Tunc Icoz

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

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.

Fluid-Structure Interaction Model for Low-Frequency Synthetic Jets

Advancements in microelectronics have increased circuit board heat densities to the point where active cooling is required. Synthetic jets offer interesting capabilities for localized active cooling of electronics due to their compact size, low cost, and substantial cooling effectiveness. The design of synthetic jets for specific applications requires practical design tools that capture the strong fluid-structure interaction without computationally long run times. There is particular interest in synthetic jets that have a low operating frequency to reduce noise levels. This paper describes how common finite elements and codes can be used to calculate parameters for a synthetic jet fluid- structure interaction model that only requires a limited number of degrees of freedom and is solved using a direct approach for low-frequency synthetic jets. Extensive tests are performed with the synthetic jet in vacuum to measure deflection, in ambient air to measure pressure and exit velocity, and impinging on a heated surface to measure heat transfer enhancement. The test results are compared with the fluid-structure interaction model results for validation, and agreement is found to be good in the frequency range of interest from 200 to 500 Hz.

Evaluation on Use of Thermoelectric Devices for Electronics Cooling

As Thermoelectric devices are getting cheaper and more powerful in cooling, these devices are getting more popular for electronics cooling applications. However, due to the additional heat production inside the thermoelectric device, the application of a TE-cooler might not always be appropriate. In some applications, use of thermoelectric devices or coolers might cause higher temperature rises on heat generating electronics than conventional cooling solutions. To authors’ best knowledge, there exists no literature that studies whether thermoelectric cooling is better than traditional convective cooling without Thermoelectrics. This study aims to evaluate the performance and effectiveness of thermoelectric cooling for electronics. Two figures of merit are proposed, to compare performances of conventional and thermoelectric cooling techniques. An attempt is made to derive this figure of merit analytically with assumptions reflecting common electronics applications. Selective case studies will be presented based on constant heat flux and constant temperature difference.Copyright

Characterization of Light Weight Heat Sink Materials for Thermal Management of Electronics

Thermal management of electronics is a critical part of maintaining high efficiency and reliability. Adequate cooling must be balanced with weight and volumetric requirements, especially for passive air-cooling solutions in electronics applications where space and weight are at a premium. It should be noted that there are systems where thermal solution takes more than 95% of the total weight of the system. Therefore, it is necessary to investigate and utilize advanced materials to design low weight and compact systems. Many of the advanced materials have anisotropic thermal properties and their performances depend strongly on taking advantage of superior properties in the desired directions. Therefore, control of thermal conductivity plays an important role in utilization of such materials for cooling applications. Because of the complexity introduced by anisotropic properties, thermal performances of advanced materials are yet to be fully understood. Present study is an experimental and computational study on characterization of thermal performances of advanced materials for heat sink applications. Numerical simulations and experiments are performed to characterize thermal performances of four different materials. An estimated weight savings in excess of 75% with lightweight materials are observed compared to the traditionally used heat sinks.Copyright

Effect of 3D Diffusion Over Vertical Thin Rectangular Geometries in Natural Convection Heat Transfer

Natural convection over vertical plates is a very well known problem in heat transfer. There are many available correlations to predict Nusselt numbers for a wide range of Rayleigh numbers. These benchmark studies on natural convection for vertical plates were conducted on rather large surfaces leading to Rayleigh numbers in the range of 0.1 to 109 . In natural convection the sole driving force of fluid motion is the change in fluid density, when the diffusive limit is small compared to convective heat transfer. However, conduction to air, as well as air entrainment from sides also contributes to the heat removal from heater surfaces. An experimental study has been carried out with small and large heaters compared to published data for 2×103 <Ra<4×107 . Square surfaces of 12.5 and 25.4 mm, and rectangular heaters of sizes 25.4×101.6 and 25.4×203.2 mm were tested for a range of heat inputs such that the surface temperatures are controlled between 30 °C and 80 °C. It is found that published correlations underpredict the Nusselt numbers as much as 20%. It is observed that widely known correlations underpredict the experimental values since the 3D conduction and side air drifts on heat transfer are not accounted for in these correlations. However, the cuboid model which includes the 3D diffusion term showed much better agreement with the experimental results.© 2006 ASME

Low Frequency Piezoelectric Synthetic Jets

Active cooling is often required for circuit boards with high heat generation densities. Synthetic jets driven with piezoelectric actuators offer interesting capabilities for localized active cooling of electronics due to their compact size, low cost and substantial cooling effectiveness. The design of synthetic jets for specific applications requires practical design tools that capture the strong fluid structure interaction without long run times. There is particular interest in synthetic jets that have a low operating frequency to reduce noise levels. This paper describes how common finite element (FE) and computational fluid dynamics (CFD) codes can be used to calculate parameters for a synthetic jet fluid structure interaction (FSI) model that only requires a limited number of degrees of freedom and is solved using a direct approach for low frequency synthetic jets. Tests are performed based on impinging on a heated surface to measure heat transfer enhancement. The test results are compared to the FSI model results for validation and agreement is found to be good in the frequency range of interest from 200 to 500 Hz.Copyright