Chip Multiprocessors (CMPs) have been prevailing in the modern microprocessor market. As the significant heat is converted by the ever-increasing power density and current leakage, the raised operating temperature in a chip has already threatened the system?s reliability and led the thermal control to be one of the most important issues needed to be addressed immediately in chip designs. Due to the cost and complexity of designing thermal packaging, many Dynamic Thermal Management (DTM) schemes have been widely adopted in modern processors. In this study, we focus on developing a simple and accurate thermal model, which provides a scheduling decision for running tasks. And we show how to design an efficient DTM scheme with negligible performance overhead. First, we propose an efficient DTM scheme for multimedia applications that tackles the thermal control problem in a unified manner. A DTM scheme for multimedia applications makes soft realtime scheduling decisions based on statistical characteristics of multimedia applications. Specifically, we model application execution characteristics as the probability distribution of the number of cycles required to decode frames. Our DTM scheme for multimedia applications has been implemented on Linux in two mobile processors providing variable clock frequencies in an Intel Pentium-M processor and an Intel Atom processor. In order to evaluate the performance of the proposed DTM scheme, we exploit two major codecs, MPEG-4 and H.264/AVC based on various frame resolutions. Our results show that our DTM scheme for multimedia applications lowers the overall temperature by 4 degrees C and the peak temperature by 6 degrees C (up to 10 degrees C), while maintaining frame drop ratio under 5% compared to existing DTM schemes for multimedia applications. Second, we propose a lightweight online workload estimation using the cumulative distribution function and architectural information via Performance Monitoring Counters (PMC) to observe the processes dynamic workload behaviors. We also present an accurate thermal model for CMP architectures to analyze the thermal correlation effects by profiling the thermal impacts from neighboring cores under the specific workload. Hence, according to the estimated workload characteristics and thermal correlation effects, we can estimate the future temperature of each core more accurately. We implement a DTM scheme considering workload characteristics and thermal correlation effects on real machines, an Intel Quad-Core Q6600 system and Dell PowerEdge 2950 (dual Intel Xeon E5310 Quad-Core) system, running applications ranging from multimedia applications to several benchmarks. Experiments results show that our DTM scheme reduces the peak temperature by 8% with 0.54% performance overhead compared to Linux Standard Scheduler, while existing DTM schemes reduce peak temperature by 4% with up to 50% performance overhead.

With increasing awareness of climate change, governments and organizations have made it their mission to see a greener future. Countries like Norway, South Korea, and Canada have promised to ban internal combustion engines (ICE) by 2025-2035. Growing demand for cleaner modes of travel have taken over the market, causing everyone to look at electric vehicles for the solution. Tesla’s revenue has tripled in the past five years, 15 new electric car manufacturer shave joined, and almost all big-name ICE companies have started producing electric/hybrid cars. As the number of electric vehicles increases, a solution to long charging times will be needed to keep up with the high-power-density fuel used in ICE. Charging stations are increasing in power ratings as Tesla introduces their 250-kW supercharger and EVBox with their 350-kW Ultronig stations. These stations are comprised of power modules that stack together to reach the desired power rating. Designing, testing, and implementing power modules for electric vehicles can be a complex process due to thermal efficiency and packaging challenges. To address these issues, it is essential to establish a design methodology for power modules that takes into account validation and packaging considerations. This thesis presents a design methodology for heat exchangers that allows for rapid prototyping with sufficient accuracy, approximately below 10%. The study includes numerical simulations, reduced modeling, and experimental validation, which can increase confidence during the design phase and reduce design times. Using reduced models for quick calculations instead of relying solely on numerical models can further expedite the process. A reliable and adaptable analytical methodology for heat exchanger design is crucial for successful optimization setup. / Thesis / Master of Applied Science (MASc)

Building-integrated Concentrator Photovoltaic (BICPV) technology produces noiseless and pollution free electricity at the point of use. With a potential to contribute immensely to the increasing global need for a sustainable and low carbon energy, the primary challenges such as thermal management of the panels are overwhelming. Although significant progress has been made in the solar cell efficiency increase, the concentrator photovoltaic industry has still to go a long way before it becomes competitive and economically viable. Experiencing great losses in their electrical efficiencies at high temperatures that may eventually lead to permanent degradation over time, affects the market potential severely. With a global PV installed capacity of 303 GW, a nominal 10 °C decrease in their average temperatures could theoretically lead to a 5 % electricity efficiency improvement resulting in 15 GW increase in electricity production worldwide. However, due to a gap in the research knowledge concerning the effectiveness of the available passive thermal regulation techniques both individually and working in tandem, this lucrative potential is yet to be realised. The work presented in this thesis has been focussed on incremental performance improvement of BICPV by developing innovative solutions for passive cooling of the low concentrator based BICPV. Passive cooling approaches are selected as they are generally simpler, more cost-effective and considered more reliable than active cooling. Phase Change Materials (PCM) have been considered as the primary means to achieve this. The design, fabrication and the characterisation of four different types of BIPCV-PCM assemblies are described. The experimental investigations were conducted indoors under the standard test conditions. In general, for all the fabricated and assembled BICPV-PCM systems, the electrical power output showed an increase of 2 %-17 % with the use of PCM depending on the PCM type and irradiance. The occurrence of hot spots due to thermal disequilibrium in the PV has been a cause of high degradation rates for the modules. With the use of PCM, a more uniform temperature within the module could be realised, which has the potential to extend the lifetime of the BICPV in the long-term. Consequentially, this may minimise the intensive energy required for the production of the PV cells and mitigate the associated environmental impacts. Following a parallel secondary approach to the challenge, the design of a micro-finned back plate integrated with a PCM containment has been proposed. This containment was 3D printed to save manufacturing costs and time and for reducing the PCM leakage. An organic PCM dispersed with high thermal conductivity nanomaterial was successfully tested. The cost-benefit analysis indicated that the cost per degree temperature reduction (£/°C) with the sole use of micro-fins was the highest at 1.54, followed by micro-fins + PCM at 0.23 and micro-fins + n-PCM at 0.19. The proposed use of PCM and application of micro-finned surfaces for BICPV heat dissipation in combination with PCM and n-PCM is one the novelties reported in this thesis. In addition, an analytical model for the design of BICPV-PCM system has been presented which is the only existing model to date. The results from the assessment of thermal regulation benefits achieved by introducing micro-finning, PCM and n-PCM into BICPV will provide vital information about their applicability in the future. It may also influence the prospects for how low concentration BICPV systems will be manufactured in the future.

The oil and gas industries use sophisticated logging tools during and after drilling. These logging tools employ internal electronics for sensing viscosity, pressure, temperature, and other important quantities. To protect the sensitive electronics, which typically have a maximum allowable temperature of 100 㬠they are shielded and insulated from the harsh external drilling environment. The insulation reduces the external heat input, but it also makes rejection of the heat generated within the electronics challenging. Electronic component failures promoted by elevated temperatures, and thermal stress, require a time consuming and expensive logging tool replacement process. Better thermal management of the electronics in logging tools promises to save oil and gas companies time and money. This research focuses on this critical thermal management challenge. Specifically, this thesis describes the design, fabrication, and test of an innovative thermal management system capable of cooling commercial-off-the-shelf electronics for extended periods in harsh ambient temperatures exceeding 200 㮠Resistive heaters embedded in quad-flat-packages simulate the electronics used in oil well logging. A custom high temperature oven facilitates the evaluation of a full scale prototype of the thermal management system. We anticipate the prototype device will validate computer modeling efforts on which its design was based, and advance future designs of the thermal management system.

Gas foil bearings (GFBs) operating at high temperature rely on thermal management procedures that supply needed cooling flow streams to keep the bearing and rotor from overheating. Poor thermal management not only makes systems inefficient and costly to operate but could also cause bearing seizure and premature system destruction. To date, most of thermal management strategies rely on empirically based "make-and-break" techniques which are often inefficient. This dissertation presents comprehensive measurements of bearing temperatures and shaft dynamics conducted on a hollow rotor supported on two first generation GFBs. The hollow rotor (1.36 kg, 36.51 mm OD and 17.9 mm ID) is heated from inside to reach an outer surface temperature of 120 degrees C. Experiments are conducted with rotor speeds to 30 krpm and with forced streams of air cooling the bearings and rotor. Air pressurization in an enclosure at the rotor mid span forces cooling air through the test GFBs. The cooling effect of the forced external flows is most distinct when the rotor is hottest and operating at the highest speed. The temperature drop per unit cooling flow rate significantly decreases as the cooling flow rate increases. Further measurements at thermal steady state conditions and at constant rotor speeds show that the cooling flows do not affect the amplitude and frequency contents of the rotor motions. Other tests while the rotor decelerates from 30 krpm to rest show that the test system (rigid-mode) critical speeds and modal damping ratio remain nearly invariant for operation with increasing rotor temperatures and with increasing cooling flow rates. Computational model predictions reproduce with accuracy the test data. The work adds to the body of knowledge on GFB performance and operation and provides empirically derived guidance for successful integration of rotor-GFB systems.

Government regulations and growing concerns regarding global warming has lead to an increasing number of passenger vehicles on the roads today that are not powered by the conventional internal combustion (IC) engine. Automotive manufacturers have introduced electric powertrains over the last 10 years which have introduced new challenges regarding powering accessory loads historically reliant on the mechanical energy of the IC engine. High density batteries are used to store the electrical energy required by an electric powertrain and due to their relatively narrow acceptable temperature range, require liquid cooling. The cooling system in place currently utilizes the A/C compressor for cooling and a separate electric element for heating which is energy expensive when the source of energy is electricity. The proposed solution is a thermoelectric heat pump for both heating and cooling. A model predictive controller (MPC) is designed, implemented and tested to optimize the operation of the thermoelectric heat pump. The model predictive controller is chosen due to its ability to accept multiple constrained inputs and outputs as well as optimize the system according to a cost function which may consist of any parameters the designer chooses. The system is highly non-linear and complex therefore both physical modelling and system identi cation were used to derive an accurate model of the system. A steepest descent algorithm was used for optimization of the cost function. The controller was tested in a test bench environment. The results show the thermoelectric heat pump does hold the battery at the speci ed set point however more optimization was expected from the controller. The controller fell short of expectation due to operational restriction enforced during design meant to simplify the problem. The MPC controller is capable of much better performance through adding more detail to the model, an improved optimization algorithm and allowing more flexibility in set point selection.

A novel and new thermal management technology for advanced ceramic microelectronic packages has been developed incorporating miniature heat pipes embedded in the ceramic substrate. The heat pipes use an axially grooved wick structure and water as the working fluid. Prototype substrate/heat pipe systems were fabricated using high temperature co-fired ceramic (alumina). The heat pipes were nominally 81 mm in length, 10 mm in width, and 4 mm in height, and were charged with approximately 50-80 mL of water. Platinum thick film heaters were fabricated on the surface of the substrate to simulate heat dissipating electronic components. Several thermocouples were affixed to the substrate to monitor temperature. One end of the substrate was affixed to a heat sink maintained at constant temperature. The prototypes were tested and shown to successful and reliably operate with thermal loads over 20 Watts, with thermal input from single and multiple sources along the surface of the substrate. Temperature distributions are discussed for the various configurations and the effective thermal resistance of the substrate/heat pipe system is calculated. Finite element analysis was used to support the experimental findings and better understand the sources of the system's thermal resistance.

Temperature control is widely sought after in regards to optical systems as their optical parameters often show dependence on temperature. Examples include diode lasers, multiplexing systems, optical amplifiers, and filters all of whom have a high sensitivity to temperature. This thesis presents a temperature controlled environment chamber actuated by a thermoelectric cooler. The design of which provides a simple, multi-applicable solution for temperature control in optical devices. The final device is comprised of three sub-areas of design. Each subsystem was custom built and applied in the final assembly -- including a digitally implemented signal generator, an error correction controller, and the environment chamber heat sink structure. The signal generator is used as input for a switched-mode based Peltier driver found commercially. A feedback error controller compensates the driver for temperature control. Both systems are implemented with microcontroller units. The environment chamber heat sink assembly is designed specifically to handle the thermal energy generated by the thermoelectric cooler. All of the systems were tested collectively for functionality. The input signal generator achieved its design goals and is capable of creating specific profiles in the temperature response. Error controller performance was reasonable in set-point tracking for continuous input signals. Step input responses are tuned for minimal settling time and overshoot. Temperature resolution in the thermistor response is around 0.1•C after digital filtering. The thermal design achieved its goal of operating in an ambient environment up to 54°C. Low temperature ambient environment operation has been confirmed to 8°C. / Master of Science

Electrohydrodynamic (EHD) micropumps have been developed and used in many diverse applications such as in microscale liquid cooling and various microfluidic systems. The objective of this research is to investigate different methods of enhancing the performance of ion drag EHD micropumps. In particular, the effect of electrode surface topology, applied electric field and doping agent in the dielectric liquid were investigated. The effect of 3D sharp features on the electrodes on charge injection in HFE 7100 as dielectric fluid was studied under an applied DC electric field. Micro and nano-scale features with high aspect ratio were developed on smooth copper electrodes by chemical etching or through electrophoretic deposition of single walled carbon nanotube (SWCNT). The spacing between the electrodes was kept at 250 µm. A reduction factor of 5 was achieved for SWCNT electrodes compared to the smooth case for the onset of charge injection. This study was then extended to determine its effects on the performance of ion drag EHD micropumps with 100 pairs of interdigitated electrodes. The emitter electrodes (20 µm) were half the width of the collector electrodes (40 µm), with one pump having an inter-electrode spacing of 120 µm and the other with 40 µm. Each micropump had a width of 5 mm and a height of 100 µm. SWCNT was deposited on the emitter electrodes of the micropump to generate a maximum static pressure of 4.7 kPa at 900 V, which is a 5 fold increase compared to the pump with smooth electrodes. Flow rate at no back pressure condition was improved by a factor of 3. The effect of Ferrocene as a doping agent in the working fluid HFE 7100 was studied under DC voltages. A maximum static pressure of 6.7 kPa was achieved at 700 V with 0.2% weight based doping agent, 11 times higher than when there was no doping agent at the same applied voltage. When there was no back pressure the pump generated a maximum flow rate of 0.47 mL/min at 700 V with 0.05% doping agent which is 9 times greater than with no doping agent. The effect of pulsed voltage on the performance of ion drag EHD micropump has been studied to exploit the displacement current at the sudden change of applied voltage magnitude. A range of pulse repetition rate and duty cycle were found to significantly enhance the pump performance. Static pressure generation was up to 75% and 88% greater at an optimal pulse repetition rate and duty cycle, respectively, compared to the average of the two DC levels. The effect of external flow on the discharge characteristics of an injection micropump was studied with DC volts. Higher discharge current and lower threshold voltage for the onset of charge injection in case of co-flow compared to the static case was observed. There was an optimum flow rate to generate maximum current for both co and counter-flow cases. / Thesis / Doctor of Philosophy (PhD)

Almost all electronic devices require efficient conversion of electrical power from one form to another. Electrical power is used world wide at the rate of approximately 12 billion kW per hour. The Center for Power Electronics Systems at Virginia Tech was established with a vision to develop an integrated systems approach via integrated power electronic modules (IPEMs) to improve the reliability, cost-effectiveness, and performance of power electronics systems. IPEMs are multi-layered structures based on embedded power technology and offer the advantage of three-dimensional (3D) packaging of electronic components in a small and compact volume, replacing the traditional wire bonding technology. They have the potential to offer reduced time and effort associated with developing and manufacturing power processors. However, placing multiple heat generating chips in a small volume also makes thermal management more challenging. With the steady increase in the heat density of the electronic packages during the last few decades, thermal management is becoming a key enabling technology for the future growth of power electronics. The focus of this work is on using computational analysis tools and experimental techniques to assess fundamental and practical cooling limitations on IPEMs, developing both passive and active integrated thermal management strategies, and creating design guidelines for IPEMs based on both thermal and thermo-mechanical stress considerations. Specifically, a commercially available finite element package is used to create a 3D geometric layout of the electronic module. The baseline finite element numerical model is validated using bench-top wind tunnel experiments. The experimental setup is also employed to characterize the thermal behavior of chips in the multi-chip package and test the applicability of superposition methodology for temperature fields of chips within multi-chip modules. Using numerical models, both passive and active integrated thermal management strategies are investigated. The passive cooling strategies include advanced ceramic materials, copper trace thickness, and structural enhancements. Active cooling strategies include double-sided cooling using traditional heat sinks, and an extension of double-sided cooling concept using microchannels integrated with the module on both sides of embedded chips. The overall result of the work presented here is the better understanding of thermal issues and limitations with IPEM technology, and development of thermal design guidelines for cooling strategies that take into consideration both thermal and thermo-mechanical performance. / Ph. D.