Computer security and privacy are becoming extremely important nowadays. The task of protecting computer systems from malicious attacks and potential subsequent catastrophic losses is, however, challenged by the ever increasing complexity and size of modern hardware and software design. We propose several methods to improve computer security and privacy from architectural point of view. They provide strong protection as well as performance efficiency. In our first approach, we propose a new dynamic information flow method to protect systems from popular software attacks such as buffer overflow and format string attacks. In our second approach, we propose to deploy encryption schemes to protect the privacy of an emerging non-volatile main memory technology - phase change memory (PCM). The negative impact of the encryption schemes on PCM lifetime is evaluated and new methods including a new encryption counter scheme and an efficient error correct code (ECC) management are proposed to improve PCM lifetime. In our third approach, we deconstruct two previously proposed secure cache designs against software data-cache-based side channel attacks and demonstrate their weaknesses. We propose three hardware-software integrated approaches as secure protections against those data cache attacks. Also we propose to apply them to protect instruction caches from similar threats. Furthermore, we propose a simple change to the update policy of Branch Target Buffer (BTB) to defend against BTB attacks. Our experiments show that our proposed schemes are both security effective and performance efficient.

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<p> Understanding the behavior of liquid hydrocarbon propellants under high pressure and temperature conditions is a crucial step towards improving the performance of modern-day combustion engines (liquid rocket engines, diesel engines, gas turbines and so on) and designing the next generation ones. Under such harsh thermodynamic conditions (high P&T) propellent droplets may experience anywhere from sub-to-trans-to-supercritical regime. The focus of this research is to explore the dynamics of the vapor-liquid two phase system formed by a liquid hydrocarbon fuel (n-heptane or n-dodecane) and ambient (nitrogen) over a wide range of P&T leading up to the mixture critical point and beyond. Molecular dynamics (MD) has been used as the primary tool in this research along with other tools like: phase stability calculations based on Gibb’s work, Peng Robinson equation of state, density gradient theory and neural networks.</p>

<p>This research is concerned with CFD modelling of thermal energy storage tanks containing water with submerged phase change materials (PCM). Under appropriate operating conditions, the energy density of this hybrid system can be significantly increased (two to five times) relative to a system containing water only. However, due to low thermal conductivity of phase change materials, the geometry and configurations of the PCM capsules in the tank should be optimized. This research focused on the assessment of flat plate PCM modules submerged in a rectangular water tank. The encapsulation of the PCM within the slender flat plates resulted in a large PCM surface area and a reduction in the internal heat transfer resistance. The water was heated by coils placed at the bottom of the tank. The resulting natural convection currents acted to transfer heat from the hot coils to the PCM modules which were treated as isothermal at the PCM melt temperature. It is concluded that the charge rate of the system increases to 2.8 times by increasing the PCM volume percentage from 2.5% to 15%. However for PCM volume percentages of more than 15%, the area of the PCM became much more than the area of the coil (around 15 times) in a way that the charge rate of the system started to be controlled by the coil. In this stage, the charge rate of the system remained constant, and adding modules to the system only increased the heat capacity of the system. Therefore the charge rate of the system could only increase if the coil surface area was increased. The heat transfer coefficients of the PCM modules and coil tubes were higher than those evaluated by the experimental correlations for natural convection. This was due to the recirculation of the flow in the tank “pumping effect” created by the coil for PCM modules and by the PCM modules for the coil. It was also concluded that superheating of the PCM surface temperature decreases the heat transfer rate to the PCM significantly, and the charge rate of the system varies linearly with the temperature difference between the PCM modules and the coil.</p> / Master of Applied Science (MASc)

<p>Electrohydrodynamics (EHD) was used to enhance the thermal performance of a latent heat thermal storage cell by reducing the charging time for a given amount of latent heat stored. Paraffin wax, which is an organic dielectric commercially available material was selected as the phase change material (PCM).</p> <p>Electric field was applied into the cell by using 9 electrodes kept at -8 kV in an effort to establish EHD forces inside the PCM. The EHD effect was studied in an originally conduction dominated melting environment. That was achieved by the cell design which promoted unidirectional melting downwards to prevent natural convection from occurring by assuring a thermally stratified molten phase. The target was to study the EHD mechanisms of enhancement with less interfering physics.</p> <p>Melting was studied under constant heat flux boundary condition. The temporal thermal profile of the surface heater and the melt front location were used to assess the EHD effect by comparing it to a 0 kV (no EHD) case.</p> <p>It was found that by using EHD (-8 kV), the time required to melt 7 mm thickness of the PCM can be reduced by 40 % when compared to 0 kV case. Through a four hour experiment time, the amount of molten PCM can be increased by 29 % by using EHD compared to 0 kV. The EHD power consumption was less than 0.17 W which is equivalent to 2.4 % of the thermal energy stored in the PCM.</p> <p>A new phenomena was discovered when applying EHD in the tested cell, which is Solid Extraction, where the solid dendrites within the mushy zone were extracted from the mushy zone into the liquid bulk towards regions of higher electric field.</p> <p>A new criteria was developed to quantify the EHD enhancement level and was called EHD enhancement factor. An enhancement factor up to 13 could be reached by using EHD. The effect of changing the heat flux on the enhancement factor was investigated, and it was found that the enhancement factor decreased by increasing the heat flux.</p> <p>Numerical simulations were performed in an effort to understand the EHD mechanisms of enhancement. The static electric field distribution, the interfacial extraction forces and the body forces acting on suspended dendrites were evaluated. The results of numerical simulations were supported by the high speed imaging and the experimental data to explain the EHD mechanisms of enhancement and the regions where solid dendrites extraction happened.</p> <p>Finally an analytical model was developed to estimate the energy stored in the different components of the tested latent heat storage cell and to estimate the amount of energy lost to the surroundings in order to quantify the accuracy of the experiment and a maximum of 18 % heat loss was estimated.</p> / Master of Applied Science (MASc)

Geo-energy piles and walls have long been recognized as a promising way to reduce carbon dioxide emissions while providing renewable energy. However, enhancing the thermal performance of these structures has remained a signif-icant challenge. This thesis evaluated five different approaches to improving the thermal performance of geo-energy piles and walls, through a series of experiments using a fully instrumented testing rig. The first approach involved adding graphTHERM powder to concrete to double its thermal conductivity, boosting heat transfer efficiency by an impressive 50% to 66%. The second approach tested slag-based geopolymer concrete as a sustainable construc-tion material for geo-energy piles and walls, reducing CO2 emissions by 44.5% while improving thermal performance by 14% to 21%. The third approach in-volved testing thermally enhanced soils at the geo-energy structures/soil inter-face, resulting in an 81% improvement in heat transfer efficiency. The fourth approach utilized innovative phase change material (PCM) heat exchangers that increased heat transfer efficiency by 75% and 43% in heating and cooling operations, respectively. Finally, incorporated PCM-impregnated light weight aggregates at the interface of the structure soil, significantly increasing tem-perature difference and reducing thermal deformation of geo-energy struc-tures.Overall, these innovative approaches made a significant contribution to enhancing the thermal performance of geo-energy piles and walls. However, approaches four and five, which involve utilizing PCM heat exchangers and PCM-impregnated LWA's, respectively, showed extra benefits in dropping the thermal effect on soils and reducing the thermal damage on those structures. These techniques offer great promise for improving the thermal performance of geo-energy structures.

Das Kraftstoffdampfmanagement in PKW dient der Reduzierung von Kraftstoffdampfemissionen und umfasst deren Entstehung im Tank, sowie Verarbeitung im Adsorber. Im Hinblick auf eine effektive Emissionsreduzierung erfolgt in dieser Arbeit die Entwicklung eines multiphysikalischen Berechnungsmodells, das die Erschließung der Kraftstoffdampfmenge im Tank sowie der Adsorbercharakteristik erlaubt. Gleichzeitig wird eine Methode zur thermischen Adsorberoptimierung durch Phasenwechselmaterialien (PCM) vorgestellt. Letztere nutzen für ihren fest/flüssig-Phasenübergang im Adsorber umgesetzte Prozesswärmen und können damit dessen Arbeitskapazität erhöhen. Die Modellierung der tankinternen Kraftstoffdampfproduktion erfolgt basierend auf der Berechnung des Dampf-Flüssigkeit-Gleichgewichtes von Mehrstoffsystemen mit realen Fluidmodellen. Zudem wird eine thermodynamische Datenbank erstellt, die es erlaubt, reale ottomotorische Kraftstoffgemische durch Modellkraftstoffsysteme mit deutlich reduzierter Komponentenanzahl abzubilden. Es wird ein detailliertes nicht-isothermes 2D-rotationssymmetrisches Mehrkammeradsorbermodell für kompressible Fluidgemische entwickelt, das die temperaturabhängige Polyschichtsorption in porösen Festbetten wiedergibt und direkt über transiente Randbedingungen mit der instationären Kraftstoffverdampfung im Tank gekoppelt ist. Darin berücksichtigt sind unter anderem anisotrope Wärme- und Stofftransportprozesse innerhalb der Festbetten sowie Randeffekte infolge einer nicht-linearen Porositätsverteilung. Zwischen den Sorptionskammern wird eine dünnwandige Aluminium-Trennwand aus makroverkapseltem PCM integriert, die zur Temperierung der umliegenden Festbetten dient. Hierzu wird auf Basis einer diskontinuierlichen Form der Enthalpy-Porosity-Methode der nicht-isotherme Phasenwechsel im Latentwärmespeicher unter Berücksichtigung der konvektiven Schmelzbewegung modelliert und in Ort und Zeit mit dem Adsorbermodell gekoppelt. Das daraus resultierende partielle Differenzialgleichungssystem wird örtlich über eine Finite-Elemente-Methode und bzgl. der Zeit in Form eines impliziten Mehrschrittverfahrens diskretisiert. Die entsprechende numerische Lösung erfolgt mit Hilfe eines automatisch gedämpften Newton-Verfahrens. Anhand des Adsorbermodells lässt sich der Einfluss von Randeffekten auf das Ad- und Desorptionsverhalten erschließen, die eine Abhängigkeit von der Festbettgeometrie und des temperaturabhängigen Beladungszustandes zeigen. Diese Sorptionsprozesse werden durch experimentelle Versuchs- reihen an einem hierzu entwickelten Adsorber-Prototyp validiert. Als Ergebnis der numerischen Simulation anwendungsrelevanter Prüfzyklen zur Adsorber-Typisierung resultiert durch den Einsatz von PCM eine Effizienzsteigerung in der Arbeitskapazität des Adsorbers von ca. 14 − 19 %. Zudem kann gezeigt werden, dass auch in Betriebszuständen ohne latenten Phasenwechsel im PCM infolge der konvektiven Bewegung der Schmelze die Sorptionsfähigkeit teilweise um mehr als 11 % ansteigt. Gleichzeitig ist im Vergleich zu der einfachen Vergrößerung des chemischen Sorptionsspeichers der Effekt einer Festbetttemperierung durch PCM partiell bis zu 10 % höher. Durch das einfache Substituieren der klassischen Kunststofftrennwände zwischen den Festbetten durch dünnwandige PCM-Kammern wird die Kraftstoffdampfnachbehandlung ohne relevante Gewichts- und Volumenzunahme des Adsorbers bedeutend verbessert.

La thèse est centrée sur la modélisation de la physique du comportement d’un alliage binaire et l’implémentation du meilleur modèle mathématique pour simuler le changement de phase liquide solide en tenant compte de la vitesse de refroidissement, la vitesse de solidification, la ségrégation, la convection naturelle et la surfusion afin d’optimiser la capacité de stockage de chaleur d'un tel matériau. Dans le présent travail, les températures pour lesquelles le changement de phase s'opère sont estimées grâce aux diagrammes des phases et la méthodologie CALPHAD qui retraduisent les différentes phases d'un alliage binaire, y compris la transformation isotherme. Pour cela, la minimisation de l'énergie de Gibbs est résolue dans un code de calcul développé à cette occasion et aboutit à l'identification des phases stables du matériau. Pour un intervalle de température souhaite le code permet d'estimer rapidement la décharge de chaleur pour la composition de l'alliage sélectionné en équilibre ou hors équilibre. Dans la méthode proposée, la vitesse de refroidissement du système permet de calculer la vitesse de solidification. Puis,celle-ci établit la relation entre la cinétique globale et la macrostructure. Basé sur le modèle de non-équilibre local, qui dépend de la variation du coefficient de partition, le degré de surfusion est prédit à partir de la vitesse de refroidissement appliquée. Une étude bibliographique a été réalisée pour amener une comparaison numérique et assurer la capacité de notre méthode à reproduire le changement de phase, en incluant des phénomènes spécifiques tels que la surfusion et la recalescence. / Latent Heat Thermal Energy Storage (LHTES) shows high storage density compared to sensible thermal systems. For high temperature applications, the use of alloys as phase change materials presents many advantages. Principally, varying alloy composition allows controlling the storage\discharge of thermal energy through an expected temperature range (defined by the heat source), and the high thermal conductivity givessuitable heat transfer properties to the system that receives/supplies the energy. However, some systems need a specific temperature range to correctly operate. In such conditions, subcooling (also known as undercooling) and segregation are undesirable phenomena in alloys when they are used as PCM. In thepresent work, we propose a method to predict the latent heat release during phase transformation of a binaryalloy submitted to a controlled cooling rate, including subcooling, segregation and variation of composition.This thesis describes the physical models that apply when heat is released from such a material. We takeinto consideration the cooling rate applied to the PCM, the solidification velocity, convective phenomena,melting temperature and subcooling. In the present work, phase diagrams and the CALPHAD methodologyare used to determine the temperature range for phase change (or constant temperature value for isothermal transformation) by minimizing the Gibbs equilibrium energy. The Gibbs free energy minimization has been implemented in a homemade numerical code. The material can be screened with different compositions for equilibrium or off-equilibrium solidification allowing quick selection of the optimal material for the specific heatsource. In the proposed method, the solidification velocity is obtained from the cooling rate. Then, variationin microstructure is driven by the solidification velocity using the local non-equilibrium diffusion model. Based on the local nonequilibrium model that depends on the partition coefficient variation, the subcooling degree, wich is derived from the applied cooling rate is predicted. A bibliographic study has been carried out and anumerical comparison has been undertaken to ensure the capacity of our code to reproduce the phase change of various materials that include phenomena such as subcooling and recalescence. The results highlight that the cooling rate is one of the most important parameters in the performance of the thermal storage, having a large effect on segregation and subcooling degree. Moreover, we show the influence ofpartition coefficient on the time evolution of solid fraction, considering a constant or a composition-dependent value. We can conclude that the latent heat release can be correctly predicted provided that the method correctly predicts the phase diagram and the variable partition coefficient. This work helps to accelerate the design and development of thermal storage systems and lays the foundation to continue exploring other kinds of materials (e.g. paraffins).

The Phase Change Memories (PCM) based on chalcogenide glasses are being considered recently as a possible replacement for conventional Non Volatile Random Access Memories (NVRAM). The main advantages of chalcogenide phase change memories are their direct write/overwrite capability, lower voltages of operation, large write/erase cycles, easiness to integrate with logic, etc. The phase change random access memories work on the principle of memory switching exhibited by chalcogenide glasses during which a local structural change (between amorphous and crystalline states) occurs due to an applied electric field. The development of newer phase change materials for NVRAM applications is based on synthesizing newer glass compositions and investigating their electrical switching characteristics by applying current/voltage pulses of different waveforms. The thermal studies on chalcogenide glasses which provide information about thermal stability, glass forming ability, etc., are also important while selecting a chalcogenide glass for PCM applications. The present thesis work deals with electrical switching and thermal studies on certain silicon based ternary telluride glasses (As-Te-Si, Ge-Te-Si and Al-Te-Si). The effect of network topological thresholds on the composition dependence of switching voltages and thermal parameters such as glass transition temperature, specific heat capacity, non-reversing enthalpy, etc., of these glasses has been investigated. The first chapter of the thesis provides an introduction to various properties of chalcogenide glasses, including their applications in phase change memories. The fundamental aspects of amorphous solids such as glass formation, glass transition, etc., are presented. Further, the concepts of rigidity percolation and self organization in glassy networks and the influence of local structural effects on the properties of glassy chalcogenides are discussed. Also, a brief history of evolution of phase change memories is presented. The second chapter deals with the experimental techniques employed in this thesis work; for sample preparation and for electrical switching studies, Alternating Differential Scanning Calorimetry (ADSC), Raman spectroscopy, NMR spectroscopy, etc. The third chapter discusses the electrical switching and thermal studies on As30Te70-xSix (2 ≤ x ≤ 22) and As40Te60-xSix (2 ≤ x ≤ 17) glasses. The composition dependence of electrical switching voltage (VT) and thermal parameters such as glass transition temperature (Tg), crystallization temperature (Tc), thermal stability (Tc-Tg), etc., reveals the occurrence of extended rigidity percolation and chemical thresholds in As30Te70-xSix and As40Te60-xSix glasses. Chapter 4 presents the electrical switching and thermal studies on Ge15Te85-xSix glasses (2 ≤ x ≤ 12). These glasses have been found to exhibit memory type electrical switching. While Ge15Te85-xSix glasses with x ≤ 5 exhibit a normal electrical switching, an unstable behavior is seen in the I-V characteristics of Ge15Te85-xSix glasses with x > 5 during the transition to ON state. Further, the switching voltage (VT) and initial resistance (R) are found to increase with addition of Si, exhibiting a change in slope at the rigidity percolation threshold of the Ge15Te85-xSix system. The ADSC studies on these glasses indicate the presence of an extended stiffness transition and a thermally reversing window in Ge15Te85-xSix in the composition range of 2 ≤ x ≤ 6. The fifth chapter deals with electrical switching investigations, thermal and structural studies on Al15Te85-xSix glasses (2 ≤ x ≤ 12). These glasses have been found to exhibit two crystallization reactions (Tc1 and Tc2) for compositions with x < 8 and a single stage crystallization is seen for compositions above x = 8. Also, a trough is seen in the composition dependence of non-reversing enthalpy (ΔHNR), based on which it is proposed that there is a thermally reversing window in Al15Te85-xSix glasses in the composition range 4 ≤ x ≤ 8. Further, Al15Te85-xSix glasses are found to exhibit a threshold type electrical switching at ON state currents less than 2 mA. The start and the end of the thermally reversing window seen in the thermal studies are exemplified by a kink and saturation in the composition dependence of switching voltages respectively. 27Al Solid State NMR measurements reveal that in Al15Te85-xSix glasses, Al atoms reside in 4-fold as well as 6-fold coordinated environments. Unlike in Al-As-Te glasses, there is no correlation seen between the composition dependence of the fraction of 4-fold and 6-fold coordinated aluminum atoms and the switching behavior of Al-Te-Si samples. Chapter 6 provides a comparison of the properties of the three glassy systems studied (As-Te-Si, Ge-Te-Si and Al-Te-Si), made to identify the system better suited for phase change memory applications. It is found that the Ge-Te-Si glassy system has better electrical/thermal properties for phase change memory applications. The seventh chapter describes easily reversible SET-RESET processes in Ge15Te83Si2 glass which is a promising candidate for phase change memory applications. This sample exhibits memory switching at a comparatively low threshold electric field (Eth) of 7.3 kV/cm. The SET and RESET processes have been achieved with 1 mA triangular current pulse for the SET process and 1 mA rectangle pulse (of 10 msec width) for RESET operation respectively. Further, a self-resetting effect is seen in this material upon excitation with a saw-tooth/square pulse. About 6.5x104 SET-RESET cycles have been achieved without any damage to the device. In chapter 8, results of in-situ Raman scattering studies on the structural changes occurring during the SET and RESET processes in Ge15Te83Si2 sample, are presented. It is found that the degree of disorder in the glass is reduced from OFF to SET state. The local structure of the sample under RESET condition is similar to that in the OFF state. The Raman results are found to be consistent with the switching results which indicate that the Ge15Te83Si2 glass can be SET and RESET easily. Further, Electron Microscopic studies on switched samples indicate the formation of nanometer sized particles of cSiTe2. A summary of the results obtained and the scope for future work are included in the chapter 9 of the thesis.