Inorganic salts are very promising as high-temperature heat transfer fluids and thermal storage media in solar thermal power production. The dual-tank molten salt storage system, for example, has been demonstrated to be effective for continuous operation in solar power tower plants. In this particular storage regime, however, much of the thermal storage potential of the salts is ignored. Most inorganic salts are characterized by high heats of fusion, so their use as phase-change materials (PCMs) allows for substantially higher energy storage density than their use as sensible heat storage alone. For instance, use of molten sodium-potassium eutectic salt over a temperature range of 260 to 560°C (the approximate operating parameters of a proposed utility-scale storage system) allows for a volumetric energy storage density of 212 kWhth/m3, whereas the use of pure sodium nitrate (T_m = 307°C) over the same temperature range (utilizing both sensible and latent heat) yields a storage density of 347 kWhth/m3. The main downside to these media is their relatively low thermal conductivity (typically on the order of 1 W/m-K). While low conductivity is not as much an issue with heat transfer fluids, which, owing to convective heat transfer, are not as reliant on conduction as a heat transfer mode, it can become important for PCM storage strategies, in which transient charging behavior will necessarily involve heating the solid-phase material up to and through the process of melting. This investigation seeks to develop new methods of improving heat transfer in inorganic salt latent heat thermal energy storage (TES) media, such as sodium / potassium nitrates and chlorides. These methods include two basic strategies: first, inclusion of conductivity-enhancing additives, and second, incorporation of infrared absorptive additives in otherwise transparent media. Also, in the process, a group of chloride based salts for use as sensible storage media and/or heat transfer fluids has been developed, based on relevant cost and thermophysical properties data. For direct conductivity enhancement, the idea is simple: a PCM with low conductivity can be enhanced by incorporation of nanoparticulate additives at low concentration (~5 wt %). This concept has been explored extensively with lower temperature heat transfer fluids such as water, ethylene glycol, etc. (e.g., nanofluids), as well as with many lower temperature PCMs, such as paraffin wax. Extension of the concept to high temperature inorganic salt thermal storage media brings new challenges—most importantly, material compatibility. Also, maintenance of the additive distribution can be more difficult. Promising results were obtained in both these regards with nitrate salt systems. The second heat transfer enhancement strategy examined here is more novel in principle: increasing the infrared absorption of a semitransparent salt PCM (e.g., NaCl) with a suitable additive can theoretically enhance radiative heat transfer (for sufficiently high temperatures), thereby compensating for low thermal conductivity. Here again, material compatibility and maintenance of additive dispersion become the focus, but in very different ways, owing to the higher temperatures of application (>600°C) and the much lower concentration of additives required (~0.5 wt %). Promising results have been obtained in this case, as well, in terms of demonstrably greater infrared absorptance with inclusion of additives.

<p>Thermal radiation, an omnipresent phenomenon characterized by electromagnetic wave emission from objects above absolute zero, has consistently intrigued scientific exploration throughout history and profoundly influences various technological applications. Traditionally, the primary utilization of thermal radiation has been limited to fields such as lighting, cooling, and energy harvesting. However, the true potential of thermal radiation extends far beyond these energy-oriented applications. Every object imprints a unique signature within its emitted thermal radiation. These signatures, distinguished by their wide-ranging spectral and polarimetric characteristics, represent a rich information source about the emitting objects. The goal of this dissertation is to offer novel prospective and platforms to expand our perception and utilization of the spectral and polarimetric attributes of thermal radiation. It seeks to augment the conventional understanding of thermal radiation as merely an energy source, underlining its immense potential as an information carrier.</p> <p>This dissertation explores the spectral and polarimetric features left within the thermal radiation and how these features can be manipulated. The research uncovers that the macroscopic spectral, spatial, and particularly spin properties of thermal radiation are intimately connected to the underlying symmetry of the microscopic emitters within a nanophotonic system. This close relationship between symmetry and thermal radiation introduces a universal strategy to gain thorough control over the spectral-polarimetric properties of thermal radiation. The control of these properties may spur pioneering developments in encoding information within thermal radiation.</p> <p>Furthermore, platforms to decode these spectral and polarimetric properties in thermal radiation are as pivotal as the encoding platforms. These decoding platforms allow us to uncover hidden messages within this invisible light and enable us to push the boundaries of fully passive and physics-aware machine perception. Nevertheless, contemporary methods for spectrum and polarization resolved detection of thermal radiation, especially in imaging form, are cumbersome, lacking robustness, and prohibitively expensive. Hence, this dissertation explores two fundamentally innovative spectral separation schemes: the nonlocal super-dispersion enabled by optically active crystals and the dispersive dichroism in 2D infrared metasurfaces. These methods present compact, cost-effective, and high-performance solutions for spectral-polarimetric thermal imaging, thereby enhancing its efficacy in diverse applications.</p>

The primary objective of the work reported here is to develop a methodology to predict the uncertainty associated with radiation heat transfer problems solved using the Monte Carlo ray-trace method (MCRT). Four equations are developed to predict the uncertainty of the distribution factor from one surface to another, the global uncertainty of all the distribution factors in an enclosure, the uncertainty of the net heat flux from a surface, and the global uncertainty of the net heat flux from all the surfaces in an enclosure, respectively. Numerical experiments are performed to successfully validate these equations and to study the impact of various parameters such as the number of surfaces in an enclosure, the number of energy bundles traced in the MCRT model, the fractional uncertainty of emissivity and temperature, and the temperature distribution in the enclosure. Finally, the methodology is successfully applied to a detailed MCRT model of a CERES-like radiometer. / Ph. D.

The work presented here addresses an important topic in thermal radiation detection when gold-black is used as an absorber. Sought is a model to simulate the absorption of thermal radiation by gold-black. Fractal geometry is created to simulate the topology of gold-black. Then electrical circuits based on the topology are identified that capture the physics of the interaction between the gold-black material and incident electro-magnetic radiation. Parameters of the model are then adjusted so results obtained are comparable to absorption data from the literature. Potential next-generation radiometric instruments will likely involve thermal radiation detectors using gold-black as an absorbing medium. A model that accurately simulates gold-black absorption will be an important tool in their design. / Master of Science

As the Earth's atmosphere evolves due to human activity, today's modern industrial society relies significantly on the scientific community to foresee possible atmospheric complications such as the celebrated greenhouse effect. Scientists, in turn, rely on accurate measurements of the Earth Radiation Budget (ERB) in order to quantify changes in the atmosphere. The Thermal Radiation Group (TRG), a laboratory in the Department of Mechanical Engineering at Virginia Polytechnic Institute and State University, has been at the edge of technology designing and modeling ERB instruments. TRG is currently developing a new generation of thermoelectric detectors for ERB applications. These detectors consist of an array of thermocouple junction pairs that are based on a new thermopile technology using materials whose electro-thermophysical properties are not completely characterized. The objective of this investigation is to design experiments aimed at determining the electro-thermophysical properties of the detector materials. These properties are the thermal conductivity and diffusivity of the materials and the Seebeck coefficient of the thermocouple junctions. Knowledge of these properties will provide fundamental information needed for the development of optimally designed detectors that rigorously meet required design specifications. / Master of Science

This dissertation focuses on developing new technologies for ultra-low-cost sapphire fiber-optic high-temperature sensors. The research is divided into three major parts, the souceless sensor, the simple Fabry-Perot (F-P) interrogator, and the sensor system. Chapter 1 briefly reviews the background of thermal radiation, fiber optic F-P sensors, and F-P signal demodulation. The research goal is highlighted. In Chapter 2, a temperature sensing system is introduced. The environmental thermal radiation was used as the broadband light source. A sapphire wafer F-P temperature sensor head was fabricated, with an alumina cap designed to generate a stable thermal radiation field. The radiation-induced optical interference pattern was observed. We demodulated the temperature sensor by white-light-interferometry (WLI). Temperature resolution better than 1°C was achieved. Chapter 3 discusses a novel approach to demodulate an optical F-P cavity at low-cost. A simple interrogator is demonstrated, which is based on the scanning-white-light-interferometry (S-WLI). The interrogator includes a piece of fused silica wafer, and a linear CCD array, to transform the F-P demodulation from the optical frequency domain to the spatial domain. By using the light divergence of an optical fiber, we projected a tunable reference F-P cavity onto an intensity distribution along a CCD array. A model for S-WLI demodulation was established. Performance of the new S-WLI interrogator was investigated. We got a good resolution similar to the well-known traditional WLI. At last, we were able to combine the above two technologies to a sapphire-wafer-based temperature sensor. The simple silica wafer F-P interrogator was optimized by focusing light to the image sensor. This approach improves the signal to noise ratio, hence allows the new integrator to work with the relatively weak thermal radiation field. We, therefore, proved in the experiment, the feasibility of the low-cost sourceless optical Fabry-Perot temperature sensor with a simple demodulation system. / PHD / Temperature measurements for high temperature harsh environments is a challenge industrial task. In this work, a low-cost sapphire fiber high temperature sensor is introduced which uses single crystal sapphire fiber as the light guiding and a sapphire-wafer-based Fabry-Perot (F-P) interferometer as the temperature sensing element. The research goal is to provide an optical sensing system whose price is competitive to the high temperature thermocouples. Two technologies were developed to reduce the cost of the sensing system, the sourceless sensor head design and the low-cost wafer-based F-P interrogator. The sourceless sensor head makes use of the environmental thermal radiation as a broadband light source, together with the white light interferometry signal demodulation method, for temperature measurements. In this case, the system avoids using not only an external light, but also the light driver and the light coupling element. A low-cost F-P cavity interrogation method was introduced to demodulate the sapphire-wafer-based temperature sensing F-P cavity. The signal demodulation is based on the scanning white light interferometry, but a reliable and low-cost reference F-P cavity is introduced. It includes only a piece of transparent wafer and a CCD array to transfer the interference fringe from the spectra domain to the spatial domain and therefore a low cost CCD can be directly applied to identify the optical path distance of the sensing OPD. Eventually, the above two technologies were able to put together and an extremely low-cost F-P temperature sensing system was built. It has a good potential for further applications and commercialization.

The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region. Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated. / MSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region. Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated. / MSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

Fire spread between objects within a compartment is primarily due to the impingement of thermal radiation from the fire source. In order to estimate if or when a remote object from the fire will ignite, one must be able to quantify the radiative heat flux being received by the target. There are a variety of methods presented in the literature that attempt to calculate the thermal radiation to a target; each one based on assumptions about the fire. The performance of six of these methods, of varying complexity, is investigated in this research. This includes the common point source model, three different cylindrical models, a basic correlation and a planar model. In order to determine the performance of each method, the predictions made by the models were compared with actual measurements of radiant heat flux. This involved taking heat flux readings at numerous locations surrounding a propane gas burner. Different fire scenarios were represented by varying the burner geometry and heat release rate. Video recordings of the experiments were used to determine the mean flame heights using video image analysis software. After comparing the measured data with predictions made by the theoretical radiation methods, the point source model was found to be the best performing method on average. This was unexpected given the relative simplicity of the model in comparison to some of its counterparts. Additionally, the point source model proved to be the most robust of the six methods investigated, being least affected by the experimental variables. The Dayan and Tien method, one of the cylindrical models, was the second most accurate over the range of conditions tested in this work. Based on these findings, recommendations are made as to the most appropriate method for use in a radiation sub-model within an existing zone model software. The accuracy shown by the point source model, coupled with its ease of implementation, means that it should be suitable for such a use.

La vulnérabilité est la composante du risque d’incendie de forêt la moins connue. Elle est généralement évaluée à dires d’experts. Plus objectivement elle peut être évaluée a posteriori en mesurant les dommages causés par un sinistre, si le détail des caractères de ce sinistre est connu. Nous proposons un modèle d’évaluation de la vulnérabilité formulé par une analyse multicritères des dires d’experts. Ce modèle est validé en utilisant un modèle physique d’exposition. Ses entrées sont fournies par un modèle de propagation. La calibration se fonde sur une analyse des dommages engendrés par un sinistre réel. Les résultats valident l’approche de modélisation de la vulnérabilité par des variables spatiales. / Vulnerability is not a well-known component of forest fire risk. It is usually assessed through experts’ opinions. It can be assessed more objectively after a disaster par measuring damages, if the attributes of the disaster are known. We propose a model for vulnerability assessment formulated with a multi-criteria analysis of experts’ opinions. This one is validated by using a physical model for exposure assessment. Its inputs are provided by a fire propagation model. The system is calibrated based on the analysis of damages induced by a real wildfire. Results demonstrate the consistency of a vulnerability model based on spatial variables.