Avram Bar-Cohen

Bio: Avram Bar-Cohen is an academic researcher from University of Maryland, College Park. The author has contributed to research in topics: Heat transfer & Heat sink. The author has an hindex of 50, co-authored 329 publications receiving 8329 citations. Previous affiliations of Avram Bar-Cohen include Auburn University & DARPA.

Thermofluid characteristics of high-quality flowin chip-scale microgap channels

Abstract: Two-phase flow in chip-scale microgap channels offers highly potent thermal management capability and is the foundation for the emerging ”embedded cooling” paradigm of electronic cooling. While heat transfer and pressure drop in such flows are intimately tied to distinct forms of vapor-liquid aggregation, insufficient attention has been paid to characterizing the behavior of high-quality, short channel, microgap flow. This paper focuses on the results of simultaneous photographic and infrared visualization of such two-phase flows, under diabatic conditions, in a single 10 mm × 12 mm× 200 μm microgap channel cooled by low mass fluxes of FC-72. Churn flow, displaying pulsatile flow of confined vapor bubbles and slugs, is found to be the dominant flow regime for this short microgap channel configuration, yielding heat transfer coefficients that are an order-of-magnitude higher than all-liquid flow but are relatively insensitive to flow quality and heat flux. With increasing heat flux, local dryout and regions of elevated wall temperature grow and, periodically, expand to cover a majority of the microgap channel area. The average heat transfer coefficient is found to be in approximate agreement with the Shah correlation for low qualities and the Chen correlation for midlevel flow qualities, but both correlations are found to considerably overestimate the heat transfer coefficient at higher qualities, especially when local dryout becomes more frequent.

Theoretical aspects of nucleate pool boiling with dielectric liquids

Abstract: Direct cooling with inert, dielectric liquids may well become the technique of choice for the thermal management of future electronic systems. Due to the efficiency of phase-change processes and the simplicity of natural circulation, nucleate pool boiling is of great interest for this application. This paper examines the characteristics of vapor bubbles and nucleate pool boiling of the dielectric liquids. The results provide a theoretical foundation for understanding and interpreting the often complex empirical results reported in the literature.

Design and Optimization of Forced Convection Heat Sinks for Sustainable Development

Abstract: The development of forced convection heat sinks for microelectronic applications, which are compatible with sustainable development, involves the achievement of a subtle balance between a superior thermal design, minimum material consumption, and minimum pumping power. Due to the rapid proliferation of electronic systems, substantial material streams and energy consumption rates are associated with the cooling of computers, as well as other categories of electronic equipment. The study reported herein explores the potential for the least-energy optimization of forced convection cooled rectangular plate heat sinks. Analytical models are used to determine the thermal performance of such heat sinks, for pressure drops in the range of 20 to 80Pa and flow rates between 0.01 and 0.04 m3/s. The results are evaluated in terms of a heat sink Coefficient of Performance, relating the cooling capability to the energy invested. Guidelines for "sustainable" heat sink designs are suggested.

Modeling and Validation of a Prototype Thermally-Enhanced Polymer Heat Exchanger

Abstract: Polymer heat exchangers (PHXs) have received considerable attention since their invention more than 40 years ago due to their corrosion resistance, low density and low manufacturing cost. New polymer composites with higher strengths, thermal conductivities and thermal stability promise to bridge the performance gap between polymers and corrosion resistant metals. In the present study, PHX components were injection molded using thermally enhanced polyamide 12 resin and assembled into a crossflow finned-plate heat exchanger prototype. The prototype was implemented in an air-to-water experimental test apparatus and the heat transfer results were compared to an analytical model. This comparison confirmed that a polymer composite heat exchanger (PCHX) can offer significantly enhanced heat transfer relative to a pure polymer. A thermomechanical finite element model of the PCHX was developed and validated using experimental results. At fluid pressures near ambient, the heat transfer rate of the PCHX was 28% less than could be attained with an identical titanium heat exchanger. As fluid pressures increased, the through wall conduction resistance had a larger effect on heat transfer rate, reducing the performance of the PCHX relative to the titanium heat exchanger. Stress analysis of the thermally enhanced PCHX revealed that the stresses due to pressure loading were more sensitive to heat exchanger geometry, while the stresses due to thermal loading were more sensitive to material property anisotropy.Copyright © 2011 by ASME

Light-emitting diodes

Abstract: Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

A review of Ga2O3 materials, processing, and devices

Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

Low loss dielectric materials for LTCC applications: a review

Abstract: Small, light weight and multifunctional electronic components are attracting much attention because of the rapid growth of the wireless communication systems and microwave products in the consumer electronic market. The component manufacturers are thus forced to search for new advanced integration, packaging and interconnection technologies. One solution is the low temperature cofired ceramic (LTCC) technology enabling fabrication of three-dimensional ceramic modules with low dielectric loss and embedded silver electrodes. During the past 15 years, a large number of new dielectric LTCCs for high frequency applications have been developed. About 1000 papers were published and ∼500 patents were filed in the area of LTCC and related technologies. However, the data of these several very useful materials are scattered. The main purpose of this review is to bring the data and science of these materials together, which will be of immense help to researchers and technologists all over the world. The comme...