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We present the characterization of a compact, high performance air-cooled heat sink with an integrated loop heat pipe. In this configuration, heat enters the heat sink at the evaporator base and is transferred within the heat pipe by the latent heat of vaporization of a working fluid. From the condensers, the heat is transferred to the ambient air by an integrated fan. Multiple condensers are used to increase the surface area available for air-cooling, and to ensure the equal and optimal operation of the individual condensers, an additional wick is incorporated into the condensers. We demonstrated with this design (10.2 cm × 10.2 cm × 9 cm), a total thermal resistance of less than 0.1 °C/W while dissipating a heat load of 500 W from a source at 75 °C. Furthermore, constant thermal resistance was observed in the upright as well as sideways orientations. This prototype is a proof-of-concept demonstration of a high performance and efficient air-cooled heat sink design that can be readily integrated for various electronics packaging and data center applications., United States. Defense Advanced Research Projects Agency. Microsystems Technology Office. Microtechnologies for Air-Cooled Exchangers (Grant Number W31P4Q-09-1-0007)

A high-performance air-cooled heat sink that incorporates a novel heat pipe with multiple parallel condenser layers and interdigitated blower impellers is presented. A flow circuit model was developed in order to predict the air flow performance of a 15-layer impeller system using experimental measurements from a single layer. A 15-layer impeller system was constructed to validate the flow circuit model. The performance of the multi-layer system was investigated by using a hot wire anemometer to compare flow between layers and by measuring the inflation rate of a bag enclosing the air outlets. This work addresses important issues that allow the extension of the air flow modeling and experimental results from a single impeller design to a multilayer stack of impellers operating in parallel and sharing a common inlet. Topics: Impellers , Design , Heat sinks, United States. Defense Advanced Research Projects Agency. Microsystems Technology Office. Microtechnologies for Air-Cooled Exchangers (Grant W31P4Q-09-1-0007)

Thermal management of modern electronics is rapidly becoming a critical bottleneck of their computational performance. Air-cooled heat sinks offer ease and flexibility in installation and are currently the most widely used solution for cooling electronics. We report the characterization of a novel loop heat pipe (LHP) with a wick in the condenser, developed for the integration into an air-cooled heat sink. The evaporator and condenser are planar (102 mm × 102 mm footprint) and allow for potential integration of multiple, stacked condensers. The condenser wick is used to separate the liquid and vapor phases during condensation by capillary menisci and enables the use of multiple condensers with equal condensation behavior and performance. In this paper, the thermal–fluidic cycle is outlined, and the requirements to generate capillary pressure in the condenser are discussed. The LHP design to fulfill the requirements is then described, and the experimental characterization of a single-condenser version of the LHP is reported. The thermal performance was dependent on the fan speed and the volume of the working fluid; a thermal resistance of 0.177 °C/W was demonstrated at a heat load of 200 W, fan speed of 5000 rpm and fluid volume of 67 mL. When the LHP was filled with the working fluid to the proper volume, capillary pressure in the condenser was confirmed for all heat loads tested, with a maximum of 3.5 kPa at 200 W. When overfilled with the working fluid, the condenser was flooded with liquid, preventing the formation of capillary pressure and significantly increasing the LHP thermal resistance. This study provides the detailed thermal–fluidic considerations needed to generate capillary pressure in the condenser for controlling the condensation behavior and serves as the basis of developing multiple-condenser LHPs with low thermal resistance., United States. Defense Advanced Research Projects Agency (W31P4Q-09-1-0007)

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012., Cataloged from PDF version of thesis., Includes bibliographical references (p. 205-211)., A higher quality version of this thesis is available at: http://hdl.handle.net/1721.1/80414, Modern computer processors require significant cooling to achieve their full performance. The "efficiency" of heat sinks is also becoming more important: cooling of electronics consumes 1% of worldwide electricity use by some estimates. Unfortunately, current cooling technologies often focus on improving heat transfer at the expense of efficiency. The present work focuses on a unique, compact, and efficient air cooled heat sink which addresses these shortcomings. While conventional air cooled heat sinks typically use a separate fan to force air flow over heated fins, the new design incorporates centrifugal fans directly into the body of a loop heat pipe with multiple planar condensers. These "integrated fans" rotate between the planar condensers, in close proximity to the hot surfaces, establishing a radially outward flow of cooling air. The proximity of the rotating impellers to the condenser surfaces results in a marked enhancement in the convective heat transfer coefficient without a large increase in input power. To develop an understanding of the heat transfer in integrated fan heat sinks, a series of experiments was performed to simultaneously characterize the fan performance and average heat transfer coefficients. These characterizations were performed for 15 different impeller profiles with various impeller-to-gap thickness ratios. The local heat transfer coefficient was also measured using a new heated-thin-film infrared thermography technique capable of applying various thermal boundary conditions. The heat transfer was found to be a function of the flow and rotational Reynolds numbers, and the results suggest that turbulent flow structures introduced by the fans govern the transport of thermal energy in the air. The insensitivity of the heat transfer to the impeller profile decouples the fan design from the convection enhancement problem, greatly simplifying the heat sink design process. Based on the experimental results, heat transfer and fan performance, by Wayne L. Staats, Jr., Ph.D.

Objective: To test a method for performing electrical impedance myography (EIM) in the mouse hind limb for the assessment of disease status in neuromuscular disease models. Methods: An impedance measuring device consisting of a frame with electrodes embedded within an acrylic head was developed. The head was rotatable such that data longitudinal and transverse to the major muscle fiber direction could be obtained. EIM measurements were made with this device on 16 healthy mice and 14 amyotrophic lateral sclerosis (ALS) animals. Repeatability was assessed in both groups. Results: The technique was easy to perform and provided good repeatability in both healthy and ALS animals, with intra-session repeatability (mean ± SEM) of 5% ±1% and 12% ±2%, respectively. Significant differences between healthy and ALS animals were also identified (e.g., longitudinal mean 50 kHz phase was 18±0.6° for the healthy animals and 14±1.0° for the ALS animals, p = 0.0025). Conclusions: With this simple device, the EIM data obtained is highly repeatable and can differentiate healthy from ALS animals. Significance: EIM can now be applied to mouse models of neuromuscular disease to assess disease status and the effects of therapy.

Includes bibliographical references (p. 72-76)., Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008., In this work, a supercritical hydrogen liquefaction cycle is proposed and analyzed numerically. If hydrogen is to be used as an energy carrier, the efficiency of liquefaction will become increasingly important. By examining some difficulties of commonly used industrial liquefaction cycles, several changes were suggested and a readily scalable, supercritical, helium-cooled hydrogen liquefaction cycle was proposed. A novel overlap in flow paths of the two coldest stages allowed the heat exchanger losses to be minimized and the use of a single-phase liquid expander eliminated the pressure reduction losses associated with a Joule-Thomson valve. A simulation program was written in MATLAB to investigate the effects of altering component efficiencies and various system parameters on the cycle efficiency. In addition to performing the overall cycle simulations, several of the system components were studied in greater detail. First, the required volume of the ortho-para catalyst beds was estimated based on published experimental data. Next, the improvement in cycle efficiency due to the use of a single-phase liquid expander to reduce the pressure of the hydrogen stream was estimated. Finally, a heat exchanger simulation program was developed to verify the feasibility and to estimate the approximate size of the heat exchangers in the cycle simulation. For a large, 50-ton-per-day plant with reasonable estimates of achievable component efficiencies, the proposed cycle offered a modest improvement in efficiency over the current state of the art. In comparison to the 30-40% Second Law efficiencies of today's most advanced industrial plants, efficiencies of 39-44% were predicted for the proposed cycle, depending on the heat exchange area employed., by Wayne Lawrence Staats, Jr., S.M.