National Science Foundation
Cooling Technologies Research Center
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CTRC Breakthroughs

Exploratory and Novel Concepts

3D Inverse Heat Conduction Method for Microelectronics

Heat Conduction breakthroughs figure Developed a 3D inverse heat conduction methods to evaluate spatially-varying, transient temperatures and power dissipation maps when direct optical access or instrumentation on the surface of interest is experimentally not feasible. Designed/optimized a discrete sensor placement scheme for use with the inverse heat conduction numerical tool in realistic rack configurations. Two different inverse methodologies were developed and combined with a Kriging interpolation optimization to optimize the number and location of the sensors. The optimization works for any 3D microchip geometry and the methodology can be used for design and active thermal management purposes.

Flexible High Thermal Conductivity Fabrics for Application in Wearable Electronic Devices

Conductivity Fabrics breakthroughs figure Low-power, wearable electronic devices require methods of attachment to non-flat surfaces, specifically the human form, as well as thermal management techniques to meet ergonomics-driven temperature constraints. Thermally conductive polymer fabrics can potentially serve as the device substrate while simultaneously providing heat spreading functionality. In this project, a high thermal conductivity polymer material is identified, and its thermal properties are characterized in the fiber, yarn and fabric form by developing an IR-microscopy based experimental measurement technique. A complementary reduced-order thermal model is also developed to predict effective fabric thermal conductivity as a function of fiber and weave properties. The project also includes design of bend testing metrologies to assess the mechanical flexibility of the woven fabrics and characterize the effect of bend cycling on their thermal behavior. The developed methods and models can be used to evaluate thermal and mechanical properties of various fabric weave geometries, and help explore the potential design space for applications of these materials as flexible heat spreaders.

Multi-Frame Super Resolution Thermal Imaging

Multi-frame super-resolution imaging is a technique that allows for increasing the resolution of any existing optical setup without camera hardware modifications. This technique is particularly relevant and promising for improved thermomechanical characterization and emissivity/temperature mapping using infrared imaging, due to the low native resolution of typical infrared optical systems. This project developed a multi-frame super-resolution infrared imaging algorithm, demonstrated the feature resolution improvement, and quantified the measurement uncertainty. The resolution enhancement enabled new measurement capabilities, such as measurement of strain through digital image correlation, simultaneous with thermography. The technique provides a simple means for users of existing infrared cameras (or other imaging systems) to improve their measurement resolution with little added effort.

Performance of Finned Heat Exchangers and Heat Sinks after Air-Side Fouling and Cleaning

Finned Exchangers breakthroughs figure Fouling of heat exchangers and heat sinks is a routinely occurring phenomenon and significantly reduces their performance.Quantitative knowledge of this loss in performance as a function of dimensional and operating parameters will inform the process of designing a heat exchanger or heat sink leading to a component that is less susceptible to fouling. The model being developed as part of this project will be used to predict the impact of fouling on the heat exchanger or heat sink as a component and on the system that it belongs to as a whole. This information will then be used to predict the duration of time after which a unit installed in the field will be so greatly impacted by fouling that normal operation becomes impossible. Cleaning schedules designed to prevent undue stress being placed on the system to which the heat exchanger belongs will help reduce down-time of those systems due to a failure. Alternatively, for the heat exchanger or heat sink which cannot be cleaned, the maximum life of the unit will be the point at which the impact of fouling results in system failure.

Acoustically Enhanced Heat Transfer

Acoustic Heat Transfer breakthroughs figure The aim of the project is the enhancement of heat transfer with an acoustic sound field with, ideally, a smaller pressure drop in comparison to other surface enhancements. This project involved experimental investigation of acoustically enhancing the heat transfer from a fluid flow to the solid wall of a plain tube and determining the influence of an acoustic field on the rate of heat transfer. A prototype test stand had been constructed in which air was the working fluid. The figure plots dimensionless heat transfer enhancement (Q/Q0) against the frequency at various sound levels (top) showing that heat transfer can be enhanced with an acoustic field by up to 30%. Differential pressure inside of the test section plotted against the frequency (below) shows that with a particular frequency, the differential pressure is negative.

Investigation of Nucleate Boiling Mechanisms in Forced Convection Using Laser-Induced Flourescence Thermography

LIFT breakthroughs figure Laser-Induced Fluorescence Thermography (LIFT) is used to obtain the fluid temperature field around growing and collapsing vapor bubbles generated during subcooled nucleate boiling. LIFT relies on the temperature-dependent fluorescence intensity exhibited by certain dye molecules. Preliminary LIFT measurements during flow boiling indicate noticeable changes in fluorescence intensity, which is believed to be the result of a spatially and temporally-varying temperature field. The figures show the fluorescence intensity field following a vapor bubble collapse during subcooled nucleate boiling: a temperature-sensitive fluorescent dye (upper image) and a temperature-insensitive fluorescent dye (lower image). The circled region in both images indicates the region where the bubble collapsed. Changes in the fluorescence intensity field, which are apparent in the upper image but not the lower image, are believed to indicate changes in the liquid temperature field.

Direct Cooling by Electron Field Emmissions

Direct Cooling breakthroughs figure Emission of electrons from a cathode has long been known to produce a heating or cooling effect at the emission site depending on the conditions of the emission, such as the emitter work function and temperature and the applied electric field. The present concept provides a method by which an emission gap distance in the nanometer range can be maintained over areas as large as several square millimeters using a carbon nanotube array emitter and a liquid metal anode. Liquid indium and other liquid metals wet carbon nanotubes very poorly. Because of this characteristic, a narrow gap may be maintained between a densely packed array of carbon nanotubes and a liquid metal by placing thin dielectric spacers at strategic locations between the carbon nanotube-liquid metal interface (see the accompanying figure). One advantage of this approach is that the emission gap distance is not affected by thermal expansion, and another is that the gap distance may be adjusted by varying the thickness of the dielectric spacers.

Electromechanical Transport of Nanofluids: Microfluidic Pumping and Heat Transfer Enhancement

Nanofluids breakthroughs figure In this work, a thorough investigation was conducted to study the fundamental mechanisms of single-phase forced convection of Al2O3-water nanofluids in a minichannel. Friction factor and convection heat transfer coefficient were measured for nanofluids of various volume fractions. Convective heat transfer of nanofluids is mildly enhanced in the laminar region, however, deteriorates in the transition and turbulent regions. A novel pumping method was also developed in this work which exploits dielectrophoresis-induced particle-fluid interaction to actuate nanofluids without needing external pumps. With this approach, the flow is generated by inducing strong electromechanical effects in the fluid using integrated microelectrodes. The fluidic driving mechanisms due to the particle-fluid and particle-particle interactions under twDEP were analyzed and the induced flow field was obtained from numerical simulations. Experimental measurements of the flow velocity in a prototype DEP micropumping device show satisfactory agreement with the numerical predications. The figure shows a comparison of the average Nusselt number measured for nanofluids and the base fluid over the entire range of Reynolds number studied.