Periodic cellular metals with honeycomb and corrugated topologies are widely used for the cores of light weight sandwich panel structures. Honeycombs have closed cell pores and are well suited for thermal protection while also providing efficient load support. Corrugated core structures provide less efficient and highly anisotropic load support, but enable cross flow heat exchange opportunities because their pores are continuous in one direction. Recent advances in topology design and fabrication have led to the emergence of lattice truss structures with open cell structures. These three classes of periodic cellular metals can now be fabricated from a wide variety of structural alloys. Many topologies are found to provide adequate stiffness and strength for structural load support when configured as the cores of sandwich panels. Sandwich panels with core relative densities of 2-10% and cell sizes in the millimetre range are being assessed for use as multifunctional structures. The open, three-dimensional interconnected pore networks of lattice truss topologies provide opportunities for simultaneously supporting high stresses while also enabling cross flow heat exchange. These highly compressible structures also provide opportunities for the mitigation of high intensity dynamic loads created by impacts and shock waves in air or water. By filling the voids with polymers and hard ceramics, these structures have also been found to offer significant resistance to penetration by projectiles.

Multifunctional periodic cellular metals

Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs)

Theoretical prediction of effective properties for multiphase material systems is very important not only to analysis and optimization of material performance, but also to new material designs. This review first examines the issues, difficulties and challenges in prediction of material behaviors by summarizing and critiquing the existing major analytical approaches dealing with material property modeling. The focus then shifts to some recent advances in numerical methodology that are able to predict more accurately and efficiently the effective physical properties of multiphase materials with complex internal microstructures. A random generation-growth algorithm is highlighted for reproducing multiphase microstructures, statistically equivalent to the actual systems, based on the geometrical and morphological information obtained from measurements and experimental estimations. Then a high-efficiency lattice Boltzmann solver for the corresponding governing equations is described which, while assuring energy conservation and the appropriate continuities at numerous interfaces in a complex system, has demonstrated its numerical power in yielding accurate solutions. Various applications are provided to validate the feasibility, effectiveness and robustness of this new methodology by comparing the predictions with existing experimental data from different transport processes, accounting for the effects due to component size, material anisotropy, internal morphology and multiphase interactions. The examples given also suggest even wider potential applicability of this methodology to other problems as long as they are governed by the similar partial differential equation(s). Thus, for given system composition and structure, this numerical methodology is in essence a model built on sound physics principles with prior validity, without resorting to ad hoc empirical treatment. Therefore, it is useful for design and optimization of new materials, beyond just predicting and analyzing the existing ones.

Predictions of effective physical properties of complex multiphase materials

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Principles Of Enhanced Heat Transfer

Preparation and thermal characterization of paraffin/metal foam composite phase change material

The paper presents numerical results on an innovative concept of an efficient, site-specific cooling technology for microprocessor hot spots using Alternating Current Electrothermal Flow (ACET). ACET deploys an electrokinetic transport mechanism without requiring any external prime mover and can be shown to be highly effective in reducing hot spot temperatures below their allowable limits. The proposed technique leverages the heat source(s) itself to drive the fluid in the ACET cooling mechanism, thereby making this a highly energy efficient active technique. Our parametric analyses present the optimal range of fluid properties, viz. electrical conductivity where the cooling efficiency is maximum. Further, the impact of geometrical parameters as well as input voltages has been characterized. Our results show a reduction of hot spot temperature by more than 30% for flow through a channel of 600 microns under appropriate conditions at an applied AC voltage of 10-12 V. Comparison with other micro-cooler technologies show the proposed technique to be more energy efficient in its range of operation. These results establish the potential of the ACET cooling and can open up a new avenue to be explored for thermal management of miniaturized devices and systems.

Alternating Current Electrothermal Flow for Energy Efficient Thermal Management of Microprocessor Hot Spots.

This work addresses the characterization of the subcritical electroconvection instability that occurs in two plane layers of two miscible dielectric liquids subjected to strong unipolar injection. It appears that electro-convection is a very efficient way to increase the mixing of two miscible liquids. It is shown that the Lacey index, which characterizes the level of mixing between the liquids, is directly impacted by the electrical Rayleigh number T. Linear and non-linear instability criteria, Tc and Tf, featuring a typical hysteresis loop, are determined. We have exhibited a very intriguing phenomenon in the time evolution of the maximum velocity Vmax in the domain. Above the linear criterion Tc, for given values of the Schmidt number Sc, one can observe the emergence of some ripples in the Vmax vs time, curve. This unexpected behavior is analyzed and an explanation in terms of modal dynamic is proposed.

Mass Transfer Enhancement Induced by Electro-Soluto Convection within Two Parallel Electrodes Due to Unipolar Charge Injection

In this paper, a novel design of centrifugal blower called Plated Blowers (PB) is proposed for cooling of laptop computers where the conventional rotor blade is replaced by a new design comprising of a sheet metal rotor with punched holes and residual hanging chads. A numerical study was conducted on these designs to determine the flow rates under operational speed of 6000 rpm. The results showed that the PB designs can outperform the traditional blade fan design as well as the Volumetric Resistance Blower (VRB) [1] in terms of air flow rate at iso-rpm conditions. Similar to VRB which has a porous rotor, the PBs are also expected to offer significant acoustic advantage over bladed fans (claim to be validated through tests), which can result in a boost in overall platform power in these mobile computing systems.

Numerical Modelling and Simulation of Plated Blowers for Cooling of Mobile Computing platforms

Chimney-based cooling method, apparatus and systen for computing devices

This paper investigates the effects of the presence of unheated and heated shrouds on the thermal performance of longitudinal finned heat sinks. A comprehensive numerical study was conducted to determine the impact of the shroud clearance from the tip of the fins and shroud heating. The first part of the study deals with the effects of an unheated shroud on finned heat sinks of different fin height, fin pitch and length in an attempt to cover a wide range of geometry. The numerical results reveal an optimum clearance for maximum heat transfer. For all heat sinks studied the unheated shroud improved the performance by as much as 15% until the shroud was very close when the performance decreased by as much as 10%. In the second part of the paper, the effects of heating of the shroud were considered. In these numerical runs, an isothermal boundary condition was imposed on the shroud. For the heating levels considered, it was found that heating of the shrouds can increase or lower the thermal performance of the heat sink depending on the heat sink geometry and shroud clearance. Finally, the numerical results also revealed a systematic dependence of the normalized Nusselt number on the Rayleigh number for a given heat sink geometry.Copyright © 2005 by ASME

Anandaroop Bhattacharya

Papers

Alternating Current Electrothermal Flow for Energy Efficient Thermal Management of Microprocessor Hot Spots.

Mass Transfer Enhancement Induced by Electro-Soluto Convection within Two Parallel Electrodes Due to Unipolar Charge Injection

Numerical Modelling and Simulation of Plated Blowers for Cooling of Mobile Computing platforms

Chimney-based cooling method, apparatus and systen for computing devices

Numerical Modeling of Buoyancy Induced Convection in Finned Heat Sinks in Presence of Unheated and Heated Shrouds