Hakan Erturk

Comparison of three regularized solution techniques in a three-dimensional inverse radiation problem

Inverse methods provide a good alternative to traditional trial-and-error methods for design of thermal systems. The inverse boundary condition estimation problem in radiating enclosures involves the solution of an ill-posed system that requires regularization to obtain a reasonable physical solution. This study compares three regularized solution techniques that can be used in the inverse boundary condition estimation problems in a three-dimensional radiating enclosure. The regularized solution techniques covered in this study are the conjugate gradient method, bi-conjugate gradient method and truncated singular value decomposition.

THERMAL PERFORMANCE AND KEY CHALLENGES FOR FUTURE CPU COOLING TECHNOLOGIES

Over the past few years, thermal design for cooling microprocessors has become increasingly challenging mainly because of an increase in both average power density and local power density, commonly referred to as “hot spots”. The current air cooling technologies present diminishing returns, thus it is strategically important for the microelectronics industry to establish the research and development focus for future non air-cooling technologies. This paper presents the thermal performance capability for enabling and package based cooling technologies using a range of “reasonable” boundary conditions. In the enabling area a few key main building blocks are considered: air cooling, high conductivity materials, liquid cooling (single and two-phase), thermoelectric modules integrated with heat pipes/vapor chambers, refrigeration based devices and the thermal interface materials performance. For package based technologies we present only the microchannel building block (cold plate in contact with the back-side of the die). It will be shown that as the hot spot density factor increases, package based cooling technologies should be considered for more significant cooling improvements. In addition to thermal performance, a summary of the key technical challenges are presented in the paper. This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.Copyright

Thermal Devices Integrated With Thermoelectric Modules With Applications to CPU Cooling

Over the past few years, the air cooling technology improvements present diminishing returns for microprocessors cooling applications. Presently most of the proposed future cooling technologies (i.e. pumped liquid cooling or vapor compressor refrigeration) may need some fluid moving device and a large remote heat exchanger which requires additional volume. Due to the complexity, reliability issues and space requirements it is preferred to extend the air cooling within the current form factors and using passive devices. This paper will show that optimized thermoelectric modules combined with two-phase (liquid/vapor) passive devices can further improve the cooling capability compared to conventional air cooling technologies at reasonable thermoelectric cooler (TEC) power consumption. Current computational fluid dynamics programs are not yet well equipped to find out the most optimized TEC geometry (for a given COP and given thermal requirements) in a reasonable amount of computation time. Therefore, two modeling steps are proposed: find out the preliminary TEC geometry using an ID analytical program (based on uniform heat flux and a given COP) and use it as an input to CFD programs (i.e. Icepak®) for detailed predictions. Using this model, we confirmed that the conventional TEC technology must use some spreading device to dissipate the CPU heat to the TEC cold side. Different spreading devices are considered: solid metal, heat pipe, vapor chambers and single/two phase pumped cooling. Their individual performance integrated with TEC will be presented. In addition, we propose that the TEC performance to be controlled as a function of instantaneous CPU power consumption, ambient temperature and other parameters. This controller offers extra flexibility which can be used for either noise reduction or TEC power reduction. However, such power cycling of the TEC may affect the TEC reliability. Power cycling accelerated test data (>500,000 accelerated cycles) have been performed together with the life predictions will be presented in the paper.Copyright

Efficient Signal Transport Model for Remote Thermometry in Full-Scale Thermal Processing Systems

The rapid thermal processing of semiconductor devices is very temperature sensitive and requires precise temperature measurement. Light pipe radiation thermometers are widely used for temperature control during manufacturing by industry, and there is concern about errors associated with light pipe measurements. Modeling in simplified systems has helped in understanding the signal transport process in light pipes and errors associated with measurements in the past. Considering the small sensor area compared to the size of the semiconductor wafer and the remaining system components, modeling of the complete system has not been done due to the computational demand. A reverse Monte Carlo model can be used efficiently to model the signal transported to the photodetector in conjunction with a thermal model of the system to better characterize the system. The proposed method is demonstrated in a full-scale instrumented system with a light pipe thermometer, and the results are compared against previously published measurements from the system.

Prediction of Thermal Conductivity and Shear Viscosity of Water-Cu Nanofluids Using Equilibrium Molecular Dynamics

Nanofluids are new class of fluids which can be used for many engineering applications due to their enhanced thermal properties. The macroscopic modeling tools used for flow simulations usually rely on effective thermal and rheological properties of the nanofluids that can be predicted through various effective medium theories. As these theories significantly under-predict, using correlations based on experimental data is considered as the only reliable means for prediction of these effective properties. However, the behavior might change significantly once the particle material or base fluid change due to different particle fluid interactions in the molecular level. One of the most promising means of modeling effective properties of the nanofluids is the molecular dynamics simulations where all the intermolecular effects can be modeled. This study investigates equilibrium molecular dynamics simulation of the water-Cu nanofluids to predict the thermal and rheological properties. The molecular dynamics simulation is carried out to achieve a thermodynamic equilibrium, based on a state that is defined by targeted thermodynamic properties of the system. The Green-Kubo method is used to predict the thermal conductivity and viscosity of the system. The study considers the use of different combining rules such as Lorentz-Berthelot and sixth-power rules for defining the inter-atomic potentials for water modeled by SPC/E and nanoparticles modeled by Lennard-Jones potential. The predicted effective properties that are thermal conductivity and shear viscosity are then compared with experimental data from literature. The predicted transport properties at different temperatures and particle concentrations are compared to experimental data from literature for model validation.Copyright

Nanolayering around and thermal resistivity of the water-hexagonal boron nitride interface

The water-hexagonal boron nitride interface was investigated by molecular dynamics simulations. Since the properties of the interface change significantly with the interatomic potential, a new method for calibrating the solid-liquid interatomic potential is proposed based on the experimental energy of the interface. The result is markedly different from that given by Lorentz-Berthelot mixing for the Lennard-Jones parameters commonly used in the literature. Specifically, the extent of nanolayering and interfacial thermal resistivity is measured for several interatomic potentials, and the one calibrated by the proposed method gives the least thermal resistivity.

A new interlayer potential for hexagonal boron nitride

A new interlayer potential is developed for interlayer interactions of hexagonal boron nitride sheets, and its performance is compared with other potentials in the literature using molecular dynamics simulations. The proposed potential contains Coulombic and Lennard-Jones 6-12 terms, and is calibrated with recent experimental data including the hexagonal boron nitride interlayer distance and elastic constants. The potentials are evaluated by comparing the experimental and simulated values of interlayer distance, density, elastic constants, and thermal conductivity using non-equilibrium molecular dynamics. The proposed potential is found to be in reasonable agreement with experiments, and improves on earlier potentials in several respects. Simulated thermal conductivity values as a function of the number of layers and of temperature suggest that the proposed LJ 6-12 potential has the ability to predict some phonon behaviour during heat transport in the out-of-plane direction.

Synthesis and Experimental Investigation of Rheological Behavior of EG and Water Based hBN Nanofluids

Hexagonal boron nitride (hBN) is a ceramic material with high thermal conductivity and superior chemical stability that makes it a suitable candidate for nanofluid synthesis. The studies that focus on hBN nanofluids are mostly limited to hBN-oil dielectric nanofluids. However, using hBN with other base fluids has also significant potential applications. This study focuses on stability and rheological behavior of hBN-water and hBN-ethylene glycol nanofluids that have not been investigated in detail. Nanofluids with hBN nanoparticles of an effective diameter of 70 nm are synthesized using the two-step method. The main problem in this method is sedimentation as a result of disproportionate amount of clustering of nanoparticles within the nanofluids. In the preparation procedure of nanofluids, sonication and pH level control are used. The stability of the hBN-water nanofluids is determined either by quantitative methods such as zeta potential measurements and/or effective particle size measurements via dynamic light scattering, or qualitative methods such as SEM (Scanning Electron Microscopy) and STEM (Scanning Transient Electron Microscopy). Following the investigation of the effect of several parameters, such as sonication time, pH level of the suspension on the stability of the nanofluids, rheological behavior is investigated experimentally. Rheology experiments are conducted with a cone–plate rheometer. In these experiments different volume concentrated (0.5–1%) nanofluids are considered. Validation studies for preparation and characterization methods are carried out with alumina (Al2O3), nanofluids which are synthesized by dispersing γ-Al2O3 nanoparticles of 15 nm into deionized water.© 2013 ASME

Assessment of Single and Two-Phase Models for Nanofluid Flow at the Entrance Region of a Uniformly Heated Tube

Hydrodynamic and thermal characteristics of Al2O3 – water nanofluid flow at entry region of a uniformly heated pipe are studied applying finite control volume method (FCV). Single phase and Eulerian-Eulerian two-phase models were used in modelling of nanofluid flow and heat transfer. The two methods are evaluated by comparing predicted convective heat transfer coefficients and friction factor with experimental results from literature. Solutions with two different velocity pressure coupling algorithms, Full Multiphase Coupled, and Phase Coupled Semi-Implicit Method for Pressure Linked Equations are also compared in terms of accuracy and computational cost. Two-phase model predicts convective heat transfer coefficient and friction factor more accurately at the entry region. Moreover, computational cost can be reduced by implementing Full Multiphase Coupled scheme.Copyright

Characterization of Electronic Packages by Thermal Diffusion Tomography

One of the most important functions of an electronic package is thermal management, as package is responsible from removing the heat generated by the transistors to ensure reliability. The quality of the package is very important for proper thermal management and it is important to have minimal flaws that increase thermal resistance of the package. Therefore, detection of flaws in the multi-layered package is critical during the assembly process development to monitor the package quality. This is achieved by techniques such as computerized tomography (CT) using x-rays, or scanning acoustic microscopy (SAM), all of which require very expensive equipment and significant processing time. Thermal diffusion tomography (TDT) can be used for detecting the flaws as a lower cost alternative to these imaging techniques. The feasibility of TDT as a fault detection technique for electronic packages with IR thermometry is considered in the current study. Two reconstruction algorithms considered; an iterative perturbation approach and Levenberg-Marquard method were found to be capable of detecting the flaws in the thermal interface layer.Copyright