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Composites, though used in a variety of applications from chairs and office supplies to structures of U.S. Navy ships and aircrafts, are not all designed to hold up to extreme heat flux and high temperature. Fiber-reinforced polymeric composites (FRPC) have been proven to provide the much needed physical and mechanical properties under fire exposure. FRPC notable features are its combination of high specific tensile strength, low weight, along with good corrosion and fatigue resistance. However FRPC are susceptible to thermal degradation and decomposition, which yields flammable gas, and are thus highly combustible. This property restricts polymeric material usage. This study developed a numerical model that simulated the degradation rate and temperature profiles of a fiber-reinforced polyester resin composite exposed to a constant heat flux and hydrocarbon fire in a cone calorimeter. A numerical model is an essential tool because it gives the composite designer the ability to predict results in a time and cost efficient manner. The goal of this thesis is to develop a numerical model to simulate a zonal-layer polyester resin and fiberglass mat composite and then validate the model with experimental results from a cone calorimeter. By inputting the thermal properties of the layered composite of alternating polymer and polymer-infused glass fiber mat layers, the numerical model is one step closer to representing the experimental data from the cone calorimeter test. The final results are achieved through adding a simulated heat flux from the pilot ignition of the degraded gas of the polyester resin. The results can be coupled into a mechanical model, which may be separately constructed for future study on the mechanical strength of composites under fire conditions.

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With the increase dependency on electricity to provide correct form of electricity for lightning, machines, and home and office appliances, the need for the introduction of high reliability power electronics in converting the raw form of electricity into efficient electricity for these applications is uprising. One of the most common failures in power electronics is temperature related failure such as overheating. To address the issue of overheating, thermal management becomes an important mission in the design of the power electronics to ensure the functional power electronics. Different approaches are taken by academia and industry researchers to provide efficient power electronics. In particular, the Center for Power Electronics System (CPES) at Virginia Tech and four other universities presented the IPEM approach by introducing integrated power electronics modules (IPEM) as standardized units that will enable greater integration within power electronics systems and their end-use application. The IPEM approach increases the integration in the components that make up a power electronics system through novel a packaging technique known as Embedded Power technology. While the thermal behavior of commonly used packages such as pin grid arrays (PGA), ball grid array (BGA), or quad flat pack (QFP) are well-studied, the influence of the Embedded Power packaging architecture on the overall thermal performance of the IPEMs is not well known. This motivates the presentation of this dissertation in developing an in-depth understanding on the thermal behavior of the Embedded Power modules. In addition, this dissertation outlines some general guidelines for the thermal modeling and thermal testing for the Embedded Power modules. Finally, this dissertation summarizes a few thermal design guidelines for the Embedded Power modules. Hence, this dissertation aims to present significant and generalized scientific findings for the Embedded Power packaging from the thermal perspective. Both numerical and experimental approaches were used in the studies. Three-dimensional mathematical modeling and computational fluid dynamics (CFD) thermal analyses were performed using commercial numerical software, I-DEAS. Experiments were conducted to validate the numerical models, characterize the thermal performance of the Embedded Power modules, and investigate various cooling strategies for the Embedded Power modules. Validated thermal models were used for various thermal analyses including identifying potential thermal problems, recognizing critical thermal design parameters, and exploring different integrated cooling strategies. This research quantifies various thermal design parameters such as the geometrical effect and the material properties on the thermal performance of the Embedded Power modules. These parameters include the chip-to-chip distance, the copper trace area, the polyimide thickness, and the ceramic materials. Since the Embedded Power technology utilizes metallization bonding as interconnection, specific design parameters such as the interconnect via holes pattern and size, the metallization thickness, as well as the metallization materials were also explored to achieve best results based on thermal and stress analyses. With identified potential thermal problems and critical thermal design parameters, different integrated cooling strategies were studied. The concept of integrated cooling is to incorporate the cooling mechanisms into the structure of Embedded Power modules. The results showed that simple structural modifications to the current Embedded Power modules can reduce the maximum temperature of the module by as much as 24%. Further improvement can be achieved by employing double-sided cooling to the Embedded Power modules. Based on the findings from the thermal analyses, general design guidelines were developed for future design of such Embedded Power modules. In addition, thermal modeling and testing guidelines for the Embedded Power modules were also outlined in this dissertation. / Ph. D.

The effect of thermal cycling on the state of stress in polymeric materials bonded to stiff elastic substrates was investigated using numerical techniques, including finite element methods. The work explored the relationship between a cyclic temperature environment, temperature-dependent viscoelastic behavior of polymers, and thermal stresses induced in a constrained system. Due to the complexity of developing a closed-form solution for a system with time, temperature, material properties, and boundary conditions all coupled, numerical techniques were used to acquire approximate solutions. Descriptions of attempted experimental verification are also included. The results of the numerical work indicate that residual stresses in an elastic-viscoelastic bimaterial system incrementally shift over time when subjected to thermal cycling. Tensile axial and peel stresses develop over a long period of time as a result of viscoelastic response to thermal stresses induced in the polymeric layer. The applied strain energy release rate at the crack tip of layered specimens is shown to similarly increase. The rate of change of the stress state is dependent upon the thermal cycling profile and the adhesive’s thermo-mechanical response. Discussion of the results focuses on the probability that the incrementing tensile residual stresses induced in an adhesive bond subjected by thermal cycling may lead to damage and debonding, thus reducing durability. / Master of Science

This thesis documents research done in the development and the exploration of feasibility for a high-throughput method to measure local thermal properties. The present capabilities in the measurement of local thermophysical properties such as thermal conductivity, thermal diffusivity, and Kapitza resistance are very inefficient and impractical to fully understand and characterize heat transport through certain materials and features. This work follows up on past work in local thermal property measurement via the spatial domain thermoreflectance (SDTR) method, and explores the possibility of parallelizing the process. The parallelized SDTR (P-SDTR) method involves using laser projector sources to periodically heat and measure the changes of reflectivity of a sample surface at multiple locations simultaneously. These measurements are made possible by the development of a lock-in camera that can measure the characteristics of modulated light using lock-in amplification at several spots across an area with an advanced camera sensor. This method allows for the measurement of local thermal properties across features such as grain boundaries, or directional properties in anisotropic materials. An experimental setup is developed to determine at which heating and probing parameters a thermoreflectance signal can be measured. A finite element model is also made to simulate the P-SDTR process, and validate that the assumptions made in SDTR can be made in P-SDTR measurements. It is shown that at an appropriate separation of heating/measurement locations, the solutions from the simulation approach that of a single measurement spot. An initial device design is proposed and tested. Future work in the development of the P-SDTR device is also laid out.

The aim of this thesis is to study the energy fluctuations (leading to thermal fluctuations) during thermal and enzymatic denaturation of biological molecules and to study the variation in fluctuations between simple molecules like the DNA (which have only a secondary structure) to molecules with higher order structures and packaging. We have developed a new technique - Thermal Fluctuation Spectroscopy (TFS) to study these fluctuations. The technique of Thermal Fluctuation Spectroscopy (TFS) is a combination of microcalorimetry and noise measurement techniques. The combination of these two powerful techniques has never been exploited before. In this technique any energy exchange between sample and the substrate is reflected as a thermal fluctuation of the substrate. The system resolution is few parts per billion (ppb) and fluctuations in energy ~ 100nJ (which correspond to temperature fluctuations ~ K) can be measured. Chromatin is the basic building block of chromosome and this thesis focuses on the constituents this fundamental building block - DNA, histones and nucleosomes. Heteropolymeric dsDNA shows extremely large non-Gaussian fluctuation around its melting temperature. For homopolymeric DNA the fluctuations during denaturation are smaller. The thermal fluctuation during denaturation of a heteropolymer in buffer is several orders larger than when the DNA is on a substrate while that for a homopolymer is comparable in both cases. Our measurements established that heteropolymeric dsDNA denaturation occurs in two stages. Initially, at around 330 K, bubbles are formed in the AT rich regions. At higher temperatures, the GC rich regions binding them denature in a cooperative transition causing extremely large fluctuations. TFS on histone monomers showed that H1 monomer shows an increase in thermal fluctuation in the temperature range studied, while the core histones did not. We infer that this is due to the fact that the core histones may not be properly folded when they exist as monomers. It was seen that H1 crosses an energy barrier of 17 kcal/mol to go from its native to denatured state. The transition was kinetically driven with a fixed barrier till 352 K. At 352K, the barrier softened by ~ 1 kcal/mol leading to faster denaturation. The core histones when assembled as dimers/oligomers showed an increase in fluctuation at temperatures below 350 K. The assembling of these histones and DNA into a mononucleosome causes a very large increase in fluctuation over the entire temperature range studied. TFS showed that the fluctuation during mononucleosome denaturation was much larger than a simple sum of the fluctuations of its constituents. From the data we were able to identify that the denaturation starts with dissociation and unfolding of the core histones and the denaturation of AT rich regions of the DNA which leads to the breaking of some of the histone-DNA contacts. At higher temperatures the linker histone H1 and the GC rich regions of the DNA denature, leading to a collapse of the entire nucleosome structure. The broadness of the transition region (the fact that the fluctuation is large over the entire temperature range) was attributed to the presence of different types of contacts and interactions (with different energies) stabilizing the nucleosome structure. The nucleosome was found to favour large energy jumps over smaller ones indicating that the denaturation has an element of cooperativity involved. Using TFS we have been able to determine the fluctuations involved in the denaturation of biomolecules like DNA, histones and nucleosomes. The energy barriers to denaturation have been determined. We have also been able to give models for the denaturation of these biomolecules. We have also shown that it is possible to study enzymatic digestion using TFS. Thus, the technique of TFS is a viable tool for the study of fluctuations in reactions, in biomolecules, during transitions and in any process where there is an energy exchange involved.

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In the residential and commercial buildings, most of the energy is used to provide the thermal comfort environment to the occupants. The recent research towards Green Buildings is focusing on reduction of energy consumption by air-conditioners and fans used for producing the thermal comfort environment. The thermal comfort is defined as the condition of mind which expresses human satisfaction with the thermal environment. The human body is continuously producing metabolic heat and it should be maintained within the narrow range of core temperature. The heat generated inside the body should be lost to the environment to maintain the thermal equilibrium with each other. The heat loss from the body is taking place in different modes such as conduction, convection, radiation and evaporation through the skin and respiration. These heat losses are influenced by the environmental factors (air temperature, air velocity, relative humidity and mean radiant temperature), physiological factors (activity level, posture and sweat rate) and clothing factors (thermal insulation value, evaporative resistance and microenvironment volume). When the body is in thermally equilibrium with its surrounding environment, the heat production should be equal to heat loss to maintain the thermal comfort. The level of thermal comfort can be measured by the different indices which combine many parameters. Of these, the Fanger’s PMV (Predicted Mean Vote) – PPD (Percentage of People Dissatisfied) index was universally suggested by ASHRAE and ISO. The PMV – PPD index was derived based on the experiment conducted on acclimated European and American subjects. Many researchers have criticized that the PMV – PPD index is not valid for tropical regions and some researchers have well agreed with this index for the same region. The validation of PMV – PPD index for thermal comfort Indians has not yet been examined. The validation of PMV – PPD index can be done by the human heat balance experiment and the individual heat losses have to be calculated from the measured parameters. In the human heat balance, the convective heat transfer plays the major role when the air movement exists around the human body. The convective heat loss is dependent on the convective heat transfer coefficient which is the function of the driving force of the convection. Using Computational Fluid Dynamics techniques, an attempt has been made in this work to determine the convective heat transfer coefficient of the human body at standing posture in natural convection. The CFD technique has been used to analyze the heat and fluid flow around the human body as follows: The anthropometric digital human manikin was modeled in GAMBIT with a test room. This model was meshed by tetrahedral elements and exported to FLUENT software to perform the analysis. The simulation was done at different ambient temperatures (16 oC to 32 oC with increment of 2 oC). The Boussinesq approximation was used to simulate the natural convection and the Surface to Surface model was used to simulate the radiation. The surrounding wall temperature was assigned equal to the ambient temperature. The sum of convective and radiative heat losses calculated based on the ASHRAE model was set as heat flux from the manikin’s surface. From the simulation, the local skin temperatures have been taken, and the temperature and velocity distributions analyzed. The result shows that the skin temperature is increasing with an increase in ambient temperature and the thickness of the hydrodynamic and thermodynamic boundary layers is increasing with height of the manikin. From the Nusselt number analogy, the convective heat transfer coefficients of the individual manikin’s segments have been calculated and the relation with respect to the temperature differences has been derived by the regression analysis. The relation obtained for the convective heat transfer coefficient has been validated with previous experimental results cited in literature for the same conditions. The result shows that the present relation agrees well with the previous experimental relations. The characteristics of the human thermal plume have been studied and the velocity of this plume is found to increase with the ambient temperature. Using the Grashof number, the flow around the human manikin has been examined and it is observed to be laminar up to abdomen level and turbulent from shoulder level. In between these two levels, the flow is found to be in transition. The validation of PMV model for tropical countries, especially for Indians, was done by heat balance experiment on Indian subjects. The experiment was conducted on forty male subjects at different ambient temperatures in a closed room in which low air movement exists. The local skin temperature, relative humidity, air velocity and globe temperature were measured. The sensation vote was received from all the subjects at all the conditions. The convective heat loss was calculated from its coefficient obtained from the present computational simulation. The radiation heat loss was calculated for two cases: In case one, the mean radiant temperature was taken equal to the ambient temperature and in case two, the mean radiant temperature was calculated from the globe temperature. The other heat losses were calculated from the basic formulae and the relations given by ASHRAE based on Fanger’s assumption. From these calculations, the validity of the Fanger’s assumption was examined. The collected sensation votes and the calculated PMV were compared to validate the PMV – PPD index for Indians. The experimental results show that there was much variation in the calculated comfort level using the measured parameters and the Fanger’s assumption. For the case of mean radiant temperature equal to the ambient temperature for indoor condition, the comfort level was varying more than the actual. In addition, the calculated comfort level from the globe temperature agreed well with the comfort level from the collected sensation votes. So it was concluded that the ASHRAE model is valid for Indians if the radiation was measured exactly. Using the ASHRAE model, the required wall emissivity of the surrounding wall at different ambient temperatures was determined from the CFD simulation. In the ASHRAE model, the surrounding wall emissivity plays the major role in the radiative heat loss from the human body. Hence in recent years, research on low emissive wall paints is focused. The computational study was done to determine the required wall emissivity to obtain the thermal comfort of the occupant at low energy consumption. The simulation was done with the different ambient temperatures (16 oC to 40 oC with increment of 4 oC) with the different surrounding wall emissivity (0.0 to 1.0 with increment of 0.2). From this simulation, the change in mean skin temperature with respect to wall emissivity was obtained for all ambient temperature conditions. The required mean skin temperature for a particular activity level was compared with the simulation results and from that, the required wall emissivity at the different ambient conditions was determined. If the surrounding walls are having the required emissivity, it leads to decrease in heat/cold strain on the human body, and the thermal comfort can be obtained with low energy consumption.(please note that title in the CD is given as COMPUTATION OF REQUIRED WALL EMISSIVITY FOR LOW ENERGY CONSUMPTION IN BUILDINGS USING ASHRAE MODEL VALIDATED FOR INDIAN THERMAL COMFORT)

Orientador: Alcides Padilha / Banca: Celso Luiz da Silva / Banca: Marcos Pinotti Barbosa / Resumo: Devido à extrema necessidade de se diversificar as fontes de energia renováveis, torna-se necessário a busca por métodos de reciclagem de energia pela utilização de rejeitos térmicos de equipamentos. Assim, o aproveitamento desses rejeitos pode ser utilizado, como uma nova fonte de energia, para o aquecimento de água e armazenamento da mesma em reservatório para uso doméstico. Devido a isso, é proposta a construção de um aparato experimental de um reservatório de armazenamento térmico cilíndrico, onde o objetivo da pesquisa será o levantamento dos coeficientes de desempenho relativo aos refrigeradores convencional e modificado, além de realizar uma análise da armazenagem da água quente, através da técnica da estratificação térmica utilizando um refrigerador com condensador modificado. O rejeito térmico coletado, através do princípio do termosifão, será armazenado na forma de energia térmica. Os resultados mostraram que os coeficientes de desempenho dos sistemas, calculados pelas técnicas relativas às perdas térmicas teóricas e experimentais, apresentaram grande diferença. Observou-se o comportamento dinâmico das termoclinas, através do efeito da estratificação térmica e a evolução das temperaturas em função do tempo, mostrando maior viabilidade do refrigerador modificado, gerando assim maior conforto térmico aos usuários além de produzir água quente para uso doméstico. / Abstract: Due to the extreme necessity to diversify renewable energy sources, the search for energy recycling methods through the utilization of thermal losses from equipment has become fundamental. Thus, these losses can be used as new source of energy for water heating and storage in Domestic Hot Water Storage Tanks (DHWST). For this reason, the construction of an experimental apparatus with a cylindrical thermal storage tank is proposed, in which the objective of the study will be a survey of the Coefficient of Performance concerning conventional and modified refrigerators, as well as to perform an analysis of hot water, through the thermal stratification technique using a refrigerator with a modified condenser. The collected thermal loss, as per the thermosiphon principle, will be stored as thermal energy. The results showed that the coefficient of performance for the systems, calculated using techniques for theoretical and experimental thermal losses, presented great differences. The dynamic behavior of the thermal distribution was observed through the thermal stratification effect and temperature evolution in terms of time, showing greater variability of the modified refrigerator generating more thermal comfort to users in addition to providing domestic hot water. / Mestre