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Showing papers by "Avram Bar-Cohen published in 2010"


Journal ArticleDOI
TL;DR: In this paper, the two-phase thermofluid characteristics of a dielectric liquid, FC-72, flowing in an asymmetrically heated chip-scale microgap channel, 10 mm wide × 37 mm long, with channel heights varying from 110 μm to 500 μm and channel wall heat fluxes of 200 kW/m2.
Abstract: Rapidly increasing light emitting diode (LED) heat fluxes necessitate the development of aggressive thermal management techniques that can intercept the dissipated heat directly in the submount. Microgap coolers, which eliminate solid-solid thermal interface resistance and provide direct contact between chemically inert, dielectric fluids and the back surface of an active electronic component, offer a most promising approach for cooling high-power LEDs. This paper focuses on the two-phase thermofluid characteristics of a dielectric liquid, FC-72, flowing in an asymmetrically heated chip-scale microgap channel, 10 mm wide × 37 mm long, with channel heights varying from 110 μm to 500 μm and channel wall heat fluxes of 200 kW/m2. The experimental two-phase, area-averaged heat transfer coefficients of FC-72 reached 10 kW/m2·K, significantly higher than the single-phase FC-72 values, thus providing cooling capability in the range associated with water under forced convection. Data obtained for single-phase water yielded very good agreement with predictions for the convective heat transfer coefficients and served to validate the accuracy of the experimental apparatus and measurement technique. It is shown that this two-phase cooling approach could be used to dissipate in excess of 600 kW/m2 in the submount of high-power LEDs.

49 citations


Journal ArticleDOI
TL;DR: In this paper, a hierarchical model is proposed to assess the lifetime of an actively cooled LED-based luminaire that can provide light output equivalent to a 100 W incandescent lamp.
Abstract: The lifetime of an actively-cooled light emitting diode (LED)-based luminaire is dependent not only on the junction temperature of LEDs, but also on the reliability of active cooling devices. We propose a novel hierarchical model to assess the lifetime of an actively cooled LED-based luminaire that can provide light output equivalent to a 100 W incandescent lamp. After design considerations for LED-based luminaires with active cooling are discussed, the proposed model is described using component-level sub-physics-of-failure models. The model is implemented to predict the lifetime of a LED-based recessed downlight with synthetic jet cooling. The effects of the time-dependent performance degradation mechanisms of the active cooling device on the lifetime of the luminaire are also discussed.

36 citations


Book ChapterDOI
01 Jan 2010
TL;DR: A review of the relevant passive and active thermal management techniques, the physical phenomena underpinning the most promising on-chip thermal management approaches are described in this article, where the authors focus on thin-film and miniaturized thermoelectric coolers, orthotropic TIMs/heat spreaders, and phase change microgap coolers for hot-spot remediation and thermal management of these nanoelectronic chips.
Abstract: The rapid emergence of nanoelectronics, with the consequent rise in transistor density and switching speed, has led to a steep increase in die heat flux and growing concern over the emergence of on-chip “hot spots.” The application of on-chip high heat flux cooling techniques provides a viable direction for the thermal management of such nanoelectronic components. Following a review of the relevant passive and active thermal management techniques, the physical phenomena underpinning the most promising on-chip thermal management approaches are described. Attention is devoted to thin-film and miniaturized thermoelectric coolers, orthotropic TIMs/heat spreaders, and phase-change microgap coolers for hot-spot remediation and thermal management of these nanoelectronic chips.

30 citations


Journal ArticleDOI
TL;DR: In this paper, the authors extend the available data on mixture CHF enhancement, as well as pool boiling, on polished silicon surfaces to FC-72/FC-40 mixture ratios of 10, 15, and 20% of FC-40 by weight, a pressure range of between 1 and 3 Â atm, and fluid temperature from 22 to 45 Â c.

21 citations


Journal ArticleDOI
TL;DR: In this article, the effect of thermal and mechanical design issues in a high power light emitting diode (LED) package platform is investigated using numerical models, and the desired parameters of adhesives for high power LED applications are identified.
Abstract: Coupled thermal and mechanical design issues in a high power light emitting diode (LED) package platform are investigated using numerical models. A thermal resistance network model and a 3-D finite element model are built for thermal and stress analyses. They are validated with the experimental data and subsequently utilized to study the effect of key parameters on the junction temperature and the thermal strains. An extensive parametric analysis is conducted to assess the effect of design and material parameters on the junction temperature and thermal strains of the high power LED under study. Based on the results, the desired parameters of adhesives for high power LED applications are identified and an example of an LED thermo-mechanical design protocol is presented.

19 citations


Journal ArticleDOI
TL;DR: In this paper, a thermally conductive polymer material, filled with carbon fibers, was used to enhance thermal conductivity by an order of magnitude or more, and a prototype polymer seawater-methane heat exchanger was used in the liquefaction of natural gas on offshore platforms.
Abstract: The compression process necessary for the liquefaction of natural gas on offshore platforms generates large amounts of heat, usually dissipated via sea water cooled plate heat exchangers. To date, the corrosive nature of sea water has mandated the use of metals, such as titanium, as heat exchanger materials, which are costly in terms of life cycle energy expenditure. This study investigates the potential of a commercially available, thermally conductive polymer material, filled with carbon fibers to enhance thermal conductivity by an order of magnitude or more. The thermofluid characteristics of a prototype polymer seawater-methane heat exchanger that could be used in the liquefaction of natural gas on offshore platforms are evaluated based on the total coefficient of performance (COP T ), which incorporates the energy required to manufacture a heat exchanger along with the pumping power expended over the lifetime of the heat exchanger, and compared with those of conventional heat exchangers made of metallic materials. The heat exchanger fabricated from a low energy, low thermal conductivity polymer is found to perform as well as, or better than, exchangers fabricated from conventional materials, over its full lifecycle. The analysis suggests that a COP T nearly double that of aluminum, and more than ten times that of titanium, could be achieved. Of the total lifetime energy use, 70% occurs in manufacturing for a thermally enhanced polymer heat exchanger compared with 97% and 85% for titanium and aluminum heat exchanges, respectively. The study demonstrates the potential of thermally enhanced polymer heat exchangers over conventional ones in terms of thermal performance and life cycle energy expenditure.

17 citations


Proceedings ArticleDOI
01 Jan 2010
TL;DR: In this article, the authors used infrared thermography to locate the nascent dryout regions and operating conditions of a 210 micron microgap channel, operated with a mass flux of 195.2 kg/m2 -s and heat fluxes of 10.3 to 26 W/cm2.
Abstract: Heat transfer to an evaporating refrigerant and/or dielectric liquid in a microgap channel can provide very high heat transfer coefficients and volumetric cooling rates. Recent studies at Maryland have established the dominance of the annular flow regime in such microgap channels and related the observed high-quality peak of an M-shaped heat transfer coefficient curve to the onset of local dryout. The present study utilizes infrared thermography to locate such nascent dryout regions and operating conditions. Data obtained with a 210 micron microgap channel, operated with a mass flux of 195.2 kg/m2 -s and heat fluxes of 10.3 to 26 W/cm2 are presented and discussed.Copyright © 2010 by ASME

13 citations


Journal ArticleDOI
TL;DR: In this paper, the results of 3-D, electrothermal, finite element modeling of a superlattice microcooler were provided, focusing on the hot spot temperature and surface temperature reductions, respectively.
Abstract: Proposed uses of solid-state thermoelectric microcoolers for hot spot remediation have included the formation of a superlattice layer on the back of the microprocessor chip, but there have been few studies on the cooling performance of such devices. This paper provides the results of 3-D, electrothermal, finite element modeling of a superlattice microcooler, focusing on the hot spot temperature and superlattice surface temperature reductions, respectively. Simulated temperature distributions and heat flow patterns in the silicon, associated with variations in microcooler geometry, chip thickness, hot spot size, hot spot heat flux, and superlattice thickness are provided. Comparison is made to hot spot cooling achieved by the Peltier effect in the silicon microprocessor chip itself. The numerical results suggest that, for a variety of operating conditions and geometries, while increasing the superlattice thickness serves to decrease the exposed superlattice surface temperature, it is ineffective in reducing the hot spot temperature below that due to the silicon Peltier effect.

8 citations


Proceedings ArticleDOI
01 Jan 2010
TL;DR: In this paper, the authors explore the parametric dependence of annular flow thermal transport in microgaps including the effects of channel diameter, mass flux, and working fluid on the two-phase heat transfer coefficients.
Abstract: Forced flow of refrigerants and dielectric liquids, undergoing phase change in a heated microgap channel between chips or in parallel microchannels in a compact cooler, is a promising candidate for the thermal management of advanced semiconductor devices It has been found that Annular flow is the dominant flow regime in such miniature channels and that relatively high heat transfer coefficients are encountered in the moderate-to-high quality sections of such channels Following a discussion of flow regimes and thermal characteristics of miniature channels, attention turns to exploring the parametric dependence of annular flow thermal transport in microgaps including the effects of channel diameter, mass flux, and working fluid on the two-phase heat transfer coefficientsCopyright © 2010 by ASME

8 citations


01 Jan 2010
TL;DR: In this paper, the results of 3-D, electrothermal, finite element modeling of a superlattice micro-cooler were provided, focusing on the hot spot temperature and surface temperature reductions, respectively.
Abstract: Proposed uses of solid-state thermoelectric micro- coolers for hot spot remediation have included the formation of a superlattice layer on the back of the microprocessor chip, but there have been few studies on the cooling perfor- mance of such devices. This paper provides the results of 3-D, electrothermal, finite element modeling of a superlattice micro- cooler, focusing on the hot spot temperature and superlattice surface temperature reductions, respectively. Simulated temper- ature distributions and heat flow patterns in the silicon, associated with variations in microcooler geometry, chip thickness, hot spot size, hot spot heat flux, and superlattice thickness are provided. Comparison is made to hot spot cooling achieved by the Peltier effect in the silicon microprocessor chip itself. The numerical results suggest that, for a variety of operating conditions and geometries, while increasing the superlattice thickness serves to decrease the exposed superlattice surface temperature, it is ineffective in reducing the hot spot temperature below that due to the silicon Peltier effect.

6 citations


Proceedings ArticleDOI
01 Jan 2010
TL;DR: An integrated molding-heat transfer modeling methodology is used to study the thermal characteristics of polymer composite fins subjected to convective heat transfer coefficients as discussed by the authors. But this method does not consider the thermal anisotropic variation of thermal conductivity in the fin.
Abstract: An integrated molding-heat transfer modeling methodology is used to study the thermal characteristics of polymer composite fins subjected to convective heat transfer coefficients. Numerical predictions of the fiber orientation in a representative, injection-molded plate fin, based on the Folgar-Tucker model, are used, via the classic Nielsen model, to determine the anisotropic variation of thermal conductivity in the fin. Thermal simulations are then performed to determine the effect of both global and local thermal anisotropy on the temperature distribution and heat transfer rate of the anisotropic fin. It is also shown that the harmonic mean conductivity, in the axial direction, can be used to represent the heat loss of an anisotropic fin to better than 10% accuracy.Copyright © 2010 by ASME


Proceedings ArticleDOI
01 Jan 2010
TL;DR: In this article, three-dimensional thermo-electric simulations are used to investigate the self-cooling of hot spots on a germanium chip for a wide range of input current, doping concentration, hot spot heat flux, micro cooler size, and GPGA thickness.
Abstract: Growing interest in germanium solid-state devices is raising concern over the effects of on-chip, micro-scaled, high flux hot spot on the reliability and performance of germanium chips. Current thermal management technology offers few choices for such on-chip hot spot remediation. However, the good thermo-electric properties of single crystal germanium support the development of a novel thermal management approach, relying on thermoelectric self-cooling by an electric current flowing in a thin planar layer on the back of the germanium chip. Use of metal-on-germanium fabrication techniques can yield a very low thermal contact resistance at the micro cooler/chip interface and the current flow can transfer the energy absorbed from a hot spot to the edge of the chip, thus substantially reducing the detrimental effect of thermoelectric heating on the temperature of the active circuitry. In this paper three-dimensional thermo-electric simulations are used to investigate the self-cooling of hot spots on a germanium chip for a wide range of input current, doping concentration, hot spot heat flux, micro cooler size, and germanium chip thickness. Results suggest that localized thermoelectric self-cooling on the germanium chip can significantly reduce the temperature rise resulting from micro-scaled high-flux hot spots.Copyright © 2010 by ASME


Journal ArticleDOI
TL;DR: A modified coupled-mode (CM) model is proposed for the optical behavior of thermally chirped Bragg gratings that accounts for the axial gradient in the modulation wavenumber, which has been ignored in the classical CM model.
Abstract: A modified coupled-mode (CM) model is proposed for the optical behavior of thermally chirped Bragg gratings. The model accounts for the axial gradient in the modulation wavenumber, which has been ignored in the classical CM model. The model is used to characterize the optical behavior of a polymethyl methacrylate-based polymer Bragg grating subjected to nonisothermal conditions. The validity of the proposed method is verified by comparing the results of the modified CM model with those obtained from the exact numerical solution.