Prathib Skandakumaran

Experiments and Modeling of Multilayer Copper Minichannel Heat Sinks in Single-Phase Flow

One of the most promising configurations for indirect liquid cooling of electronic systems is in the use of heat sinks or cold plates where a liquid is forced to flow through channels embedded in a solid matrix. Traditional microchannel heat sinks consist of a single layer of parallel, high-aspect ratio rectangular channels microfabricated in silicon or copper. These heat sinks can achieve very high heat fluxes due to: 1) high heat transfer coefficients from microchannels, and 2) large surface areas from high-aspect ratio channels. When manufacturing techniques prohibit the fabrication of high-aspect ratio channels, stacking or creating multilayers of single layered channels can be an alternative to increasing surface area. In this work, square channel copper minichannel heat sinks were fabricated with single and multiple layers. It was experimentally shown that multilayer heat sinks have significant advantages over single layer equivalents with reductions in thermal resistance and pressure drop. Numerical simulations using CFD were performed and comparisons were made with experimental results. An approximate one-dimensional resistance network model for both single and multilayered heat sinks was also developed. Both numerical and resistance network models compared well with experiments

Passive, lightweight thermal solutions for Remote Radio Head (RRH) electronics

The ever-increasing size of wireless networks has led to the development of new base station architectures. Traditional telecommunication base stations have baseband and radio-frequency (RF) components mounted inside an air-conditioned hut with co-axial cables transmitting signals to remote antennas. The industry is moving to distributed network and remote radio head (RRH) architectures where the baseband components are digitally connected to a group of RF components mounted on top of antenna towers. Operators who maintain these units typically desire a lightweight, low-volume passive thermal solution. Thermal issues are challenging with power amplifier (PA) powers ranging from 100-200 Watts. Traditional base station heat sinks that cool high-powered PAs are made of a die-cast aluminum material having a low thermal conductivity. This paper introduces a unique two-phase heat spreader technology to improve the thermal performance of die-cast aluminum heat sinks used in RRH application. Experiments and CFD models were used to compare the thermal performance of baseline aluminum heat sinks to thermal solutions that included an embedded or directly-attach two-phase heat spreader. Results are presented that show a heat sink combined with a two-phase heat spreader solution can provide a 20% reduction in PA temperature or a 17% reduction in heat sink weight.

A modified effectiveness-NTU approach for analysis of low-aspect ratio mini-channel heat sinks using novel shape factor formulations

In the electronics cooling literature, the use of the overall thermal resistance to describe the thermal performance of cooling technologies, has hindered the adoption of more robust and powerful thermal performance metrics, most notably the heat exchanger effectiveness, epsiv , that arises in the theory of heat exchangers. This paper presents a theoretical treatment of the heat transfer in cold plates with low aspect ratio channels which leads to a modified form of the epsiv-NTU equation for single-sided heat exchangers. The heat transfer from the surface where heat is applied to the coolant channels is separated into the thermal impedance caused by conduction through the solid matrix, and that caused by the convection in the coolant channels. The heat conduction through the solid and into the surfaces of the channel is modeled using novel analytical shape factor formulations. This generalized conduction treatment leads to a modification of the traditional epsiv-NTU formulation that includes a generalized Biot number, defined in terms of the conduction shape factor. The new formulation robustly correlates recently reported experimental data from compact copper-water heat sinks used to cool small 1 cm times 1 cm heat sources.

Heat Transfer in Water-Cooled Silicon Carbide Milli-Channel Heat Sinks for High Power Electronic Applications

Heat transfer and fluid flow in a novel class of water-cooled milli-channel heat sinks are investigated. The heat sinks are manufactured using an extrusion freeform fabrication (EFF) rapid prototyping technology and a water-soluble polymer material. EFF permits the fabrication of geometrically complex, three-dimensional structures in non-traditional materials. Silicon carbide, SiC, is TEC-matched to silicon and is an ideal material for heat exchangers that will be mounted directly to heat dissipating electronic packages. This paper presents experimental results on the heat transfer and flow in small SiC heat exchangers with multiple rows of parallel channels oriented in the flow direction. Rectangular heat exchangers with 3.2 cm × 2.2 cm planform area and varying thickness, porosity, number of channels, and channel diameter were fabricated and tested. Overall heat transfer and pressure drop coefficients in single-phase flow regimes are presented and analyzed. The per channel Reynolds number places the friction coefficients in the developing to developed hydrodynamic regime, and showed excellent agreement with laminar theory. The overall heat transfer coefficients for a single row SiC heat exchanger compared favorably with a validation heat exchanger fabricated from copper, however the heat transfer coefficient in multiple row heat sinks did not agree well with the laminar theory.Copyright

Thermal Performance of Natural Graphite Heat Spreaders With Embedded Thermal Vias

Natural graphite heat spreaders are in use in electronic cooling applications where heat flux density is low. Natural graphite is an anisotropic material, with a high thermal conductivity in the plane of the spreader combined with a much lower thermal conductivity through its thickness. This low through-thickness thermal conductivity poses a problem when attempting to cool heat sources with relatively high heat flux densities. This problem can be overcome by embedding a thermal via in the graphite material. This via is made from an isotropic material with a thermal conductivity significantly higher than the through-thickness graphite conductivity. This paper examines the thermal performance of a natural graphite heat spreader with an embedded thermal via. The work is primarily experimental although numerical models were used to guide the experiments. The thermal performance of these spreaders is compared to that of spreaders made from conventional isotropic materials. The effect of accelerated aging tests on the performance of these graphite spreaders is reviewed. Finally, two applications are examined; first cooling an ASIC module and second, cooling an FB-DIMM memory card.Copyright

P-70: A New Chassis Design Concept for Plasma Display Panels

Plasma display panel (PDP) manufacturers are motivated to reduce costs to broaden consumer acceptance and to fend off competition from other flat panel and projection display technologies. This paper introduces a novel approach to designing the plasma display panel chassis that represents the lowest cost and lightest weight chassis system in the industry while significantly reducing the average screen temperature compared to conventional chassis designs. The new chassis design is enabled by the use of a natural graphite based thermal spreader sheet that renders redundant the thermal spreading function of the solid aluminum chassis.