Experiments were conducted to investigate the macroscopic thermal behaviour of 2m diameter Jet A fires in crosswinds of 3m/s to 13m/s. Two scenarios were considered: with and without a 2.7m diameter, 10.8m long, blocking object situated 3.4m downwind of the fire. These scenarios simulated transportation accidents with the fire representing a burning pool of aviation fuel and the object simulating an aircraft fuselage. To date, the limited number of experiments that have been conducted to examine wind effects on fire behaviour have been performed at small scale, which does not fully simulate the physics of large fires, or in outdoor facilities, with poorly controlled wind conditions. This thesis presents the first systematic characterization of the thermal environment in a large, turbulent fire under controlled wind conditions, with and without a large downwind blocking object. In experiments without the object, flame geometry was measured using temperature contour plots and video images, and the results compared to values predicted using published correlations. Results were greatly affected by the method used to measure flame geometry and by differences in boundary conditions between experiments. Although the presence of the blocking object prevented direct measurement of flame geometry due to interaction between the fire plume and object, temperature and heat flux measurements were analyzed to describe overall effects of the object on fire plume development. The fire impinged on the blocking object at wind speeds below 7m/s and interacted with the low-pressure wake region behind the object. Laboratory-scale experiments were also conducted to examine the responses of different heat flux gauges to controlled heating conditions simulating those found in wind-blown fires. Schmidt-Boelter, Gardon and Hemispherical Heat Flux gauges and a Directional Flame Thermometer were exposed to a convective flow and to radiation from a cone calorimeter heater. Measurements were influenced by differences between the calibration and measurement environments, differences in sensor surface temperature, and unaccounted thermal losses from the sensor plate. Heat flux results from the fires were consistent with those from the cone calorimeter, but were additionally affected by differences in location relative to the hot central core of the fire.

Experiments were conducted to investigate the macroscopic thermal behaviour of 2m diameter Jet A fires in crosswinds of 3m/s to 13m/s. Two scenarios were considered: with and without a 2.7m diameter, 10.8m long, blocking object situated 3.4m downwind of the fire. These scenarios simulated transportation accidents with the fire representing a burning pool of aviation fuel and the object simulating an aircraft fuselage. To date, the limited number of experiments that have been conducted to examine wind effects on fire behaviour have been performed at small scale, which does not fully simulate the physics of large fires, or in outdoor facilities, with poorly controlled wind conditions. This thesis presents the first systematic characterization of the thermal environment in a large, turbulent fire under controlled wind conditions, with and without a large downwind blocking object. In experiments without the object, flame geometry was measured using temperature contour plots and video images, and the results compared to values predicted using published correlations. Results were greatly affected by the method used to measure flame geometry and by differences in boundary conditions between experiments. Although the presence of the blocking object prevented direct measurement of flame geometry due to interaction between the fire plume and object, temperature and heat flux measurements were analyzed to describe overall effects of the object on fire plume development. The fire impinged on the blocking object at wind speeds below 7m/s and interacted with the low-pressure wake region behind the object. Laboratory-scale experiments were also conducted to examine the responses of different heat flux gauges to controlled heating conditions simulating those found in wind-blown fires. Schmidt-Boelter, Gardon and Hemispherical Heat Flux gauges and a Directional Flame Thermometer were exposed to a convective flow and to radiation from a cone calorimeter heater. Measurements were influenced by differences between the calibration and measurement environments, differences in sensor surface temperature, and unaccounted thermal losses from the sensor plate. Heat flux results from the fires were consistent with those from the cone calorimeter, but were additionally affected by differences in location relative to the hot central core of the fire.

<p>Recent technological advances in the field of electronics and the accompanying trend of device miniaturization with enhanced functionality has led to growing interest in new methods of electronic device integration. As a result, flexible, wearable, and portable electronic devices have emerged as a way of providing a multifunctional infrastructure to facilitate various consumer needs, creating new challenges for materials development. Polymers possess a unique combination of desirable properties such as mechanical compliance, durability, low density and chemical stability which makes them ideally suitable as substrate materials to cater to such diverse applications. However, the low thermal conductivity of polymers hinders their heat spreading capability in thermal management applications for flexible and wearable devices. In recent years, there has been a growing interest in ultra-high molecular weight polyethylene (UHMW-PE) materials with aligned polymer chains due to their remarkably high thermal conductivity that is similar to some metals. These are commercially manufactured in large volumes as fibers using gel-spinning and ultra-drawing processes that impart a high degree of crystallinity and orientation to the polymer chains. As a result, these materials develop exceptionally high mechanical strength, elastic modulus, and thermal conductivity compared to conventional polymers. Therefore, UHMW-PE materials have found applications in commercial products like motorcycle gear and ballistic vests, but have not been commercially deployed for heat spreading and thermal management applications. While there has been much fundamental work on the development of high thermal conductivity fibers, effective translation of the high conductivity from individual fibers to macroscale (wearable) flexible fabrics has not been previously explored. The objective of this thesis is to obtain a fundamental understanding of the thermal transport properties of fabric materials constructed from the high conductivity polymer fibers, and assess their applicability for potential heat spreading applications. </p> <p>In the present work, commercially available high thermal conductivity fibers made of UHMW-PE are utilized to fabricate plain-weave fabrics prototypes, and the thermal properties of individual fibers, yarns, and woven fabrics are measured using a novel in-plane thermal measurement method. The characterization technique leverages infrared (IR) microscopy for a non-contact temperature sensing and is generally scalable for thermal characterization of the in-plane thermal-conductivity of materials across different length scales. Effective thermal conductivities on the order of ~10 Wm^-1K^-1 are achieved along the in-plane dominant heat transport direction of the woven fabric, which is exceptionally high (~2-3 orders of magnitude) compared to conventional clothing and textile-based materials. The thermal conductivity and mechanical flexibility of the UHMW-PE fabrics are benchmarked with respect to conventional materials and the effect of bend-stressing and thermal annealing of the fabrics is characterization using the developed metrology. </p> <p>Additionally, a laser-based IR thermal metrology technique leveraging both non-contact heating and temperature sensing is conceptualized and validated using a numerical thermal modeling approach. The proposed technique provides an approach to estimate the in-plane heat spreading properties of anisotropic materials with direction-depended thermal properties based on quantifying the surface temperature map of a sample subjected to periodic heating. Numerical simulations are leveraged to demonstrate the applicability of this method to enable measurement of a wide range of thermal properties indicating great potential to develop this further as a standardized robust method for in-plane anisotropic thermal characterization of materials such as fabrics and films.</p> <p>This work sheds light on the high thermal conductivity of UHMW-PE materials that can be achieved using a scalable manufacturing process and describes the thermal metrology approaches to enable their characterization, thereby providing a foundation for the conceptualization and design of flexible substrate based thermal solutions in future wearable/flexible electronic devices.</p> <p> </p>

This dissertation presents thermal and thermoelectric measurements of nanostructures. Because the characteristic size of these nanostructures is comparable to and even smaller than the mean free paths or wavelengths of electrons and phonons, the classical constitutive laws such as the Fourier’s law cannot be applied. Three types of nanostructures have been investigated, including nanoscale constrictions patterned in a sub-100 nm thick silicon film, monatomic thick graphene ribbons supported on a silicon dioxide (SiO₂) beam, and indium antimonide (InSb) nanowires. A suspended measurement device has been developed to measure the thermal resistance of 48-174 nm wide constrictions etched in 35-65 nm thick suspended silicon membranes. The measured thermal resistance is more than ten times larger than the diffusive thermal resistance calculated from the Fourier’s law. The discrepancy is attributed to the ballistic thermal resistance component as a result of the smaller constriction width than the phonon-phonon scattering mean free path. Because of diffuse phonon scattering by the side walls of the constriction with a finite length, the phonon transmission coefficient is 0.015 and 0.2 for two constrictions of 35 nm x 174 nm x220 nm and 65 nm x 48 nm x 50 nm size. Another suspended device has been developed for measuring the thermal conductivity of single-layer graphene ribbons supported on a suspended SiO₂ beam. The obtained room-temperature thermal conductivity of the supported graphene is about 600 W/m-K, which is about three times smaller than the basal plane values of high-quality pyrolytic graphite because of phonon-substrate scattering, but still considerably higher than for common thin film electronic materials. The measured thermal conductivity is in agreement with a theoretical result based on quantum mechanical calculation of the threephonon scattering processes in graphene, which finds a large contribution to the thermal conductivity from the flexural vibration modes. A device has been developed to measure the Seebeck coefficients (S) and electrical conductivities ([sigma]) of InSb nanowires grown by a vapor-liquid-solid process. The obtained Seebeck coefficient is considerably lower than the literature values for bulk InSb crystals. It was further found that decreasing the base pressure during the VLS growth results in an increase in the Seebeck coefficient and a decrease in the electrical conductivity, except for a nanowire with the smallest diameter of 15 nm. This trend is attributed to preferential oxidation of indium by residual oxygen in the growth environment, which could cause increased n-type Sb doping of the nanowires with increasing base pressure. The deviation in the smallest diameter nanowire from this trend indicates a large contribution from the surface charge states in the nanowire. The results suggest that better control of the chemical composition and surface states is required for improving the power factor of InSb nanowires. On approach is to use Indium-rich source materials for the growth to compensate for the loss of indium due to oxidation by residual oxygen. / text

Cette thèse est une étude expérimentale du comportement thermo-mécanique superélastique d'un fil nanocristallin Ti-50.9Ni at.% Ni en alliage à mémoire de forme (SMA) (diamètre 0.5 mm), après subir un cold work (CW). Les AMF sont capables d'induire des changements de température importants lorsqu'ils sont chargés mécaniquement. Ce phénomène est dû à un important couplage thermomécanique présent dans cette transformation de phase solide entre les phases Austénite (A) et Martensite (M).La chaleur latente par unité de masse (ΔH) tout au long de la transformation de phase est l'énergie responsable de cette variation de température. La détermination de ΔH est généralement effectuée par calorimétrie à balayage différentiel (DSC). Cependant, pour les SMA nanocristallins, les résultats DSC obtenus ne sont pas concluants sur la détermination de cette propriété.Dans ce travail, une méthode utilisant la corrélation d'image numérique (DIC) et les mesures de champ thermique (TFM) a été utilisée pour analyser les couplages thermomécaniques lors d'une transformation de phase induite par contrainte. Des champs cinématiques et thermiques ont été acquis lors d'essais de traction superélastiques réalisés sur des fils CW NiTi soumis à différentes températures de traitements thermiques (TTT) allant de 523 à 598 K pendant 30 min. Un tel traitement thermique à basse température favorise une boucle totalement superélastique sans plateau de contrainte et sans déformation de type Lüders. En supposant un modèle thermique uniforme, les sources de chaleur impliquées lors du chargement cyclique ont été estimées. Cette puissance thermique par unité de masse a été comparée à la puissance mécanique et intégrée au fil du temps pour obtenir l'équilibre énergétique. De plus, grâce à une analyse thermodynamique basée sur l'énergie libre de Gibbs, les valeurs de ΔH, ainsi que la fraction de martensite, ont été estimées au cours des transformations de phase A-M directe et inverse M-A. L'analyse des résultats a conduit aux conclusions suivantes: (1) Les puissances et énergies thermiques et mécaniques présentaient une dépendance significative vis-à-vis du TTT. (2) Malgré l'effet important des valeurs du TTT sur les réponses mécaniques et thermiques, les ΔH obtenues étaient très proches pour tous les TTT et dans la même gamme de valeurs fondée dans la littérature pour un alliage Ti-50.9Ni at.% Ni entièrement recuit testé par technique DSC. (3) Pour une deformation donnée, la fraction de martensite augmente avec l'augmentation de TTT. (4) Pour une contrainte imposée de 4,5%, la fraction de martensite augmente de 30% à 40% en augmentant le TTT de 523K à 598K. / This PhD thesis is an experimental study of the thermomechanical superelastic behaviour of a Ti-50.9Ni at.% Ni Shape Memory Alloy (SMA) nanocrystalline thin wire (diameter 0.5 mm), in a Cold Worked (CW) state. SMAs are capable of inducing important temperature change when they are mechanically loaded. This phenomenon is due to an important thermomechanical coupling present in this solid phase transformation between Austenite (A) and Martensite (M) phases. The latent heat per unit of mass (∆H) throughout the phase transformation is the energy responsible of this temperature variation. The determination of ∆H is generally performed by differential scanning calorimetry (DSC). However, for nanocrystalline SMAs, the obtained DSC results are non conclusive on the determination of this property.In this work, a method using digital image correlation (DIC) and thermal field measurements (TFM) was used to analyse the thermomechanical couplings during a stress induced phase transformation (SIPT). Kinematics and thermal full fields were acquired during superelastic tensile tests performed on the CW NiTi wire submitted to different heat treatments temperatures (HTT) ranging from 523 to 598 K during 30 min. Such a heat treatment at low temperature promoted a fully superelastic loop without stress plateau and no Lüders-like deformation. Assuming a uniform thermal model, the heat sources involved during the cyclic loading were estimated. This thermal power per unit of mass was compared to the mechanical one and integrated over the time to get energy balance. Further, through a thermodynamic analysis based on the Gibbs free energy, the values of ∆H, as well as the martensite fraction, were estimated during the forward A-M and reverse M-A phase transformations. The analysis of the results led to the following conclusions: (1) Thermal and mechanical powers and energies presented a significant dependence on the HTT. (2) Despite the strong effect of the values of the HTT on mechanical and thermal responses, the obtained ∆H were very close for all HTT and in the same range of values founded in the literature for a fully annealed Ti-50.9Ni at.% Ni alloy tested via DSC technique. (3) For a given strain, martensite fraction increases with increasing HTT. (4) For an imposed strain of 4.5%, the martensite fraction increases from 30% to 40% when increasing HTT from 523K to 598K.