Rozprawy doktorskie na temat „Radiative Heat Flux Rate”

All substances above zero kelvin temperature emit fluctuating electromagnetic waves due to the random motions of charge carriers. Controlling the spectral and directional radiative properties of surfaces has wide applications in energy harvesting and thermal management. Artificial metamaterials have attracted much attention in the last decade due to their unprecedented optical and thermal properties beyond those existing in nature. This dissertation aims at tailoring radiative properties at infrared regime and enhancing the near-field radiative heat transfer by employing metamaterials. A comprehensive study is performed to investigate the extraordinary transmission, negative refraction, and tunable perfect absorption of infrared light. A polarizer is designed with an extremely high extinction ratio based on the extraordinary transmission through perforated metallic films. The extraordinary transmission of metallic gratings can be enhanced and tuned if a single layer of graphene is covered on top. Metallic metamaterials are not the unique candidate supporting exotic optical properties. Thin films of doped silicon nanowires can support negative refraction of infrared light due to the presence of hyperbolic dispersion. Long doped-silicon nanowires are found to exhibit broadband tunable perfect absorption. Besides the unique far-field properties, near-field radiative heat transfer can be mediated by metamaterials. Bringing objects with different temperatures close can enhance the radiative heat flux by orders of magnitude beyond the limit set by the Stefan-Boltzmann law. Metamaterials provide ways to make the energy transport more efficient. Very high radiative heat fluxes are shown based on carbon nanotubes, nanowires, and nanoholes using effective medium theory (EMT). The quantitative application condition of EMT is presented for metallodielectric metamaterials. Exact formulations including the scattering theory and Green’s function method are employed to investigate one- and two-dimensional gratings as well as metasurfaces when the period is not sufficiently small. New routes for enhancing near-field radiative energy transport are opened based on proposed hybridization of graphene plasmons with hyperbolic modes, hybridization of graphene plasmons with surface phonon modes, or hyperbolic graphene plasmons with open surface plasmon dispersion relation. Noncontact solid-state refrigeration is theoretically demonstrated to be feasible based on near-field thermal radiation. In addition, the investigation of near-field momentum exchange (Casimir force) between metamaterials is also conducted. Simultaneous enhancement of the near-field energy transport and suppress of the momentum exchange is theoretically achieved. A design based on repulsive Casimir force is proposed to achieve tunable stable levitation. The dissertation helps to understand the fundamental radiative energy transport and momentum exchange of metamaterials, and has significant impacts on practical applications such as design of nanoscale thermal and optical devices, local thermal management, thermal imaging beyond the diffraction limit, and thermophotovoltaic energy harvesting.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2011.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 130-133).<br>The effects on pool boiling characteristics such as critical heat flux and the heat transfer coefficient of different surface characteristics such as surface wettability, roughness, morphology, and porosity are not well understood. Layer-by-layer nanoparticle coatings were used to modify the surface of a sapphire heater to control the surface roughness, the layer thickness, and the surface chemistry. The surface was then tested in a water boiling test at atmospheric pressure while imaging the surface with high speed infrared thermography yielding a 2D time dependent temperature profile. The critical heat flux and heat transfer coefficient were enhanced by over 100% by optimizing the surface parameters. It was found that particle size of the nanoparticles in coating, the coating thickness, and the wettability of the surface have a large impact on CHF and the heat transfer coefficient. Surfaces were also patterned with hydrophobic "islands" within a hydrophilic "sea" by coupling the Layer-by-layer nanoparticle coatings with an ultraviolet ozone technique that patterned the wettability of the surface. The patterning was an attempt to increase the nucleation site density with hydrophobic dots while still maintaining a large hydrophilic region to allow for rewetting of the surface during the ebullition cycle and thus maintaining a high critical heat flux. The patterned surfaces exhibited similar critical heat fluxes and heat transfer coefficients to the surfaces that were only modified with layer-by-layer nanoparticle coatings. However, the patterned surfaces also exhibited highly preferential nucleation from the hydrophobic regions demonstrating an ability to control the nucleation site layout of a surface and opening an avenue for further study.<br>by Bren Andrew Phillips.<br>S.M.

"This dissertation documents two interrelated studies that were conducted to more fundamentally understand the scalability of flame heat flux. The first study used an applied heat flux in the bench scale horizontal orientation which simulates a large scale flame heat flux. The second study used enhanced ambient oxygen to actually increase the bench scale flame heat flux itself. Understanding the scalability of flame heat flux more fully will allow better ignition and combustion models to be developed as well as improved test methods. The key aspect of the first study was the use of real scale applied heat flux up to 200 kW/m2. An unexpected non-linear trend is observed in the typical plotting methods currently used in fire protection engineering for ignition and mass loss flux data for several materials tested. This non-linearity is a true material response. This study shows that viewing ignition as an inert material process is inaccurate at predicting the surface temperature at higher heat fluxes and suggests that decomposition kinetics at the surface and possibly even in-depth may need to be included in an analysis of the process of ignition. This study also shows that viewing burning strictly as a surface process where the decomposition kinetics is lumped into the heat of gasification may be inaccurate and the energy balance is too simplified to represent the physics occurring. The key aspect of the second study was direct experimental measurements of flame heat flux back to the burning surface for 20.9 to 40 % ambient oxygen concentrations. The total flame heat flux in enhanced ambient oxygen does not simulate large scale flame heat flux in the horizontal orientation. The vertical orientation shows that enhanced ambient oxygen increases the flame heat flux more significantly and also increases the measured flame spread velocity."

A new method of power production, called pressurized oxy-fuel combustion, burns coal with CO2 and oxygen, rather than air, bringing us closer to the end goal of developing zero emission coal-fired utility boilers. However, high-pressure, high-temperature systems such as these are under-studied, and their behavior is difficult to measure. An accurate model for previously untested conditions requires data for validation. The heat release profile of flames and their radiative intensity is one of the key data sets required for model validation of an oxy-coal combustion system. A radiometer can be used to obtain the necessary radiative heat flux data. However, several studies show significant measurement errors of past radiometer designs. This work focuses on developing a narrow angle radiometer that can be used to describe radiative heat transfer from a pressurized oxy-coal flame. The sensitivity of the instrument to outside environmental influences is thoroughly examined, making it possible to obtain the axial radiative heat flux profile of the flame in a 100kW pressurized facility by accurately converting the measured quantities into radiative heat flux. Design aspects of the radiometer are chosen to improve the accuracy of radiative heat flux measurements as well as conform to the physical constraints of the 100kW pressurized facility. The radiometer is built with a 0.079-inch aperture, an 8.63-inch probe internally coated with high emissivity coating, four baffles spaced evenly down the length of the probe, no optic lens, a thermopile as the sensor, argon purge gas, and a water-cooled jacket. The radiometer has a viewing angle of 1.33 degrees. The instrument is calibrated with a black body radiator, and these calibration data are used in combination with radiation models to convert the radiometer signal in mV to radiative heat flux in kW/m2. Environmental factors affecting accuracy are studied. The results of the calibration data show that the radiometer measurements will produce a calculated heat flux that is accurate to within 5.98E-04 kW/m2.

L'objectif de ce travail est d’évaluer le positionnement et le potentiel des récepteurs à lit fluidisé à changement de section par rapport aux autres méthodes de chauffage de gaz à haute température par voie solaire. A cette fin, une connaissance approfondie des phénomènes thermiques et hydrodynamiques du récepteur est nécessaire. Pour acquérir cette connaissance, nous avons modélisé les transferts thermiques dans le récepteur en portant une attention particulière sur les transferts radiatifs en prenant en compte les diffusions multiples de la lumière dans le milieu particulaire, les effets de parois sur les transferts radiatifs et la directionnalité du rayonnement solaire concentré. La détermination précise de la distribution de particules dans le ciel du lit fluidisé s'est avérée un paramètre critique pour le calcul des transferts thermiques. Ces modèles, plus tard affinés par une confrontation avec des références expérimentales, nous ont permis d'explorer l'effet de la géométrie sur les transferts thermiques dans le récepteur. Il ont permis entre autres de mettre en évidence l'intérêt d'utiliser une colonne de fluidisation à changement de section et l'importance de l'optimisation du couple concentrateur solaire / récepteur afin d'éviter d'éventuelles surchauffes au niveau des parois du récepteur. De même, il semble que l'homogénéisation de la température dans le lit fluidisé contenu dans le récepteur soit favorable à son rendement<br>The aim of this work is to evaluate the position and the potential of solar fluidized bed receivers compared to other methods for the solar heating of gases at high temperature. To this end, a thorough knowledge of the heat transfer and hydrodynamic of the receiver is necessary. To acquire this knowledge, we modeled the heat transfer in the receiver with a focus on the radiative transfer by taking into account the multiple scattering of light in the particle medium, the effect of walls on radiative heat transfer and the directionality of the concentrated solar radiation. The accurate determination of the distribution of particles within the fluidized bed has been a critical parameter for the calculation of heat transfer. With these models, later refined by a confrontation with experimental references, we have studied the effect of geometry on heat transfer in the receiver. This study highlighted the necessity to use a switching section fluidization column and the importance to optimize the pair : solar concentrator / receiver to avoid any overheating at the walls of the receiver. Moreover, it appears that the homogenization of the temperature in the fluidized bed of the receiver increase its performance

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.

Este trabalho tem como objetivo implementar e testar modelos de gás cinza atualizados na rotina de radiação térmica do software Fire Dynamics Simulator (FDS), além da utilização do próprio modelo de gás cinza disponível no software, para a predição do fluxo de calor radiativo. Os modelos de gás cinza estudados foram o modelo padrão do software FDS (aqui denominado como GC1), e os modelos de gás cinza mais atuais: o GC2, no qual o coeficiente de absorção do meio participante é dado por relações polinomiais, e o GC3, sendo este um modelo de gás cinza que baseia o cálculo do coeficiente de absorção no modelo WSGG. Os novos modelos de gás cinza foram implementados no código fonte do software FDS, o qual é um código aberto, e a verificação da implementação foi realizada através da solução numérica do equacionamento utilizando os valores reportados pelo software. Com os novos modelos de gás cinza já corretamente implementados, passou-se então para a simulação computacional dos casos previamente selecionados. Para todos os modelos de gás cinza, foram simulados incêndios em poças, para diferentes combustíveis (etanol, n-heptano e metanol) em diferentes cenários de incêndio, considerando ou não a presença de fuligem no sistema. Os cenários de incêndio eram: (i) totalmente fechado, (ii) totalmente aberto e (iii) com uma condição intermediária, fechado, porém com uma abertura para o meio externo. Um estudo de análise de malha e de diferentes parâmetros, como o estudo da quantidade necessária de ângulos sólidos discretos, foram realizados para correta padronização dos parâmetros. As simulações computacionais foram validadas para o modelo de gás cinza padrão do FDS através da comparação de resultados com aqueles reportados na literatura específica de cada caso. Com os modelos já validados simulou-se novamente cada cenário de incêndio com os diferentes modelos de gás cinza anteriormente implementados. A partir da análise dos resultados obtiveram-se boas concordâncias para os campos de temperatura, frações molares tanto de CO2 quanto de H2O e para as frações volumétricas de fuligem. Os fluxos de calor radiativos foram corretamente preditos para todos os modelos de gás cinza implementados. O modelo GC2 apresentou resultados com desvios médios na faixa de 15%, o modelo de gás cinza baseado no WSGG (GC3) apresentou os melhores resultados, com erros médios inferiores a 10%, enquanto que o modelo padrão do software, GC1, apresentou resultados intermediários.<br>This work aims to implement and test updated gray gas models in the thermal radiation routine of the Fire Dynamics Simulator (FDS) software, as well as the use of the gray gas model available in the software to the prediction of radiative heat flux. The gray gas models studied were the default model of the FDS software (determined GC1), and the most current gray gas models: the GC2, in which the absorption coefficient of the participant medium is given by a polynomial relations, and the GC3, which is a gray gas model that was based on the calculation of the absorption coefficient in the WSGG model. The most recently gray gas models were implemented in the source code, which is an open source, and the verification of the implementation was performed by the numerical solution of the equations from the reported values of the software. With the new gray gas models already implemented, the next step was the computational simulation of the previously selected cases. For all the gray gas models, pool fires were simulated different scenarios of fire for different fuels (ethanol, nheptane and methanol), with and without considering soot presence in the system. The fire scenarios were: (i) fully closed, (ii) fully open and (iii) with an intermediate condition, closed but with an opening to the external environment. A study of a mesh analysis and different parameters, such as the study of the required amount of discrete solid angles, were performed to correct the standard parameters. The computational simulations were verified for the default gray gas model of the FDS by comparing the simulations results with those reported in the specific literature of each case. With the models already verified, each fire scenario was simulated with the different gray gas models previously implemented. From the analysis of the results, good agreements were obtained for the fields of temperature, molar fraction of CO2 and H2O and soot volume fraction. The radiative heat fluxes were correctly predicted for all gray gas models early implemented. The GC2 model present results with average deviation in the range of 15%, the gray gas model based on WSGG (GC3) presented the best results, with average deviation lower than 10%, while the default software model (GC1) presented intermediate results.

Current methods for measuring energy flow rate in a pipe use a variety of invasive sensors, including temperature sensors, turbine flow meters, and vortex shedding devices. These systems are costly to buy and install. A new approach that uses non-invasive sensors that are easy to install and less expensive has been developed. A thermal interrogation method using heat flux and temperature measurements is used. A transient thermal model, lumped capacitance method LCM, before and during activation of an external heater provides estimates of the fluid heat transfer coefficient h and fluid temperature. The major components of the system are a thin-foil thermocouple, a heat flux sensor (PHFS), and a heater. To minimize the thermal contact resistance R" between the thermocouple thickness and the pipe surface, two thermocouples, welded and parallel, were tested together in the same set-up. Values of heat transfer coefficient h, thermal contact resistance R", time constant �[BULLET], and the water temperature �[BULLET][BULLET], were determined by using a parameter estimation code which depends on the minimum root mean square RMS error between the analytical and experimental sensor temperature values. The time for processing data to get the parameter estimation values is from three to four minutes. The experiments were done over a range of flow rates (1.5 gallon/minute to 14.5 gallon/minute). A correlation between the heat transfer coefficient h and the flow rate Q was done for both the parallel and the welded thermocouples. Overall, the parallel thermocouple is better than the welded thermocouple. The parallel thermocouple gives small average thermal contact resistance average R"=0.00001 (m2.�[BULLET][BULLET]/W), and consistence values of water temperature and heat transfer coefficient h, with good repeatability and sensitivity. Consequently, a non-invasive energy flow rate meter or (BTU) meter can be used to estimate the flow rate and the fluid temperature in real life.<br>MS

The purpose of this study was to analyze the dropwise condensation on a cylindrical surface including the sweeping effect theoretically. For this purpose, first the problem of the equilibrium shape and departure size of drops on the outer surface of a cylinder was formulated. The equations of the surface of the drop were obtained by minimizing (for a given volume) the total energy of the drop which consists of surface and gravitational energy by using the techniques of variational calculus. The departure size of the droplets on a surface at varies angle of inclinations were also determined experimentally. Drop departure size is observed to decrease up to as the surface inclination was decreased up to 90 degree and then it increased up to 180 degree. Mean base heat flux, drop departure rate, sweeping frequency, fraction of covered area, sweeping period, local heat flux and average heat flux for the dropwise condensation on a cylindrical surface including the sweeping effect is formulated and the resulting integral equation was solved by using the finite difference techniques. The results show that drop departure rate and sweeping frequency was strongly affected by the angular position and reached asymptotic value at large angular positions. Comparing the results of the average heat flux values at different diameters show that at larger diameters the average heat flux becomes larger. This is due to the increased sweeping effect at larger diameters.

Le transfert de chaleur entre un fluide et un milieu poreux revêt un intérêt important dans de nombreux processus industriels tels que les régénérateurs, les réacteurs catalytiques et nucléaires et plus récemment les collecteurs solaires. Les travaux présentés constituent une contribution à l'étude des transferts couples de chaleur au sein d'un milieu poreux soumis à un flux radiatif et traversé par un fluide gazeux. Une fois exposées, les approches classiques, on détermine, à partir d'une analyse théorique, les champs de température du milieu solide et du fluide au cours de leurs phases d'évolution dynamique. Par la suite, dans le cadre d'une représentation concrète, l'étude détaillée d'un capteur à absorbeur poreux conduit à la formulation mathématique de trois modèles d'évolution spécifiques au régime transitoire. Pour confirmer la validité de l'analyse, des expériences sont conduites dans les conditions correspondant au modèle. Par ailleurs, une importance particulière est donnée au coefficient d'échange volumique interne au milieu poreux. On examine ainsi à travers des développements simples quelques méthodes conduisant à sa détermination en admettant qu'elles représentent une approche immédiate dans un cadre de conditions aux limites très restrictives. Les équations d'énergie couplées pour le gaz et le milieu poreux sont linearisées pour être résolues analytiquement par leur formulation intégrale, plus numériquement sur une base plus complète

A novel type of precast, prestressed concrete structural element is being implemented in load-bearing systems in buildings. These structural elements combine the use of high-performance, self-consolidating concrete (HPSCC) and non-corroding carbon fibre reinforced polymer (CFRP) prestressing tendons; this produces highly optimized, slender structural elements with excellent serviceability and (presumed) extended service lives. More widely, the use of new construction techniques, innovative materials, and ground-breaking designs is increasingly commonplace in today's rapidly evolving building construction industry. However, the performance of these and other structural elements in fire is in general not well known and must be understood before these can be used with confidence in load-bearing applications where structural fire resistance is a concern. Structural fire testing has traditionally relied on the use of the standard fire resistance test (i.e. furnace test) for assuring regulatory compliance of structural elements and assemblies, and in many cases also for developing the scientific understanding of structural response to fire. Conceived in the early 1900s and fundamentally unchanged since then, the standard testing procedure is characterized by its high cost and low repeatability. A novel test method, the Heat-Transfer Rate Inducing System (H-TRIS), resulting from a mental shift associated with controlling the thermal exposure not by temperature (e.g. temperature measured by thermocouples) but rather by the time-history of incident heat flux, was conceived, developed, and validated within the scope of the work presented in this thesis. H-TRIS allows for experimental studies to be carried out with high repeatability, imposing rationally quantifiable thermal exposure, all at low economic and temporal cost. The research presented in this thesis fundamentally seeks to examine and understand the behaviour of CFRP prestressed HPSCC structural elements in fire, with emphasis placed on undesired 'premature' failure mechanisms linked to the occurrence of heat-induced concrete spalling and/or loss of bond between the pretensioned CFRP tendons and the concrete. Results from fire resistance tests presented herein show that, although compliant with testing standards, temperature distributions inside furnaces (5 to 10% deviation) appear to influence the occurrence of heat-induced concrete spalling for specimens tested simultaneously during a single test; fair comparison of test results is therefore questionable if thermal exposure variability is not explicitly considered. In line with the aims of the research presented in this thesis, H-TRIS is used to carry out multiple comprehensive studies on the occurrence of concrete spalling and bond behaviour of CFRP tendons; imposing a quantified, reproducible and rational thermal exposure. Test results led to the conclusion that a "one size fits all" approach for mitigating the risk of heat-induced concrete spalling (e.g. prescribed dose of polypropylene (PP) fibres included in fresh concrete), appears to be ineffective and inappropriate in some of the conditions examined. This work demonstrates that PP fibre cross section and individual fibre length can have an influence on the risk of spalling for the HPSCC mixes tested herein. The testing presented herein has convincingly shown, for the first time using multiple repeated tests under tightly controlled thermal and mechanical conditions, that spalling depends not only on the thermal gradients in concrete during heating but also on the size and restraint conditions of the tested specimen. Furthermore, observations from large scale standard fire resistance tests showed that loss of bond strength of pretensioned CFRP tendons occurred at a 'critical' temperature of the tendons in the heated region, irrespective of the temperature of the tendons at the prestress transfer length, in unheated overhangs. This contradicts conventional wisdom for the structural fire safety design of concrete elements pretensioned with CFRP, in which a minimum unheated overhang is generally prescribed. Overall, the research studies presented in this thesis showed that a rational and practical understanding of the behaviour of CFRP prestressed HPSCC structural elements during real fires is unlikely to be achieved only by performing additional standard fire resistance tests. Hence, H-TRIS presents an opportunity to help promote an industry-wide move away from the contemporary pass/fail and costly furnace testing environment. Recommendations for further research to achieve the above goal are provided.

This work presents the development of a novel and non-invasive method that measures fluid flow rate and temperature in pipes. While current non-invasive flow meters are able to measure pipe flow rate, they cannot simultaneously measure the internal temperature of the fluid flow, which limits their widespread application. Moreover, devices that are able to determine flow temperature are primarily intrusive and require constant maintenance, which can shut down operation, resulting in downtime and economic loss. Consequently, non-invasive flow rate and temperature measurement systems are becoming increasingly attractive for a variety of operations, including for use in leak detection, energy metering, energy optimization, and oil and gas production, to name a few. In this work, a new solution method and parameter estimation scheme are developed and deployed to non-invasively determine fluid flow rate and temperature in a pipe. This new method is utilized in conjunction with a sensor-based apparatus--"namely, the Combined Heat Flux and Temperature Sensor (CHFT+), which employs simultaneous heat flux and temperature measurements for non-invasive thermal interrogation (NITI). In this work, the CHFT+ sensor embodiment is referred to as the British Thermal Unit (BTU) Meter. The fluid's flow rate and temperature are determined by estimating the fluid's convection heat transfer coefficient and the sensor-pipe thermal contact resistance. The new solution method and parameter estimation scheme were validated using both simulated and experimental data. The experimental data was validated for accuracy using a commercially available FR1118P10 Inline Flowmeter by Sotera Systems (Fort Wayne, IN) and a ThermaGate sensor by ThermaSENSE Corp. (Roanoke, VA). This study's experimental results displayed excellent agreement with values estimated from the aforementioned methods. Once tested in conjunction with the non-invasive BTU Meter, the proposed solution and parameter estimation scheme displayed an excellent level of validity and reliability in the results. Given the proposed BTU Meter's non-invasive design and experimental results, the developed solution and parameter estimation scheme shows promise for use in a variety of different residential, commercial, and industrial applications.<br>MS

Nous étudions dans cette thèse la rectification thermique de diodes thermiques radiatives ou conductive constituées de matériaux à changement de phase.Cette thèse est divisée en trois parties. Dans les premières parties, nous modélisons comparativement les performances d’une diode thermique conductive sphérique et cylindrique constitués de VO2 présentant un transition de phase et des matériaux n’en présentant pas. Des expressions analytiques aux bornes des diodes sont dérivées. Des flux thermiques, des facteurs de rectifications ainsi que les profils de température à l’intérieur de la diode sont obtenus. Nos résul-tats montrent que les différentes géométries de diodes ont un impact significatif sur les profils de température et les flux thermiques, mais moins un sur les facteurs de rectification. Dans ce travail, nous avons obtenu des facteurs de rectification maximaux allant jusqu’à 20.8% et 20.7%, qui sont supérieurs à celui prédit pour une diode plane constituée de VO2. Nous montrons également que des facteurs de rectification similaires à ceux obtenus avec le VO2 dans les géométries sphériques et cylindriques peuvent être atteints avec des matériaux à changement de phase dont le contraste de conductivité est plus important que dans le cas du VO2. Dans la deuxième partie, nous étudions la rectification de diodes thermiques constituées de deux matériaux à changement de phase. Avec, l’idée de générer un facteur de redressement plus élevé que dans le cas d’une diode thermique conductive ne comprenant qu’un matériau à changement de phase unique. Là encore, le travail a conduit à l’établissement d’expressions explicites pour les profils de température, les flux thermiques et le facteur de rectification. Nous avons obtenu un facteur de rectification optimal de 60% avec une variation de température de 250 K couvrant les transitions métal-isolant des deux matériaux. Dans la troisième partie de notre travail, nous avons modélisé et optimisé la rectification thermique de diodes thermiques planes, cylindriques et sphériques radiatives à base de deux matériaux à changement de phase. Nous savons calculer et analyser les facteurs de rectification de ces trois diodes et obtenu les facteurs de rectification optimaux respectifs pour les trois géométries 82%, 86% et 90.5%. Nos résultats montrent que la géométrie sphérique est la meilleure pour optimiser la rectification des courants thermiques radiatifs. De plus, des facteurs de rectification potentiellement supérieurs à ceux prédits ici peuvent être réalisés en utilisant deux matériaux à changement de phase avec des contrastes d’émissivités plus élevés que ceux proposés ici. Ces résultats analytiques et graphiques fournissent un guide utile pour optimiser les facteurs de rectification des diodes thermiques conductives et radiatifs basées sur des matériaux à changement de phase de géométries différentes<br>The thermal rectification of conductive and radiative thermal diodes based on phase-change materials, whose thermal conductivities and effective emissivities significant change within a narrow range of temperatures, is theoretically studied and optimized in different geometries. This thesis is divided into three parts. In the first part, we comparatively model the performance of a spherical and cylindrical conductive thermal diodes operating with vanadium dioxide (VO2) and non-phase-change materials, and derive analytical expressions for the heat flows, temperature profiles and optimal rectification factors for both diodes. Our results show that different diode geometries have a significant impact on the temperature profiles and heat flows, but less one on the rectification factors. We obtain maximum rectification factors of up to 20.8% and 20.7%, which are higher than the one predicted for a plane diode based on VO2. In addition, it is shown that higher rectification factors could be generated by using materials whose thermal conductivity contrast is higher than that of VO2. In the second part, on the other hand, we theoretically study the thermal rectification of a conductive thermal diode based on the combined effect of two phase-change materials. Herein, the idea is to generate rectification factors higher than that of a conductive thermal diode operating with a single phase-change material. This is achieved by deriving explicit expressions for the temperature profiles, heat fluxes and rectification factor. We obtain an optimal rectification factor of 60% with a temperature variation of 250 K spanning the metal-insulator transitions of VO2 and polyethylene. This enhancement of the rectification factor leads us to the third part of our work, where we model and optimize the thermal rectification of a plane, cylindrical and spherical radiative thermal diodes based on the utilization of two phase-change materials. We analyze the rectification factors of these three diodes and obtain the following optimal rectification factors of 82%, 86% and 90.5%, respectively. The spherical geometry is thus the best shape to optimize the rectification of radiative heat currents. In addition, potential rectification factors greater than the one predicted here can be realized by utilizing two phase-change materials with higher emissivities contrasts than the one proposed here. Our analytical and graphical results provide a useful guide for optimizing the rectification factors of conductive and radiative thermal diodes based on phase-change materials with different geometries

This master thesis project was performed in collaboration with Scania CV AB, Engine Materials group. The purpose with the project was to investigate different ceramic TBC (Thermal Barrier Coating) thermal insulation properties inside the combustion chamber. Experimental testing was performed with a Single-Cylinder engine with TBC deposited on selected components. A dummy-valve was developed and manufactured specifically for this test in order to enable a water cooling system and to ease the testing procedure. The dummy-valve consists of a headlock, socket, valve poppet and valve shaft. Additionally, a copper ring is mounted between the cylinder head and the valve poppet to seal the system from combustion gases. Thermocouples attached to the modified valve poppet and valve shaft measured the temperature during engine test to calculate the heat flux. The TBCs consisted of three different materials: 7-8% yttrium-stabilized zirconia (8YSZ), gadolinium zirconia and lanthanum zirconia. The 8YSZ TBC was tested as standard, but also with microstructural modifications. Modifications such as pre-induced segmented cracks, nanostructured zones and sealed porosity were used. The results indicated that the heat flux of 8YSZ-standard, 8YSZ-nano and 8YSZ-segmented cracks was in level with the steel reference. In the case of 8YSZ-sealed porosity the heat flux was measured higher than the steel reference. Since 8YSZ-standard and 8YSZ-sealed porosity are deposited with the same powder it is believed that the high heat flux is caused by radiative heat transfer. The remaining samples have had some microstructural changes during engine testing. 8YSZ-nano had undergone sintering and its nanostructured zones became fewer and almost gone after engine testing leading to less heat barrier in the top coat of the TBC. However, for 8YSZ-segmented cracks and gadolinium zirconia lower heat flux was measured due to the appearance of horizontal cracks. These cracks are believed to act as internal barriers as they are orientated perpendicular to the heat flow. During long-time (5 hour) engine tests the 8YSZ-standard exhibited the same phenomena: a decrease in heat flux due to propagation of horizontal cracks. One-dimensional heat flux was not achieved and the main reason for that was caused by heating and cooling of the shafts outer surface. However, the dummy-valve system has proven to be a quick, easy and stable to perform tests with a Single-Cylinder engine. Both water-cooling and long-time engine tests were conducted with minor issues. The dummy-valve has been further developed for future tests. Changes to the valve shaft are the most remarkable: smaller diameter to reduce heat transfer and smaller pockets to ensure better thermocouple positioning. Another issue was gas leakage from the combustion chamber through the copper ring and valve poppet joint. The copper ring will be designed with a 1 mm thick track to improve sealing, hence better attachment to the valve poppet.

Cette thèse propose une modélisation avancée des transferts thermiques dans les écoulements turbulents pour tous les types de conditions aux limites sur la température aux parois. Ces travaux reposent sur un double constat : d'une part, les modèles de turbulence traitant la thermique de l'écoulement dans la plupart des applications industrielles sont basés sur de simples relations algébriques incapables de représenter des physiques complexes, comme la convection naturelle et la thermique de la zone proche-paroi. D'autre part, la condition aux limites sur la température à la paroi (température fixée, flux de chaleur imposé ou transfert thermique conjugué) influence le comportement proche-paroi des variables thermiques turbulents. La formulation du modèle bas-Reynolds du second ordre des flux thermiques turbulents EBDFM (Elliptic Blending Differential Flux Model), développée à l'origine pour traiter des cas où une température est fixée à la paroi, a été étendue à des cas de flux de chaleur imposé et de transfert thermique conjugué. Cette nouvelle formulation se fonde sur des analyses asymptotiques rigoureuses des termes des équations de transport des flux thermiques turbulents pour chaque condition aux limites sur la température. Un des éléments essentiels de la nouvelle formulation de l'EBDFM est le ratio des échelles de temps thermique et dynamique R. Le comportement asymptotique de ce ratio dépend fortement de la condition aux limites : R tend vers le nombre de Prandtl à la paroi lorsqu'une température est imposée, et vers l'infini sinon. Ainsi, dans le but de reproduire fidèlement ce comportement, il s'est avéré nécessaire de résoudre des équations de transport pour la variance de température ¯(θ^'2 )et pour son taux de dissipation ε_θ puisque ces deux variables pilotent le comportement asymptotique de R. Par conséquent, cette thèse propose des modèles bas-Reynolds pour les variables ¯(θ^'2 )et ε_θ valables pour toutes les conditions aux limites thermiques. La nouvelle formulation du modèle EBDFM ainsi que les modèles de ¯(θ^'2 )et ε_θ ont été validées par des simulations réalisées avec le logiciel de CFD Code_Saturne pour des écoulements dans un canal plan en convection forcée<br>Advanced modeling of turbulent heat transfer for all thermal boundary conditions is proposed. This work was motivated by two facts: first, the thermal turbulent models used in most of the industrial computations are based on eddy-viscosity models which cannot deal with complex physics such as natural convection or heat transfer in the near-wall region. Then, the thermal boundary condition at the wall (imposed temperature, imposed heat flux, conjugate heat transfer) influences the near-wall behavior of the turbulent thermal variables. The formulation of the low-Reynolds number second moment closure EBDFM (Elliptic Blending Differential Flux Model), which was originally developed for an imposed temperature at the wall, has been extended to an imposed heat flux and a conjugate heat transfer condition. This new formulation is based on rigorous asymptotic analysis of the terms of the transport equation of the turbulent heat flux for all thermal boundary conditions. One of the key elements is the thermal-to-mechanical time-scale ratio R. Its asymptotic behavior highly depends on the thermal boundary condition: R goes to the Prandtl number at the wall for an imposed temperature and tends to infinity otherwise. Thus, solving a transport equation for the temperature variance ¯(θ^'2 ) and for its dissipation rate ε_θ is necessary to reproduce the asymptotic behavior of R. Indeed, these two variables drive the behavior of R in the near-wall region. Therefore, low-Reynolds number models for ¯(θ^'2 ) and ε_θ, valid for all thermal boundary conditions, are proposed. The new formulation of the EBDFM and the models for ¯(θ^'2 ) and ε_θ have been validated by performing Code_Saturne computations of channel flows in the forced convection regime

"Review of the literature shows that the reported correlation between predictions and experimental data of flame spread vary greatly. The discrepancies displayed by the models are generally attributed to inaccurate input parameters, user effects, and inadequacy of the model. In most experiments, the metric to which the model is deemed accurate is based on the prediction of the heat release rate, but flame spread is a highly complex phenomenon that should not be simplified as such. Moreover, fire growth models are usually made up of distinctive groups of calculation on separate physical phenomena to predict processes that drive fire growth. Inaccuracies of any of these “sub-models” will impact the overall flame spread prediction, hence identifying the sources of error and sensitivity of the subroutines may aid in the development of more accurate models. Combating this issue required that the phenomenon of flame spread be decomposed into four components to be studied separately: turbulent fluid dynamics, flame temperature, flame heat transfer, and condensed phase pyrolysis. Under this framework, aspects of a CFD model may be validated individually and cohesively. However, a lack of comprehensive datasets in the literature hampered this process. Hence, three progressively more complex sets of experiments, from free plume fires to fires against an inert wall to combustible wall fires, were conducted in order to obtain a variety of measurements related to the four inter-related components of flame spread. Multiple permutations of the tests using different source fuels, burner size, and source fire heat release rate allowed a large amount of comparable data to be collected for validation of different fire configurations. FDS simulations using mostly default parameters were executed and compared against the experimental data, but found to be inaccurate. Parametric study of the FDS software shows that there are little definitive trends in the correlation between changes in the predicted quantities and the modeling parameters. This highlights the intricate relationships shared between the subroutines utilized by FDS for calculations related to the four components of flame spread. This reveals a need to examine the underlying calculation methods and source code utilized in FDS."

The quest of travelling beyond earth, preludes with ground based experimental studies, detailed analysis and accurate calculations in the aspects of having a safer design of flying vehicles. As the vehicles plunge into the dense atmosphere with greater velocities to hypersonic Mach numbers, the shockwave produced ahead of the aerodynamic body becomes highly intense producing volatile conditions at a temperature of several thousands of Kelvins. Predominantly the unsteady effects are dominated by radiations in the velocities which are greater than 6km/s. During such high enthalpy flows, the atmospheric molecules which cross the strong shockwave are excited to higher energy states. Therefore the shocked gas ahead of the space vehicle is at a state of chemical and thermal non-equilibrium. To attain equilibrium condition, energy is released from high enthalpy fluid to surroundings. The aerodynamic body which faces this energy release is heated by all modes of heat transfer. Behind the normal shock, excited molecules relax to lower levels by energy releasing mechanisms including emission of radiation. In this process, initial photons emitted are absorbed by other molecules further raising its energy levels leading to dissociation and ionization. During recombination of molecular species more photons are released. Such radiations from molecules and shock wave reach the surface of the aerodynamic body. Collective absorption of all incident radiation heats up the surface of planetary entry body. In particular, the radiative heating predominates at very high velocities. Direct measurement of total radiative heating is highly challenging due to the complexity in finding out a proper measurement device. Existing literatures show that only a partial amount of radiative heating could be measured by thin film gauges, since the efficiency of thin film based measurement technique depends on the absorption of sensing element used and the wavelength range of the radiation. In the present work, it is attempted to measure the radiative heat flux over aerodynamic body in the hypersonic flow condition. To overcome the limitations imposed by the existing measurement technique, a novel thermal sensing element based on Carbon is devised, which is denoted as Large Carbon Cluster. LCC is prepared by single step pyrolysis technique with benzene and ferrocene as precursor mixture. The ratio of precursor mixture is varied to find the proper LCC layer to be formed on a ceramic substrate to get a particular electrical resistance in order to use it as a thermal sensing element. Calibration of the devised carbon allotrope i.e. LCC is found to be having very good thermo-electric characteristics. Several thermal gauges are developed based on LCC for aerodynamic models to test them for the total heat flux rate in Mach 8 hypersonic flow generated in hypersonic shock tunnel – HST2. The performance of the gauges is compared with the existing platinum based thin-film thermal gauges. It is found that the LCC based thin film gauges perform better than platinum thin-film heat transfer gauges. The durability of LCC is also found to be better than platinum. The main aim of finding LCC is that it has good optical absorptivity than any other thermal sensing element; therefore it can be used in radiative heat flux measurements. Aerodynamic models are prepared with the radiative thermal gauges based on LCC and these models are tested in different atmospheric hypersonic test flows. The results reveal that radiative heat flux rate is significantly measured even at lower velocity hypersonic flow conditions. This gives a great confidence on using the LCC based thermal gauges for higher velocity flow conditions and to real time test flights.

碩士<br>國立交通大學<br>機械工程學系<br>98<br>An experiment is carried out in the present study to investigate the characteristics of time periodic evaporation heat transfer for refrigerant R-410A flowing in a horizontal narrow annular duct subject to an imposed time periodic mass flux oscillation or heat flux oscillation. The mass flux oscillation and heat flux oscillation are both in the form of triangular waves. The gap of the duct is fixed at 2.0 mm. In the study, the effects of the refrigerant mass flux oscillation, heat flux oscillation, R-410A saturation temperature, imposed heat flux and vapor quality of the refrigerant on the temporal evaporating flow heat transfer and the photos of the evaporating flow will be examined in detail. The present experiment is conducted for the mean refrigerant mass flux varied from 300 to 500 kg/m2s, the amplitude of the mass flux oscillation is fixed at 10, 20 and 30% with the period of the mass flux oscillation tp fixed at 20, 60 and 120 seconds. Besides, the mean imposed heat flux is varied from 0 KW/ m2 to 15 KW/ m2, the amplitude of the heat flux oscillation is chosen to vary from 10% to 50% of the mean heat flux , and the period of the q oscillation tp is also fixed at 20, 60, 120 seconds. The mean refrigerant saturation temperature is set at 5, 10 and 15 ℃ for the mean refrigerant vapor quality varied from 0.05 to 0.95. The measured evaporation heat transfer data are expressed in terms of the variations of the heated wall temperature and evaporation heat transfer coefficient with time. In the first part of the study the results for the R-410A evaporation subject to the mass flux oscillation are presented. The measured heat transfer data for the R-410A evaporating flow for a constant coolant mass flux are first compared with the time-average data for a time periodic mass flux oscillation. This comparison shows that the mass flux oscillation exerts negligible influences on the time-average evaporation heat transfer. Then, we present the data to elucidate the effects of the experimental parameters on the amplitude of Tw oscillation over wide ranges of the experimental parameters. The results indicate that the Tw oscillation is stronger for higher amplitude and a longer period of the mass flux oscillation. However, a small time lag in the Tw oscillation is also noted. Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and liquid film take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets better as the mass flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the mass flux increases the opposite processes occur. These unusual changes of the heating surface temperature and heat transfer coefficient with the mass flux oscillation are attributed to the strong effects of the mass flux oscillation on the state of the refrigerant at the duct inlet and hence on the changes of the vapor quality and liquid film thickness in the evaporating flow. In the second part of the study results for the R-410A evaporation subject to the heat flux oscillation are also presented. Effects of the mean level and oscillation amplitude and period of the heat flux on the time periodic R-410A evaporation heat transfer have been investigated in detail. We first note that the time-average heat transfer coefficients for the time periodic evaporation of R-410A are not affected to a noticeable degree by the amplitude and period of the imposed heat flux oscillation. Then, the heated pipe wall temperature and evaporating flow pattern also oscillate periodically in time and at the same frequency as the heat flux oscillation. Experiment also shows that the resulting oscillation amplitudes of the wall temperature get longer for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. A significant time lag in the heated surface temperature oscillation is also noted, which apparently results from the thermal inertia of the copper inner pipe. Moreover, at the intermediate vapor quality changes in the evaporating flow pattern between that dominated by the nucleation bubbles and liquid film take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets worse as the heat flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the heat flux increases the opposite processes occur. These changes of the heating surface temperature and heat transfer coefficient with the heat flux oscillation are attributed to the strong effects of the heat flux oscillation and hence on the changes of the vapor quality and liquid film thickness in the evaporating flow. The effects of heat flux oscillation at extremely short and long periods have been explored. Due to the existence of the thermal inertia of the heated copper duct, the resulting heated surface temperature does not oscillate with time at an extremely short period of the imposed heat flux oscillation. But the oscillation amplitude of the heated surface temperature gets noticeably stronger for an extremely long period of the imposed heat flux oscillation.

博士<br>國立交通大學<br>機械工程學系<br>98<br>Experiments have been conducted here to investigate how the imposed time periodic refrigerant flow rate or heat flux oscillation affects the saturated and subcooled flow boiling heat transfer and associated bubble characteristics for refrigerant R-134a in a horizontal narrow annular duct. Besides, the evaporation heat transfer of R-134a flow in the same duct are examined. The test section for the horizontal annular duct consists of an outer pipe made of Pyrex glass and an inner heated copper pipe, intending to measure the boiling heat transfer coefficient and to facilitate the visualization of boiling processes. A cartridge heater is installed inside the inner pipe to provide the required heat flux to the refrigerant flow in the narrow annular duct. In the study the gap of the duct is varied from 1.0 to 5.0 mm with the mean refrigerant mass flux, saturated temperature, imposed heat flux and mean vapor quality respectively ranging from 100 to 600 kg/m2s, 5 to 15℃, 0 to 45 kW/m2 and 0.05 to 0.95. The inlet subcooling is varied from 3 to 6℃. In particular, attention is focused on the time periodic flow boiling characteristics affected by the mean levels, amplitudes and periods of the flow rate or heat flux oscillation. Some results have been obtained and are reported here. In the first part of the present study, experiments have been carried out to investigate the effects of the imposed time periodic refrigerant flow rate oscillation in the form of nearly a triangular wave on the saturated and subcooled flow boiling and evaporation heat transfer and associated bubble characteristics of R-134a in a horizontal narrow annular duct. The results indicate that when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface only exists in a partial time interval of each periodic cycle. But the intermittent boiling prevails in narrower ranges of the experimental parameters in the subcooled flow boiling. At somewhat higher heat flux persistent boiling prevails. Besides, the refrigerant flow rate oscillation is found to negligibly affect the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, active nucleation site density, and evaporating flow pattern are noted to oscillate periodically in time as well and at the same frequency as the imposed mass flux oscillation. Furthermore, in the persistent boiling the resulting Tw oscillation is stronger for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in dp, f and nac are noted. Specifically, in the first half of the periodic cycle in which the mass flux decreases with time the departing bubbles are larger and the departure rate is lower but the active nucleation site density is higher. The opposite is the case in the second half of the cycle in which the mass flux increases. The effects of the mass flux oscillation on the departing bubble size and active nucleation site density dominate over the bubble departure frequency, causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing G in the flow boiling, opposing to that in the single-phase flow. But the bubble characteristics are only mildly affected by the period of the mass flux oscillation. However, a short time lag in the Tw oscillation is also noted. Finally, flow regime maps are provided to delineate the boundaries separating different boiling regimes for the R-134a saturated and subcooled flow boiling in the annular duct. Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and liquid film resulting from the refrigerant flow rate oscillation take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets better as the mass flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the mass flux increases the opposite processes occur. These unusual changes of the heating surface temperature and heat transfer coefficient with the mass flux oscillation are attributed to the strong effects of the mass flux oscillation on the state of the refrigerant at the duct inlet and hence on the changes of the vapor quality and liquid film thickness in the evaporating flow. In the second part of the present study, experiments are conducted to investigate how the imposed time periodic heat flux oscillation also in the form of nearly a triangular wave on the refrigerant R-134a saturated and subcooled flow boiling and evaporation heat transfer and associated bubble characteristics in a horizontal narrow annular duct. The results also show that when the mean imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface only exists in a partial interval of each periodic cycle. But the intermittent boiling appears in narrower ranges of experimental parameters in the subcooled flow boiling. At somewhat higher heat flux persistent boiling prevails. Besides, the heat flux oscillation does not noticeably affect the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, active nucleation site density, and evaporating flow pattern are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the persistent boiling the resulting oscillation amplitudes of the heated surface temperature, heat transfer coefficient and bubble parameters, such as dp, f and nac, get larger for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. A significant time lag in the Tw oscillation is noted. In the first half of the periodic cycle in which the heat flux decreases with time, after the time lag the heated wall temperature decreases with time, so does the bubble parameters. The opposite processes occur in the second half of the cycle in which q increases with time. Finally, flow regime maps are provided to delineate the boundaries separating different boiling regimes for the R-134a saturated and subcooled flow boiling in the annular duct. Moreover, at the intermediate vapor quality changes in the evaporating flow patterns between that dominated by the nucleation bubbles and by the liquid film resulting from the heat flux oscillation take place cyclically. Furthermore, after the time lag the heated pipe wall temperature decreases and the evaporation heat transfer gets worse as the heat flux decreases in the first half of the periodic cycle. In the second half of the cycle in which the heat flux increases the opposite processes occur. These changes of the heating surface temperature and heat transfer coefficient with the heat flux oscillation are attributed to the strong effects of the heat flux oscillation on the changes of the vapor quality and liquid film thickness in the evaporating flow.

碩士<br>國立交通大學<br>機械工程學系<br>98<br>An experiment is carried out here to investigate the heat transfer and associated bubble characteristics in time periodic flow boiling of refrigerant R-410A in a horizontal narrow annular duct subject to a time periodic mass flux or heat flux oscillation. Both the imposed mass flux and heat flux oscillations are in the form of triangular waves. Effects of the refrigerant mass flux oscillation, heat flux oscillation, and refrigerant saturated temperature on the temporal flow boiling heat transfer and bubble characteristics are examined. The bubble characteristics at the middle axial location of the duct are obtained from the flow visualization of the boiling flow, including the time variations of the bubble departure diameter and frequency and active nucleation site density. The present experiment is conducted for the mean refrigerant mass flux varied from 300 to 500 kg/m2s, the amplitude of the mass flux oscillation is fixed at 10, 20 and 30% and the amplitude of the heat flux oscillation is fixed at 10, 30 and 50% of their respective mean levels and , and the period of the G or q oscillation is fixed at 20, 60 and 120 seconds. The mean refrigerant saturation temperature is set at 5, 10 and 15 ℃. The gap of the duct is fixed at 2.0 mm. The measured boiling heat transfer data are expressed in terms of the boiling curves and boiling heat transfer coefficients along with the time variations of the heated wall temperature. In the first part of the present study the measured heat transfer data for the R-410A flow boiling for a constant coolant mass flux are first compared with the time-average data for the flow subject to a time periodic mass flux oscillation. This comparison shows that the mass flux oscillation exerts negligible influences on the time-average boiling heat transfer coefficients. Then, we present the data to elucidate the effects of the experimental parameters on the amplitude of Tw oscillation over a wide range of the imposed heat flux covering the single-phase, intermittent and persistent boiling flow regimes. The results indicate that the Tw oscillation is stronger for a higher amplitude and a longer period of the mass flux oscillation. However, the mean saturated temperature of the refrigerant exhibits much weaker effects on the Tw oscillation and the mean refrigerant mass flux exerts nonmonotonic effects on the amplitude of the Tw oscillation. Moreover, the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density are found to oscillate periodically in time and at the same frequency as the mass flux oscillation. Furthermore, the oscillations of dp, f and nac are somewhat like triangular waves. In the first half of the cycle in which the mass flux decreases linear increases in dp and nac and linear decrease in f are found. The effect of on nac oscillation is much stronger than on dp and f oscillation causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing G in the flow boiling opposed to that in the single-phase flow. But they are only slightly affected by the period of the mass flux oscillation. Besides, a small time lag in the Tw oscillation is also noted. In the second part of the present study the measured heat transfer data for the R-410A flow boiling for a constant heat flux are also first compared with the time-average data for a time periodic heat flux oscillation. This comparison shows that the time-average heat transfer coefficients are not affected by the time periodic heat flux oscillation to a significant degree. Then, we present the data to elucidate the effects of the experimental parameters on the amplitude of Tw oscillation over a wide range of the mean imposed heat flux covering the single-phase, intermittent and persistent boiling flow regimes. The results indicate that the Tw oscillation gets stronger for a higher amplitude and a longer period of the imposed heat flux oscillation and for a higher mean imposed heat flux. Moreover, a significant time lag in the heated surface temperature oscillation is also noted, which apparently results from the thermal inertia of the copper inner pipe. The effects of the heat flux oscillation at extremely short and long periods have been explored. Due to the existence of the thermal inertia of the heated copper duct, the resulting heated surface temperature does not oscillate with time at an extremely short period of the imposed heat flux oscillation. When the mean imposed heat flux is close to the heat flux corresponding to that for the onset of stable flow boiling, intermittent flow boiling appears. A flow regime map and an empirical correlation are given to delineate the boundaries separating different boiling flow regimes in the annular duct subject to imposed heat flux oscillation. Furthermore, the bubble departure diameter and frequency, and active nucleation site density also oscillate periodically in time and at the same frequency as the heat flux oscillation. The results also show that the oscillations in dp, f and nac get larger for a long period and a larger amplitude of the impose heat flux oscillation and for a higher mean imposed heat flux. Furthermore, the bubbles become smaller and more dispersed after the time lag when the imposed heat flux decreases with time. The opposite processes take place at increasing heat flux.