. Results further suggest that advective fluxes of H can be taken as an indicator concerning the presence and sign of advection of CO2. This points towards a coincident non-turbulent transport of heat and CO2. However, all investigated advective fluxes are site-specific. They are characterised by a large uncertainty due to uncertainties in the mean vertical velocity (vertical advection) and in the horizontal differences in scalar magnitude (horizontal advection). Obviously, they are influenced by the limitations of the experimental set-up (spatial resolution) and the local characteristics of the individual measurements. An overall evaluation of advective fluxes with respect to their representativeness and magnitude requires further studies / Die vorliegende Arbeit wurde als kumulative Dissertation verfasst, die auf begutachteten Publikationen beruht. Sie wird um bisher nicht veröffentlichte Daten zur Advektion latenter Wärme ergänzt. Ziel war es, vor allem die Rolle der advektiven Flüsse von sensibler und latenter Wärme in Bezug auf die Energiebilanz und das Problem der Energiebilanzschließung an der Erdoberfläche näher zu untersuchen. Unter der Energiebilanzschließungslücke wird im Allgemeinen das Phänomen verstanden, dass die Summe der gemessenen turbulenten Flüsse von sensibler und latenter Wärme zumeist nicht der gemessenen verfügbaren Energie entspricht. Als Datengrundlage für die Arbeiten dienten hierzu die Datensätze von zwei Advektionsexperimenten, die an vier verschiedenen Nadelwaldstandorten in Europa stattfanden. Das erste dieser Advektionsexperimente MORE II fand an der Ankerstation Tharandt (Deutschland) statt und das zweite (ADVEX) wurde an drei verschiedenen Standorten durchgeführt (Ritten/Renon, Italien; Wetzstein, Deutschland; Norunda, Schweden). Eine Untersuchung der verfügbaren Energie (AE), die über den sensiblen Wärmestrom (H) und den latenten Wärmestrom (LE) wieder an die Atmosphäre abgegeben wird, zeigte, dass die in der Bestimmung der verfügbaren Energie liegende Unsicherheit das Problem der Energiebilanzschließungslücke nicht ausreichend erklärt. Die mittlere absolute Unsicherheit der verfügbaren Energie war dabei mittags am größten (41 W m-2 – 52 W m-2; ca. 12 % der verfügbaren Energie). Nachts war diese kleiner (20 W m-2 – 30 W m-2). Jedoch waren dann die relativen Unsicherheiten deutlich größer, da die verfügbare Energie nachts klein ist. Von den betrachteten Speichertermen der Energiebilanz erwies sich die Speicheränderung von Wärme in der Biomasse als am wichtigsten. Für die vier untersuchten Standorte verbesserte sich die Energiebilanzschließung, wenn die Speicherterme mit einbezogen wurden. Grundsätzlich sollten alle Speicherterme bei der Bestimmung der Energiebilanz mit beachtet werden. Für den Nadelwaldstandort Tharandt wurde die Bilanz der sensiblen Wärme unter Beachtung der advektiven Flüsse und der horizontalen turbulenten Flussdivergenz erstellt. Die Einbeziehung der advektiven Flüsse und der horizontalen turbulenten Flussdivergenz führte zu einer Vergrößerung der Amplitude im mittleren Tagesgang und deutet auf eine Verbesserung der Energiebilanzschließung zumindest nachts hin. Im herkömmlichen Sinne wird die Bilanz für Energie oder Massenflüsse als Summe aus vertikalem turbulenten Fluss und Speicheränderung bestimmt. Die Gesamtsumme dieser Bilanz wurde um 30 % reduziert, wenn die advektiven Flüsse mit einbezogen wurden. Hinsichtlich der horizontalen turbulenten Flussdivergenz kann man noch keine abschließende Einschätzung geben. Die vorliegenden Ergebnisse deuten einen vernachlässigbaren Anteil an der Gesamtbilanz für diesen Term an. Erste Ergebnisse für die Bestimmung der Energiebilanz von Nadelwäldern unter Beachtung der advektiven Flüsse von sensibler und latenter Wärme zeigen eine teilweise Reduzierung der Energiebilanzschließungslücke, jedoch keine vollständige Schließung der Energiebilanz. Weiterhin wurden die advektiven Flüsse sensibler Wärme mit denen von CO2 verglichen. Die Bilanzen für den CO2-Fluss als auch für den Fluss sensibler Wärme änderten sich deutlich unter Einbeziehung der advektiven Flüsse, wenn auch unterschiedlich für verschiedene Standorte. Besonders nachts sind die advektiven Flüsse von sensibler Wärme und CO2 im Mittel durch gegensätzliche Vorzeichen gekennzeichnet. Diese Beziehung eröffnet die Möglichkeit, advektive Flüsse von CO2 auf der Basis von advektiven Flüssen sensibler Wärme hinsichtlich ihres Vorhandenseins und ihrer Richtung abzuschätzen. Dies deutet auf einen gleichzeitigen nicht-turbulenten Transport von Wärme und CO2 hin. Generell ist festzustellen, dass alle untersuchten advektiven Flüsse spezifisch für den jeweiligen Standort und durch eine große Unsicherheit gekennzeichnet sind. Diese ergibt sich zum einen aus der mittleren vertikalen Geschwindigkeit (vertikale Advektion) und zum anderen aus den horizontalen Differenzen (horizontale Advektion) der jeweiligen skalaren Größen. Die betrachteten advektiven Flüsse werden offensichtlich durch Einschränkungen, die sich aus dem experimentellen Aufbau ergeben (z.B. begrenzte räumliche Auflösung), in ähnlicher Weise beeinflusst. Eine abschließende Beurteilung der advektiven Flüsse hinsichtlich ihres Anteils an der Gesamtbilanz und ihrer Repräsentativität erfordert weitere Studien.

<p dir="ltr">Boiling processes have long had an important role in power generation and air conditioning applications. The efficient and reliable heat dissipation afforded through the phase change process in the boiling has led to their generation of a substantial body of work in this field over several decades. Despite decades of efforts, the heat transfer performance prediction in boiling has been highly empirical with models working only for a narrow range of surface/fluids or other operating conditions. The limitation in these models is a result of a lack of mechanistic understanding of the underlying heat and mass transfer process. Surface dryout or boiling crisis is a process wherein there is a spontaneous formation of vapor film on top of the surface causing a catastrophic increase in surface temperature. The heat flux at which this formation of vapor film occurs is called critical heat flux (CHF). The CHF demarcates the upper limit to the regime of stable nucleating bubbles called nucleate boiling. The mechanism causing dryout is under debate for over half a century and several conflicting theories that cause dryout have been suggested since the 1950s including hydrodynamic, irreversible dryspot expansion, macrolayer dryout/liftoff, critical bubble distributions, vapor-recoil based theories and more. The lack of consensus is due to limitation in the information collected on the dynamic multiscale and chaotic bubble interactions. Recent advances in high-fidelity spatiotemporal phase, temperature, and heat flux measurements now enable diagnostic tools that can be leveraged to understand the complex heat transfer processes emerging from bubble-surface interaction on the boiling surface. In this work, we develop such techniques to understand various transport mechanisms underlying boiling and its crisis.</p><p dir="ltr">In this work, an experimental technique for collecting synchronized through-substrate visual and infrared (IR) measurements of a boiling surface is developed. An IR and visually transparent sapphire substrate with an IR-opaque indium-tin-oxide (ITO) heater layer is used to measure the phase (liquid and vapor areas) and temperature of the ITO layer. The visual camera collects the light reflected off the substrate from a red LED and the images collected show a contrast between liquid and vapor areas that is used to generate binarized phase maps. The temperature from the IR camera is used as boundary condition to solve a conduction problem for heat fluxes going into the fluid. Four distinct heat flux signatures corresponding to liquid, contact line, vapor and rewetting regions are observed. A post-processing methodology utilizing synchronous phase measurements to identify and partition these regions is introduced. The high-fidelity phase measurements allow for detection of fine features that are not discernable using heat flux maps alone. Analysis of the heat flux and temperature maps of partitioned regions for HFE-7100 fluid on the ITO surface show qualitative agreement with the trends in mechanisms underlying those areas. The experiment and post-processing methodology introduced in this work is the first to provide partitioning of underlying heat transfer mechanisms for multi-bubbles throughout the entire range of the boiling curve during both steady and transient scenarios.</p><p dir="ltr">The technique developed is used to probe the mechanisms underlying the boiling crisis. Theories suggested in the literature for boiling crisis are carefully evaluated and evidence against hydrodynamic instability, macrolayer dryout, vapor recoil, irreversible expansion of dryspots, macrolayer liftoff model, and bifurcations from critical distributions is observed. The signature in the peak of the spatially averaged fluid heat flux is observed to precede any other signs of dryout. Beyond the peak heat flux an increase in superheat leads to reduced heat dissipated by boiling and further increases the temperature causing a thermal runaway in the substrate that eventually leads to dryout. Hence, the boiling crisis is found to be a consequence of a peak in the nucleate boiling curve. The cause for the peak in the boiling heat flux for the surface-fluid combination tested was due to degradation of heat transfer caused by the replacement of high-heat-transfer contact line region with lower-heat-transfer vapor covered regions, among the multiple competing mechanisms. Hence, we propose that mechanistically modeling the boiling crisis rests on prediction of the peak in the upper portion of the nucleate boiling curve by considering all underlying heat transfer mechanisms. A modeling framework based on heat flux partitioning, where the overall heat transferred during boiling is calculated as the sum of the heat transferred by individual mechanisms is demonstrated as potential pathway to predict the upper portion of the nucleate boiling curve and thereby critical heat flux. Based on the terms involved in summation for individual mechanisms, we propose that the boiling curve for any given surface be interpreted as a path on a multidimensional surface (boiling manifold). Estimation of such a boiling manifold allows for prediction of the boiling curve for any surface, given development of the relations between these parameters and surface-fluid properties, and can further be used to backtrack relevant thermophysical or nucleation properties for enhanced boiling performance.</p><p dir="ltr">Enhancement of pool boiling heat transfer performance using surface modifications is of major interest to applications and this work further delves into characterizing the boiling performance using traditional surface averaged measurements of microstructured surfaces using HFE-7100. We find that microlayer evaporation from the imbibed liquid layer underneath the growing vapor bubbles is the key mechanism of boiling heat transfer enhancement in microstructures. Further, this implies that characterization of microstructured surfaces for evaporative performance can serve as an important proxy to enable heat transfer coefficient enhancement prediction during pool boiling. Hence, we also developed an easily calculated Figure of Merit (FOM) that characterizes the efficacy of evaporation from microstructured surfaces.</p><p dir="ltr">To summarize, in this work we developed an experimental technique using synchronous through-substrate high-speed visual and IR imaging methods. New post-processing techniques for partitioning of different heat transfer mechanisms are proposed and used to analyze boiling on an ITO-coated sapphire substrate with HFE-7100 as the working fluid. We reveal thermal runaway in the substrate caused due to a negative-sloping boiling curve as the mechanism of dryout. Mechanistic modeling of the critical heat flux thus involves calculating the peak in the nucleate boiling curve. A framework to predict the nucleate boiling curve and subsequently critical heat flux is proposed based on the partitioning analysis. The experimental method developed lays the groundwork for measuring heat flux and superheats associated with various mechanisms, and hence, enables validation of future partitioning-based boiling heat transfer models that intrinsically enable prediction of the peak.</p>

<p>To improve the prediction of two-phase local structure and heat transfer in subcooled boiling flow, the wall nucleation phenomenon was studied to accurately model the wall source term in the interfacial area transport equation (IATE) for the use with the two-fluid model. The existing experimental datasets and modeling works of departure diameter, departure frequency and active nucleation site density were comprehensively reviewed. Since these parameters are coupled in the bubble ebullition cycles, simultaneous measurements of departure diameter, departure frequency and active nucleation site density were performed in a vertical annular test section. The ranges of the existing experimental database were extended to high pressure and high heat flux conditions. The stochastic characteristics of the departure diameter and departure frequency measured from a single nucleation site and over multiple nucleation sites were investigated. Significant variations between different nucleation sites were observed. A parametric study of departure diameter, departure frequency and nucleation site density were conducted at varying system pressure, heat flux, flow rate and subcooling conditions. The existing models of these parameters were evaluated with the experimental dataset of the existing and the present works. Significant discrepancies were observed between model predictions and experimental data, which indicates that the mechanism of nucleate boiling is not fully understood. The heat flux partitioning model was also evaluated. The results show that the heat flux at high pressure or low flow rate conditions was significantly underestimated. This may suggest that major heat transfer mechanisms are missing in the heat flux partitioning model.</p>

Forests affect climate by absorbing CO₂ but also by altering albedo, latent heat flux, and sensible heat flux. In this study we used the Community Earth System Model to assess the biophysical effect of North American temperate forests on climate and how this effect changes with location, tree type, and forest management. We calculated the change in annual temperature and energy balance associated with afforestation with either needle leaf evergreen trees (NET) or broadleaf deciduous trees (BDT) and between forests with high and low leaf-area indices (LAI). Afforestation from crops to forests resulted in lower albedo and higher sensible heat flux but no consistent difference in latent heat flux. Forests were consistently warmer than crops at high latitudes and colder at lower latitudes. In North America, the temperature response from afforestation shifted from warming to cooling between 34° N and 40° N for ground temperature and between 21° N and 25° N for near surface air temperature. NET tended to have lower albedo, higher sensible heat flux and warmer temperatures than BDT. The effect of tree PFT was larger than the effect of afforestation in the south and in the mid-Atlantic. Increasing LAI, a proxy for increased management intensity, caused a cooling effect in both tree types, but NET responded more strongly and albedo decreased while albedo increased for BDT. Our results show that forests' location, tree type, and management intensity can have nearly equal biophysical effects on temperature. A forest will have maximum biophysical cooling effect if it is in the south, composed of broadleaf PFT, and is managed to maximize leaf area index. / Master of Science

Two classes of carbon nanomaterials—carbon nanotubes and graphene—have promoted the advancement of nanoelectronics, quantum computing, chemical sensing and storage, thermal management, and optoelectronic components. Studies of the thermal radiative properties of carbon nanotube thin film arrays and simple graphene hybrid structures reveal some of the most exciting characteristic electromagnetic interactions of an unusual sort of material, called hyperbolic metamaterials. The features and results on these materials in the context of both far-field and near-field radiation are presented in this dissertation. Due to the optically dark nature of pyrolytic carbon in the wavelength range from visible to infrared, it has been suggested vertically aligned carbon nanotube (VACNT) coatings may serve as effective radiative absorbers. The spectral optical constants of VACNT are modeled using the effective medium theory (EMT), which is based on the anisotropic permittivity components of graphite. The effects of other EMT parameters such as volume filling ratio and local filament alignment factor are explored. Low reflectance and high absorptance are observed up to the far-infrared and wide range of oblique incidence angles. The radiative properties of tilt-aligned carbon nanotube (TACNT) thin films are illustrated. Energy streamlines by tracing the Poynting vectors are used to show a self-collimation effect within the TACNT thin films, meaning infrared light can be transmitted along the axes of CNT filaments. Graphene, a single layer sheet of carbon atoms, produces variable conductance in the terahertz frequency regime by tailoring the applied voltage gating or doping. Periodically embedding between dielectric spacers, the substitution of graphene provides low radiative attenuation compared to traditional metal-dielectric multilayers. The hyperbolic nature, namely negative angle of refraction, is tested on the graphene-dielectric multilayers imposed with varying levels of doping. EMT should be valid for graphene-dielectric multilayers due to the nanometers-thick layers compared to the characteristic wavelength of infrared light. For metal- or semiconductor-dielectric multilayers with thicker or lossier layers, EMT may not hold. The validity of EMT for these multilayers is better understood by comparing against the radiative properties determined by layered medium optics. When bodies of different temperatures are separated by a nanometers-size vacuum gap, thermal radiation is enhanced several-fold over that of blackbodies. This phenomenon can be used to develop more efficient thermophotovoltaic devices. Due to their hyperbolic nature, VACNT and graphite are demonstrated to further increase evanescent wave tunneling. The heat flux between these materials separated by vacuum gaps smaller than a micron is vastly improved over traditional semiconductor materials. A hybrid structure composed of VACNT substrates covered by doped graphene is analyzed and is shown to further improve the heat flux, due to the surface plasmon polariton coupling between the graphene sheets.

In this study, the cooling of a simulated blade under increasing pitch between holes was examined. The change in non-dimensional surface temperature, phi, was measured experimentally to quantify this performance loss. This critical quantification of the sensitivity of cooling to pitch between holes has not been studied previously. A range of blowing ratios and angles of attack were tested. Data are presented in terms of the laterally averaged phi, and in terms of the minimum phi, arguably more important from a design perspective. Increasing the pitch 13% produced no measureable change using either parameter. An increase of 26% in pitch produced only a 4% loss in lateral averages, while some hot points dropped by 10%. These small changes are due to compensating effects of increased internal and through-hole convective cooling. A limit to these effects was shown when increasing pitch 53%. While performance loss in the average was still relatively small at 15%, the minimum phi decreased by 27%. Heat flux gauges were used to gather data on the internal and external surface. The internal impingement used in this study represents a more accurate representation of internal cooling for an actual engine part than has been previously studied, providing a starting point for exploring the differences between engine configurations and those generally investigated in the literature. External heat flux measurements were used to measure the ratio of heat flux with and without film cooling. These results call into question the use of the net heat flux reduction parameter, which is commonly used to quantify overall film cooling performance. / text

Nanofluids as an innovative class of heat transfer fluids created by dispersing nanometre-sizedmetallic or non-metallic particles in conventional heat transfer fluids displayed the potential toimprove the thermophysical properties of the heat transfer fluids. The main purpose of this study is toinvestigate the influence of the use of nanofluids on two-phase heat transfer, particularly on thethermal performance of the heat pipes. In the first stage, the properties of the nanofluids were studied,then, these nanofluids were used as the working fluids of the heat pipes. The thermal performance ofthe heat pipes when using different nanofluids was investigated under different operating conditionsexperimentally and analytically. The influences of the concentration of the nanofluids, inclinationangles and heat loads on the thermal performance and maximum heat flux of the heat pipes wereinvestigated.This study shows that the thermal performance of the heat pipes depends not only on thermophysicalproperties of the nanofluids but also on the characteristics of the wick structure through forming aporous coated layer on the heated surface. Forming the porous layer on the surface of the wick at theevaporator section increases the wettability and capillarity and also the heat transfer area at theevaporator of the heat pipes.The thermal performance of the heat pipes increases with increasing particle concentration in all cases,except for the heat pipe using 10 wt.% water/Al2O3 nanofluid. For the inclined heat pipe, irrespectiveof the type of the fluid used as the working fluid, the thermal resistance of the inclined heat pipes waslower than that of the heat pipes in a horizontal state, and the best performance was observed at theinclination angle of 60o, which is in agreement with the results reported in the literature. Otheradvantages of the use of nanofluids as the working fluids of the heat pipes which were investigated inthis study were the increase of the maximum heat flux and also the reduction of the entropy generationof the heat pipes when using a nanofluid.These findings revealed the potential for nanofluids to be used instead of conventional fluids as theworking fluid of the heat pipes, but the commercialization of the heat pipes using nanofluids for largescale industrial applications is still a challenging question, as there are many parameters related to thenanofluids which are not well understood. / <p>QC 20170228</p>

Um dos parâmetros termo-hidráulicos de segurança mais importantes no projeto e operação de reatores a água pressurizada é o fluxo de calor crítico (FCC). O FCC ocorre quando se atinge uma região de instabilidade na mudança de mecanismo de transferência de calor de uma parede aquecida para um fluido, aumentado drasticamente a temperatura da parede. Transientes em um reator nuclear podem afetar a taxa de geração de calor ou a fluxo de refrigerante no núcleo, prejudicando a retirada de calor das varetas combustíveis. Conhecer o FCC nestas condições é essencial para evitar danos às varetas e, consequentemente, a liberação de material radioativo. O objetivo deste trabalho é analisar o FCC para o LABGENE (Laboratório de Geração Nucleoelétrica) por meio do planejamento experimental e da simulação de seções de teste em condições de operação utilizando o código COBRAIIIc/MIT-1 e a correlação EPRI para o FCC. Considerou-se primeiramente seções de teste 3×3 de dois tamanhos distintos e os resultados para 100 pontos experimentais foram mostrados por meio de superfícies de resposta, a fim de melhor visualizar e analisar o comportamento de FCC para cada condição. Dois pontos importantes são os valores máximo e mínimo do FCC encontrados. O valor máximo (1,038 MBtu/hr.ft2 ou 3,27 MW/m2) indica o fluxo de calor necessário para a realização dos experimentos e o mínimo (0,162 MBtu/hr.ft2 ou 0,51 MW/m2) indica a pior condição de operação, a qual estaria mais próxima do ponto de ebulição. As simulações e modificações no código foram verificadas utilizando o banco de dados da Universidade de Columbia. Foram selecionados 2718 pontos experimentais referentes a seções de teste 5×5 com perfil de potência uniforme. Os resultados foram apresentados pela razão entre o valor predito e o valor experimental (DNBR) e os limites de tolerância unilateral 95/95 foram calculados, estando dentro dos valores esperados. / One of the most important thermal-hydraulic safety parameters for pressurized water reactor design and operation is the critical heat flux (CHF). The CHF occurs when a region of instability reached in the change of heat transfer mechanism from a hot wall to a fluid is reached, dramatically increasing the wall temperature. Transients in a nuclear reactor can affect the heat generation rate or the coolant flow in the core, impairing the removal of heat from the fuel rods. Knowledge of the CHF on these conditions is essential to prevent fuel rod damages and therefore the release of radioactive material. The main goal of this work is to analyze the CHF for LABGENE (Nuclear-electrical Generation Laboratory) by an experimental design and test sections simulation in operating conditions by using COBRAIIIc/MIT-1 code and the EPRI correlation for CHF. 3x3 test sections were initially considered for two different heights and outcomes for 100 experimental points were shown by means of response surfaces in order to better visualize and analyze the behavior of CHF for each condition. Two important points are the maximum and minimum values of the CHF found. The maximum value (1.038 MW/m2 or 3.27 MBtu/hr.ft2) indicates the power required for the experiments and the minimum one (0.162 MBtu/hr.ft2 or 0.51 MW/m2) indicates the worst operation condition, which would be closer to the boiling point. Code simulations and modifications were verified using the CHF database of Columbia University. 2718 data points pertaining to test sections 5×5 with uniform power profile were selected. The results were presented by the ratio between the predicted value and the experimental value (DNBR) and the limits of unilateral tolerance 95/95 were calculated, being within the expected values.

<div>Nucleate boiling is an efficient heat transfer mechanism that enables the dissipation of high heat fluxes at low temperature differences. Heat transfer phenomena during nucleate boiling are closely linked to the two-phase flow morphology that evolves in time and based on the operating conditions. In particular, the critical heat flux, which is the upper limit for the nucleate boiling regime, can be triggered by hydrodynamic mechanisms resulting from interactions between the liquid and vapor phases. The aim of this thesis is to characterize the two-phase flow morphology evolution during nucleate boiling at high heat fluxes in two configurations: pool boiling, and confined and submerged two-phase jet impingement. The characterization is performed via non-invasive, high-speed optical based diagnostic tools. </div><div>Experimental characterization of liquid-vapor interfaces during boiling is often challenging because the rapidly evolving vapor structures are sensitive to invasive probes and multiple interfaces can occlude one another along a line of sight. In this thesis, a liquid-vapor interface reconstruction technique based on high-speed stereo imaging is developed. Images are filtered for feature enhancement and template matching is used for determining the correspondence of local features of the liquid-vapor interfaces between the two camera views. A sampling grid is overlaid on the reference image and windows centered at each sampled pixel are compared with windows centered along the epipolar line in the target image to obtain a correlation signal. To enhance the signatures of true matches, the correlation signals for each sampled pixel are averaged over a short time ensemble correlation. The three-dimensional coordinates of each matched pixel are determined via triangulation, which yields a set of points in the physical world representing the liquid-vapor interface. The developed liquid-vapor interface reconstruction technique is a high-speed, flexible and non-invasive alternative to the various existing methods for phase-distribution mapping. This technique also has the potential to be combined with other optical-based diagnostic tools, such as tomographic particle image velocimetry, to further understand the phase interactions.<br></div><div>The liquid-vapor interface reconstruction technique is used to characterize liquid-vapor interfaces above the heated surface during nucleate pool boiling, where the textured interface resulting from the boiling phenomena and flow interactions near the heated surface is particularly suited for reconstruction. Application of the reconstruction technique to pool boiling at high heat fluxes produces a unique quantitative characterization of the liquid-vapor interface morphology near heated surface. Analysis of temporal signals extracted from reconstructions indicate a clear transition in the nature of the vapor flow dynamics from a plume-like vapor flow to a release mode dominated by vapor burst events. Further investigation of the vapor burst events allows identification of a characteristic morphology of the vapor structures that form above the surface that is associated to the square shape of the heat source. Vapor flow morphology characterization during pool boiling at high heat fluxes can be used to inform vapor removal strategies that delay the occurrence of the critical heat flux during pool boiling.</div><div>As compared to pool boiling, nucleate boiling can be sustained up to significantly higher heat fluxes during two-phase jet impingement. The increases in critical heat flux are explained via hydrodynamic mechanisms that have been debated in the literature. The connection between two-phase flow morphology and the extension of nucleate boiling regime is investigated for a single subcooled jet of water that impinges on a circular heat source via high-speed visualization from two synchronized top and side views of the confinement gap. When boiling occurs under subcooled exit flow conditions and at moderate heat fluxes, the regular formation and collapse of vapor structures that bridge the heated surface and the orifice plate is observed, which causes significant oscillations in the pressure drop across. Under saturated exit flow conditions, the vapor agglomerates in the confinement gap into a bowl-like vapor structure that recurrently shrinks, due to vapor break-off at the edge of the orifice plate, and replenishes due to vapor generation. The optical visualizations from the top of the confinement gap provide a unique perspective and indicate that the liquid jet flows downwards through the vapor structure, impinges on the heated surface, and then flows underneath the vapor structure, as a fluid wall jet the keeps the heated surface wetted such that discrete bubbles continue to nucleate. At high heat fluxes, intense vapor generation causes the fluid wall jet to transition from a bubbly to a churn-like regime, and some liquid droplets are sheared off into the vapor structure. The origin of critical heat flux appears to result from a significant portion of the liquid in the wall jet being deflected off the surface, and the remaining liquid film on the surface drying out before reaching the edge of the heater.</div><div>The flow morphology characterizations presented in this dissertation further the understanding of flow and heat transfer phenomena during nucleate boiling. In the pool boiling configuration, the vapor release process was quantitatively described; during two-phase jet impingement, a possible mechanism for critical heat flux was identified. Opportunities for future work include the utilization of image processing techniques to extract quantitative measurements from two-phase jet impingement visualizations. Also, the developed liquid-vapor interface reconstruction technique can be applied to a boiling situation with a simpler liquid-vapor interface geometry, such as film boiling, to generate benchmark data for validation and development of numerical models.</div><div><br></div>