Improving processor efficiency through thermal modeling and runtime management of hybrid cooling strategies

One of the main challenges in building future high performance systems is the ability to maintain safe on-chip temperatures in presence of high power densities. Handling such high power densities necessitates novel cooling solutions that are significantly more efficient than their existing counterparts. A number of advanced cooling methods have been proposed to address the temperature problem in processors. However, tradeoffs exist between performance, cost, and efficiency of those cooling methods, and these tradeoffs depend on the target system properties. Hence, a single cooling solution satisfying optimum conditions for any arbitrary system does not exist. This thesis claims that in order to reach exascale computing, a dramatic improvement in energy efficiency is needed, and achieving this improvement requires a temperature-centric co-design of the cooling and computing subsystems. Such co-design requires detailed system-level thermal modeling, design-time optimization, and runtime management techniques that are aware of the underlying processor architecture and application requirements. To this end, this thesis first proposes compact thermal modeling methods to characterize the complex thermal behavior of cutting-edge cooling solutions, mainly Phase Change Material (PCM)-based cooling, liquid cooling, and thermoelectric cooling (TEC), as well as hybrid designs involving a combination of these. The proposed models are modular and they enable fast and accurate exploration of a large design space. Comparisons against multi-physics simulations and measurements on testbeds validate the accuracy of our models (resulting in less than 1C error on average) and demonstrate significant reductions in simulation time (up to four orders of magnitude shorter simulation times). This thesis then introduces temperature-aware optimization techniques to maximize energy efficiency of a given system as a whole (including computing and cooling energy). The proposed optimization techniques approach the temperature problem from various angles, tackling major sources of inefficiency. One important angle is to understand the application power and performance characteristics and to design management techniques to match them. For workloads that require short bursts of intense parallel computation, we propose using PCM-based cooling in cooperation with a novel Adaptive Sprinting technique. By tracking the PCM state and incorporating this information during runtime decisions, Adaptive Sprinting utilizes the PCM heat storage capability more efficiently, achieving 29\% performance improvement compared to existing sprinting policies. In addition to the application characteristics, high heterogeneity in on-chip heat distribution is an important factor affecting efficiency. Hot spots occur on different locations of the chip with varying intensities; thus, designing a uniform cooling solution to handle worst-case hot spots significantly reduces the cooling efficiency. The hybrid cooling techniques proposed as part of this thesis address this issue by combining the strengths of different cooling methods and localizing the cooling effort over hot spots. Specifically, the thesis introduces LoCool, a cooling system optimizer that minimizes cooling power under temperature constraints for hybrid-cooled systems using TECs and liquid cooling. Finally, the scope of this work is not limited to existing advanced cooling solutions, but it also extends to emerging technologies and their potential benefits and tradeoffs. One such technology is integrated flow cell array, where fuel cells are pumped through microchannels, providing both cooling and on-chip power generation. This thesis explores a broad range of design parameters including maximum chip temperature, leakage power, and generated power for flow cell arrays in order to maximize the benefits of integrating this technology with computing systems. Through thermal modeling and runtime management techniques, and by exploring the design space of emerging cooling solutions, this thesis provides significant improvements in processor energy efficiency.2018-07-09T00:00:00

Modelling And Optimization Of Micro-Channel And Thermal Energy Storage Heatsinks For Microelectronic Devices [TK7874. J44 2007 f rb].

Pemodelan dan pengoptimuman penyerap haba mikro-alur dan penyerap haba muatan dikaji dalam penyelidikan sini. Penyerap haba mikro-alur merupakan teknologi penyejukan yang berkesan untuk menyingkirkan tenaga haba yang tinggi daripada kawasan yang kecil dan terhad di dalam perkakasan mikroelektronik. The modelling and optimization of micro-channel and Thermal Energy Storage (TES) heatsinks in electronic cooling are investigated in the present study. The microchannels heatsinks is an efficient cooling technology to remove large amount of heat from very small and constrained areas of the high heat flux of microelectronic devices

Effect of wall resistance on the total thermal resistance of a stacked microchannel heat sink

This paper reports on the different modeling approach of the total thermal resistance in a microchannel heat sink (MCHS); with wall resistance and the frequently used fin model, in comparison with experimental results. For a single stack MCHS, the wall model caused more than 10% difference but it can be extended to a stacked MCHS while the fin model could not, due to the adiabatic top condition. The wall resistance model is idealized, assuming a 100% efficient convective heat transfer while in the fin model 70% was the maximum. Meanwhile, stacking showed that at a constant flow rate, the thermal resistance could be reduced by 3% for a double stack, while increasing beyond that will decrease the thermal performance of the MCHS. The study showed the limits of models used and possible stacking of a MCHS for improved heat removal capability

A Hierarchical Manifold Microchannel Heat Sink Array for High-Heat-Flux Two-Phase Cooling of Electronics

High-heat-flux removal is necessary for next-generation microelectronic systems to operate more reliably and efficiently. Extremely high heat removal rates are achieved in this work using a hierarchical manifold microchannel heat sink array. The microchannels are imbedded directly into the heated substrate to reduce the parasitic thermal resistances due to contact and conduction resistances. Discretizing the chip footprint area into multiple smaller heat sink elements with high-aspect-ratio microchannels ensures shortened effective fluid flow lengths. Phase change of high fluid mass fluxes can thus be accommodated in micron-scale channels while keeping pressure drops low compared to traditional, microchannel heat sinks. A thermal test vehicle, with all flow distribution components heterogeneously integrated, is fabricated to demonstrate this enhanced thermal and hydraulic performance. The 5 mm x 5 mm silicon chip area, with resistive heaters and local temperature sensors fabricated directly on the opposite face, is cooled by a 3 x 3 array of microchannel heat sinks that are fed with coolant using a hierarchical manifold distributor. Using the engineered dielectric liquid HFE-7100 as the working fluid, experimental results are presented for channel mass fluxes of 1300, 2100, and 2900 kg/m2 s and channel cross sections with nominal widths of 15 micrometers and nominal depths of 35 micrometers, 150 micrometers, and 300 micrometers. Maximum heat flux dissipation is shown to increase with mass flux and channel depth and the heat sink with 15 micrometers x 300 micrometers channels is shown to dissipate base heat fluxes up to 910 W/cm2 at pressure drops of less than 162 kPa and chip temperature rise under 47 degrees C relative to the fluid inlet temperature

Experimental and numerical thermal analysis of multi-layered microchannel heat sink for concentrating photovoltaic application

Concentrating photovoltaic has a major challenge due to the high temperature raised during the process which reduces the efficiency of the solar cell. A multi-layered microchannel heat sink technique is considered more efficient in terms of heat removal and pumping power among many other cooling techniques. Thus, in the current work, multi-layered microchannel heat sink is used for concentrating photovoltaic cooling. The thermal behavior of the system is experimentally and numerically investigated. The results show that in extreme heating load of 30 W/cm2 with heat transfer fluid flow rate of 30 mL/min, increasing the number of layers from one to four reduces the heat source temperature from 88.55 to 73.57 °C. In addition, the single layered MLM heat sink suffers from the highest non-uniformity in the heat source temperature compared to the heat sinks with the higher number of layers. Additionally, the results show that increasing the number of layers from one to four reduces the pressure drop from 162.79 to 32.75 Pa

Electro-thermal design and optimization of high-specific-power slotless PM machine for aircraft applications

A 1 MW high-frequency air-core permanent-magnet (PM) motor, with power density over 13 kW/kg (8 hp/lb) and efficiency over 96\%, is proposed for NASA hybrid-electric aircraft application. In order to maximize power density of the proposed motor topology, a large-scale multi-physics optimization, which is not favorable for current electrical machine software, is needed to obtain the best design candidates, which is not favorable for current electrical machine software. Therefore, developing electromagnetic (EM) and thermal analytical methods with computational efficiency and satisfactory accuracy is a key enabling factor for future multi-physics optimization of motor power density. This dissertation summarizes the efforts of developing an electro-thermal analysis and optimization scheme of the proposed motor for aircraft applications. Component hardware tests including windage loss, fan performance, full-scale stator temperature and litz-wire were conducted to validate the proposed prediction methods and provide calibrations in the motor design analysis. Furthermore, slotless litz wire winding geometry and strand size are optimized with the developed electro-thermal modeling including transposition effects. After gaining confidence in the developed electro-thermal models, an optimization design toolbox is built for the hybrid-electric engine systems study. The first application study is in partnership with Rolls Royce's Electrically Variable Engine Project to study thermal management system integration effects on motor sizing. The second study is in collaboration with Raytheon Technologies to study motor transient performance with phase change materials integration, which can be tailored to a hybrid-electric engine mission profile

Serpentine minichannel liquid-cooled heat sinks for electronics cooling applications

The increasing density of transistors in electronic components is leading to an inexorable rise in the heat dissipation that must be achieved in order to preserve reliability and performance. Hence, improving the thermal management of electronic devices is a crucial goal for future generations of electronic systems. Therefore, a complementary experimental and numerical investigation of single-phase water flow and heat transfer characteristics of the benefits of employing three different configurations of serpentine minichannel heat sink (MCHS) designs has been performed, to assess their suitability for the thermal management of electronic devices. These heat sinks are termed single (SPSMs), double (DPSMs) and triple path serpentine rectangular minichannels (TPSMs), and their performance is compared, both experimentally and numerically, with that of a design based on an array of straight rectangular minichannels (SRMs) in terms of pressure drop (ΔP), average Nusselt number (Nuavg) and total thermal resistance (Rth). The results showed that the serpentine channel bends are very influential in improving heat transfer by preventing both the hydrodynamic and thermal boundary layers from attaining a fully-developed state. The SPSM design provides the most effective heat transfer, followed by the DPSM and TPSM ones, both of which out-performed the SRM heat sink. The SPSM heat sink produced a 35% enhancement in Nuavg and a 19% reduction in Rth at a volumetric flow rate (Qin) of 0.5 l/min compared to the conventional SRM heat sink. These improvements in the heat transfer are, however, achieved at the expense of significantly larger ΔP. It was found that the incorporation of serpentine minichannels into heat sinks will significantly increase the heat-removal ability, but this must be balanced with the pressure drop requirement. Therefore, an experimental and numerical investigation of the benefit of introducing chevron fins has been carried out to examine the potential of decreasing pressure drop along with further thermal enhancement. This novel design is found to significantly reduce both the ΔP across the heat sink and the Rth by up to 60% and 10%, respectively, and to enhance the Nuavg by 15%, compared with the SPSM heat sink without chevron fins. Consequently, the design of the SPSM with and without chevron fins was then optimised in terms of the minichannel width (Wch) number of minichannels (Nch) and chevron oblique angle (θ). The optimisation process uses a 30 (without chevron fins) and 50 (with chevron fins) point Optimal Latin Hypercubes Design of Experiment, generated from a permutation genetic algorithm, and accurate metamodels built using a Moving Least Square (MLS) method. A Pareto front is then constructed to enable the compromises available between designs with a low pressure drop and those with low thermal resistance to be explored and appropriate design parameters to be chosen. These techniques have then been used to explore the feasibility of using serpentine MCHS and heat spreaders to cool GaN HEMT

High Efficiency Polymer based Direct Multi-jet Impingement Cooling Solution for High Power Devices

Liquid jet impingement cooling is an efficient cooling technique where the liquid coolant is directly ejected from nozzles on the chip backside resulting in a high cooling efficiency due to the absence of the TIM and the lateral temperature gradient. In literature, several Si-fabrication based impingement coolers with nozzle diameters of a few distributed returns or combination of micro-channels and impingement nozzles. The drawback of this Si processing of the cooler is the high fabrication cost. Other fabrication methods for nozzle diameters for ceramic and metal. Low cost fabrication methods, including injection molding and 3D printing have been introduced for much larger nozzle diameters (mm range) with larger cooler dimensions. These dimensions and processes are however not compatible with the chip packaging process flow. This PhD focuses on the modeling, design, fabrication and characterization of a micro-scale liquid impingement cooler using advanced, yet cost efficient, fabrication techniques. The main objectives are: (a) development of a modeling methodology to optimize the cooler geometry; (b) exploring low cost fabrication methods for the package level impingement jet cooler; (c) experimental thermal and hydraulic characterization and analysis of the fabricated coolers; (d) applying the direct impingement jet cooling solutions to different applications

Optimization of Heat Sinks in a Range of Configurations.

In this study, different heatsink geometries used for electronic cooling are studied and compared to each other to determine the most efficient. The goal is to optimize heat transfer of the heat sinks studied in a range of configuration based on fin geometry. Heat sinks are thermal conductive material devices designed to absorb and disperse heat from high-temperature objects (e.g. Computer CPU). Common materials used in the manufacturing of heat sinks are aluminum and copper due to their relatively high thermal conductivity and lightweight [1]. Aluminum is used as the material for the heatsinks studied in this research project. To start, experimental results from a wind tunnel test conducted were compared to numerical results generated to establish a validation case. Best practices in running numerical simulations on heat sinks along with suitable models for simulating real-world conditions were determined and analyzed. The two main thermal performance-evaluating parameters used in this project are pressure drop (ΔP) and thermal resistance (R). Thirteen numerical CFD simulations were run on different heatsink fin extrusion geometries including the traditional rectangular plate, arc plate, radial plate, cross pin, draft pin, hexagonal pin, mixed shape pin fin, pin and plate, separated plate, airfoil plate, airfoil pin, rectangular pin, and square zig-zag plate heat sinks. It was observed that different fin geometries and dimensions affect the performance of heat sinks to varying extents. The square zig-zag plate heat sink from results obtained had the lowest thermal resistance of 0.25 K/W with the separated plate having the lowest pressure drop of 11.94 Pa. This information is relevant in the selection of fan type, size, and model of heat sink for electronics cooling. Also, another important conclusion drawn from this project is the existence of no definite correlation between the thermal resistance (R) and pressure drop (ΔP) parameters when evaluating heatsink performance