The paper uses a two-dimensional heat conduction model to obtain the thermal performance and entropy generation in an orthotropic pin fin used in advanced light weight heat sinks. The analytical expressions for the distribution and heat transfer rates are used to generate the thermal performance graphs that are of general applicability unlike the results for specific cases reported so far. The same information is subsequently used to generate graphs of local and total volumetric entropy generation rates in the fin. The results clearly delineate the effect of radial Biot number, fin aspect ratio, and the ratio of radial to axial thermal conductivities on temperature distribution, heat transfer rate, and local and total entropy generation rates. A procedure is discussed to illustrate the simultaneous realisation of the least material and minimum entropy generation pin fin designs.

Heat transfer and entropy generation in a two-dimensional orthotropic convection pin fin

Thermal management is a critical and essential component of maintaining high efficiency and reliability of electronic components. Adequate cooling must be provided while meeting the weight and volumetric requirements, especially for passive air-cooling solutions in avionics applications where space and weight are at a premium. It should be noted that there are electronic systems where thermal solution contributes 95% or more to the total weight of the system. With recent advancements in material science and increase in demand for more aggressive cooling solutions, it has become critical to invent, manufacture, characterize, and implement advanced thermal materials for the design of compact and low weight systems. However, because of the complexity due to anisotropic properties of these materials, their thermal performances and reliability are yet to be fully understood. The present paper aims at characterization of advanced thermal materials in passive air-cooling applications. A combined computational and experimental investigation is performed on various advanced heat sink materials and manufactured prototypes. Results are compared with the traditionally used aluminum heat sink and showed that significant weight savings can potentially be achieved. A composite figure of merit is proposed for measuring mass effective cooling capabilities of heat sinks. It is defined as the cooling performance per unit mass of heat sink and shown that advanced materials can perform as much as ten times better than conventional aluminum heat sinks.

Light Weight High Performance Thermal Management With Advanced Heat Sinks and Extended Surfaces

Seamless advancements in the electronics industry lead to high heat fluxes from very limited thermal real estates. Use of natural convection air cooling is of interest to meet some of the low flux cooling needs, while active cooling techniques via liquid or forced convection are the methods of choice. In natural convection heat transfer applications, the components used for cooling may represent a significant portion of the overall weight of the system. Consequently, advanced materials are of interest in such applications, as they may substantially reduce the total size and weight of the system. Many of these advanced materials have anisotropic thermophysical properties, hence the control of thermal conductivity is crucial. This paper is motivated to address the lack of understanding of the use of anisotropic advanced materials in natural convection environments. Numerical simulations are carried out to test the performance of heat sinks made of such materials and comparisons are made with the heat sinks of traditional engineering materials under the same conditions. The results demonstrate that the total weight of the system may be reduced drastically with the use of advanced materials relative to the most commonly used heat sink materials at the same thermal performance. A figure of merit (FOM) is proposed to compare the thermal performance of different heat sinks. Total resistance, conduction and convection resistances, and performance-related FOM values for each heat sink are presented. It is shown that the conduction thermal resistance is dominant at lower fin thicknesses for sparse heat sinks while it is negligible for dense heat sinks. Pyrolytic graphite-based heat sinks demonstrate the best thermal performance, while carbon-foam heat sinks produce the highest FOM values due to the material's low density.

On the Conduction and Convection Heat Transfer From Lightweight Advanced Heat Sinks

Heat sinks constructed from natural graphite/epoxy composites are a high performance, lightweight alternative to traditional copper and aluminum designs. For applications where the heat source is the same size as the heat sink, laminated graphite/epoxy heat sinks have been shown to perform equivalently to copper heat sinks at a 75% lower weight. However, when the heat source is smaller than the heat sink, laminated graphite/epoxy heat sinks perform poorly. For this situation, a hybrid graphite epoxy heat sink has been developed. This hybrid construction incorporates graphite/epoxy fins bonded to a copper base. This paper examines the thermal performance of this heat sink and includes both computer models and experimental results. The effects of fin material and bonding techniques on thermal performance were examined. An optimized hybrid heatsink was found to have a thermal performance nearly equivalent to that of a copper heat sink combined with a 40% reduction in weight.

Comparative thermal performance evaluation of graphite/epoxy fin heat sinks

In this thesis, a multi-disciplinary investigation of using natural graphite sheet (NGS) for heat sink applications is presented with focus on thermal performance, electromagnetic performance, reliability, cost, energy efficiency, and environmental impact. NGS heat sinks are a promising alternative for weight-sensitive applications in which the heat sink is protected by a case. Contrary to the conventional metals, NGS is also predicted to be feasible at high temperatures or in corrosive environments. To provide the basis for the heat sink design, the thermal conductivity, thermal diffusivity, electrical conductivity, thermal emissivity, coefficient of thermal expansion, and compression behavior are measured and reported in an easy-to-use form. It is shown experimentally that the the thermal contact resistance at metal-NGS interfaces is comparable to metal-metal ones with thermal interface materials, and that the poor through–plane thermal conductivity can be mitigated by embedding heat pipes in NGS heat sinks. The conducted common-mode electromagnetic emissions cannot be reduced by using NGS heat sinks, but potential to reduce the radiated emission by 12 to 97 % was identified. Complex implications on reliability arising from replacing conventional metal heat sinks with NGS ones are discussed. The cost of NGS heat sinks produced in high volumes is predicted to be a double that of mass-produced conventional aluminum ones. The environmental impact of production, manufacturing, and end-of-life management of NGS is reviewed and compared to the conventional heat sink materials. A case-specific approach to evaluating the feasibility of using NGS heat sinks is recommended and the major steps are outlined. The technology is considered to be ready for a transfer to the industrial research and development stage. An audiovisual summary of the work is available at https://www.youtube.com/playlist?list=PLaX55SIXaD20NQQ2JLP-7abmET7l-6LS4.

Natural graphite sheet heat sinks for power electronics

Recent developments in natural graphite composites by GrafTech International, Ltd. have led to novel highly conductive and lightweight materials with thermally non-isotropic structures [1]. The in-plane and out-of-plane thermal conductivities for such materials differ by a factor of up to 50. The non-isotropy in thermal conductivities is found to influence heat dissipation in different ways. This paper investigates the influence of thermal conductivity on the thermal performance of graphite heat sinks. In addition to addressing technology challenges encountered in design and optimization of heat sinks, this paper also discusses the efficient ways to utilize graphite composite materials, which can be expected to play an important role in future electronics cooling applications.© 2003 ASME

Joe Capp

Papers

Optimum Design of Heat Sinks Using Non-Isotropic Graphite Composites