bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of Control Strategy Development Using an Integrated Model of a SOFC Based APU Under Transient Conditions
Authors: Rancruel Diego , von Spakovsky Michael
Published: April 2008

A typical approach to the synthesis/design optimization of energy systems is to only use steady state operation and high efficiency (or low total life cycle cost) at full load as the basis for the synthesis/design. Transient operation as reflected by changes in power demand, shut-down, and start-up are left as secondary tasks to be solved by system and control engineers once the synthesis/design is fixed. However, start-up and shut-down may be events that happen quite often and, thus, may be quite important in the creative process of developing the system. This is especially true for small power units used in transportation applications or for domestic energy supplies, where the load demand changes frequently and peaks in load of short duration are common. The duration of start-up is, of course, a major factor which must be considered since rapid system response is an important factor in determining the feasibility of solid oxide fuel cell (SOFC) based auxiliary power units (APUs). Start-up and shut-down may also significantly affect the life span of the system due to thermal stresses on all system components. Therefore, a proper balance must be struck between a fast response and the costs of owning and operating the system so that start-up or any other transient process can be accomplished in as short a time as possible yet with a minimum in fuel consumption. In this research work we have been studying the effects of control laws and strategies and transients on system performance. The results presented in this paper are based on a set of transient models developed and implemented for the components of a 5 kW net power SOFC based APU and for the high-fidelity system which results from their integration. The simulation results given below are for two different start-up approaches: one with steam recirculation and component preheating and the second without either. These start-up simulations were performed for fixed values of a number of system-level parameters (e.g., fuel utilization, steam to methane ratio, etc.) and were used to generate sufficient information to permit the development of appropriate control strategies for this critical operating point. These strategies are based on a balance between fuel consumption and response time. In addition, energy buffering hardware was added to the system configuration in order to minimize the effect of transients on fuel cell stack performance and lifetime.
3-D Modeling, Smoothing, and Finite Element Modeling of the Femur
Authors: Schonning A. , Ionescu I. , Oommen B. , Conway T.
Published: April 2008

Computed Tomography data of a male was obtained from the National Institute of Health. The data was used to create three-dimensional models of the femur. These models were subject to data smoothing to prepare them for finite element meshing. The meshing was performed using a highly controlled mesh of hexahedral elements. Initial analysis of the femur model has been has been performed to verify its reliability.
Quasi-Steady Analysis of a Magnetorheological Dashpot Damper
Authors: Choi Young-Tai , Wereley Norman M.
Published: April 2008

This paper addresses quasi-steady analysis of a magnetorheological (MR) dashpot damper. MR dashpot dampers show mixed fluid mode of flow and shear flows since a dashpot inside dampers works as a piston and a moving wall simultaneously. In this study, quasi-steady analysis of MR dashpot dampers has developed based on the utilization of the Bingham-plastic constitutive model to assess performance metrics such as damping capacity. For the mixed mode MR damper that is the sums of flow and shear flows, fluid velocity profile, shear stress profile, and damping coefficient are theoretically derived. In addition, the preyield thickness equation to characterize the relationship between the Bingham number and the preyield thickness is constructed. Through computer simulation, damping characteristics of the mixed mode MR dashpot damper are evaluated and compared with flow mode case.
Pressure Exchange in Multidimensions
Authors: Zhang Hongfang. , Garris Charles A.
Published: April 2008

Direct flow induction devices based on pressure exchange are highly efficient and very compact because of the effective use of the non-steady flow and the reversible pressure work on the fluid-fluid interfaces. These technical advantages will broaden their practical applications to such areas as air conditioning, water desalinization and gas turbine cycles etc. The current paper will discuss in detail the concept of pressure exchange. The physical mechanism of pressure exchange is explored by studying a one dimensional shock tube and a simple two-dimensional example by use of the computational analysis. A novel three-dimensional pressure exchange ejector invented by Garris will be introduced.
Development of a Fourth Generation Energy Recovery Device
Authors: Stover Richard L.
Published: April 2008

Energy Recovery, Inc’s Big Rotor Pressure Exchanger is the result of over 100 years of technology development. Work exchanger devices developed for SWRO plants are considered by many to be the most important breakthrough in desalination in the last 10 years. What has made this breakthrough possible and how have these devices affected the job of designing SWRO systems? ERI Chief Technical Officer Dr. Richard Stover presents the challenges and solutions of the design and deployment of the PX. PX performance data and engineering design considerations from operating SWRO-PX process configurations and PX arrays will be provided. Recent technical advances and future products will be discussed.
Computation of Turbulent Free Convection in Oblique Porous Enclosures Using a Macroscopic Two-Equation Model
Authors: Braga Edimilson J. , de Lemos Marcelo J. S.
Published: April 2008

Computations for turbulent natural convection within an inclined cavity totally filled with a fluid saturated porous medium are presented. The finite volume method in a generalized coordinate system is applied. The inclined walls are maintained at constant but different temperatures, while the horizontal walls are kept insulated. Governing equations are written in terms of primitive variables and are recast into a general form, Flow and heat transfer characteristics, (streamlines, isotherms and average Nusselt number), are investigated for a wide range of values of Rayleigh number and inclined angle. The turbulent model used is the standard k-ε model with a wall function. In this work, the turbulence model is first switched off and the laminar branch of the solution is found. Subsequently, the turbulence model is included so that the solution merges to the laminar branch for a reducing Ram. This convergence of results as Ram decreases can be seen as an estimate of the so-called relaminarization phenomenon. Present solutions are compared with published results and the influence of the inclination angle on Racr is analyzed. For Ram greater than around 104, both laminar and turbulent flow solutions deviate, indicating that such critical value for Ram was reached.
Flow and Heat Transfer Past a Sudden Contraction With a Porous Insert Using Linear and Non-Linear Turbulence Models
Authors: Assato Marcelo , de Lemos Marcelo J. S.
Published: April 2008

This work presents a numerical investigation for the turbulent flow and heat transfer in an abrupt contraction channel with a porous material placed in a flow passage. The channel has a contraction rate of 3:2. Results for the hybrid medium were obtained using linear and non-linear k-ε macroscopic models. It was used an inlet Reynolds number of Re = 132000 based on the height of the step. Parameters such as porosity, permeability and thickness of the porous insert were varied in order to analyze their effects on the flow pattern. The results of local heat transfer, friction coefficient and stream lines obtained by the two turbulence models were compared for the cases without and with porous insertion of thickness a/H=0.083, 0.166 and 0.250, where H is the step height. Insert porosity of varied between 0.85 and 0.95 with permeability in the range 10−6–10−2 m2.
Cutting Mechanisms and Their Influence on Dynamic Forces, Vibrations and Stability in Micro-Endmilling
Authors: Liu Xinyu , Jun Martin B. G. , DeVor Richard E. , Kapoor Shiv G.
Published: April 2008

A dynamic cutting force and vibration model of the micro-endmilling process that accounts for the dynamics of the micro-endmill, influences of the stable built-up-edge, and the effects of minimum chip thickness, elastic recovery, and the elastic-plastic nature in ploughing/rubbing has been developed. Experimental validation has been performed, and the model is shown to predict the cutting force and tool vibration within an average of 12%. Using the model developed, effects of the minimum chip thickness and elastic recovery on the cutting forces and vibrations as well as process stability of the micro-endmilling process have been examined. The results indicate that the large edge radius relative to the feedrate causes the process stability to be sensitive to feedrate, resulting in the low feedrate instability phenomenon. The elastic recovery significantly increases the peak-to-valley cutting forces and enlarges the unstable feedrate range.
Identification of Nonlinear Systems by Haar Wavelet
Authors: Chen Shy-Leh , Ho Keng-Chu
Published: April 2008

This study addresses the identification of autonomous nonlinear systems. It is assumed that the function form in the nonlinear system is known, leaving some unknown parameters to be estimated. It is also assumed that the free responses of the system can be measured. Since Haar wavelets can form a complete orthogonal basis for the appropriate function space, they are used to expand all signals. In doing so, the state equation can be transformed into a set of algebraic equations in unknown parameters. The technique of Kronecker product is utilized to simplify the expressions of the associated algebraic equations. Together with the least square method, the unknown system parameters are estimated. Several simulation examples verify the analysis.
Very Deep X-Ray Lithography for Heat and Mass Transfer Applications
Authors: Kelly Kevin W. , Becnell Charles , Desta Yohannes
Published: April 2008

Heat and mass transfer devices are being fabricated by International Mezzo Technologies that utilize micro features with increasingly aggressive combinations of both feature height and feature aspect ratio. Improvements in x-ray lithography using SU-8 now make it possible to lithographically define densely packed arrays of features with heights of 3 mm and with aspect ratios of around 25. These arrays potentially serve as the starting point for increasingly aggressive LIGA-based micro machining of heat exchangers, regenerators, recuperators, etc. that have superior performance due to length scale advantage.
In-Situ Monitoring of Phase Transformations During Welding of Steels Using Synchrotron-Based X-Ray Diffraction Techniques
Authors: Elmer J. W. , Palmer T. A.
Published: April 2008

Understanding the evolution of microstructure in welds is an important goal of welding research because of the strong correlation between weld microstructure and weld properties. To achieve this goal it is important to develop a quantitative measure of phase transformations encountered during welding in order to ultimately develop methods for predicting weld microstructures from the characteristics of the welding process. To aid in this effort, synchrotron radiation methods have been developed at Lawrence Livermore National Laboratory for direct observations of microstructure evolution during welding. Using intense, highly collimated synchrotron radiation, the atomic structure of the weld heat affected and fusion zones can be probed in real time. Two synchrotron-based techniques have been developed for these investigations, known as spatially resolved (SRXRD) and time resolved (TRXRD) x-ray diffraction, and these techniques have been used to investigate welding induced phase transformations in titanium alloys, low alloy steels, and stainless steel alloys. This paper will provide a brief overview of the application of these methods to understand microstructural evolution during the welding of low carbon (AISI 1005) and medium carbon (AISI 1045) steels, where the different levels of carbon influence the starting microstructures and the evolution of microstructures during welding.
Spatially Resolved Energy Dispersive X-Ray Diffraction (EDXRD) as a Tool for Nondestructively Providing Phase Composition Depth Profiles on Cement and Other Materials
Authors: Wilkinson Angus P. , Jupe Andrew C. , Kurtis Kimberly E. , Naik Nikhila N. , Stock Stuart R. , Lee Peter L.
Published: April 2008

The presence of sulfates in water or soils surrounding portland cement concrete structures leads to progressive degradation. Spatially resolved energy dispersive diffraction (EDXRD) in combination with computed microtomography (μCT) and mechanical measurements can provide the information needed to understand, in detail, the degradation mechanisms that are associated with sulfate attack and to validate accelerated test methods used to evaluate the sulfate resistance of cements. Highly penetrating, high-energy X-rays from synchrotron sources allow the use of EDXRD to nondestructively determine depth profiles for the crystalline phases in the cement paste specimens several millimeters below the sample surface. These depth profiles, and how they vary with sulfate exposure conditions and duration, can be correlated with mechanical changes and the crack patterns seen in the microtomographs. Spatially resolved EDXRD is in principle useful for phase composition mapping and depth profiling in a wide range of materials where the attenuation of high energy x-rays is not extreme. Suitable materials include many ceramic compositions.
Scanning X-Ray Microdiffraction for Materials Science at the Advanced Light Source
Authors: Tamura N. , Valek B. C. , MacDowell A. A. , Celestre R. S. , Padmore H. A. , Spolenak R. , Patel J. R.
Published: April 2008

With the advent of high brilliance synchrotron sources and outstanding progress made in X-ray focusing optics, intense sub-micron X-ray beams are now routinely produced at several synchrotron facilities around the world. At the Advances Light Source, a dedicated Scanning X-ray Microdiffraction beamline using either white or monochromatic X-ray focused beam has been developed and is used for mapping grain orientation, strain/stress or crystalline species distribution in various samples with micron to submicron spatial resolution. It also allows for the study of local plasticity as well as for the characterization or identification of new crystalline structures. The facility and its capabilities are described through the study of the electromigration phenomenon in Al(Cu) and Cu interconnect test structures.
X-Ray Microtomography Applications to Heterogeneous Materials
Authors: Landis Eric N.
Published: April 2008

Microtomography is a powerful tool that allows us to image materials at close to micron resolution. In the case of heterogeneous materials such as wood-based and cement-based composite materials were microstructure-property relationships are extremely difficult to quantify, microtomography enables us to make quantitative assessments of internal structure. Through 3D image analysis we can characterize a variety of parameters that affect a range of physical and mechanical properties. This paper provides an overview of synchrotron-based and laboratory-based microtomography applications to heterogeneous materials, along with the 3D image processing required to extract quantitative information.
Investigation of Thermal Properties in Thin Films With the Transient Thermo-Reflectance Technique
Authors: Hopkins Patrick , Norris Pamela
Published: April 2008

The reliability of micro-electronic devices depends on the thermal properties and energy transfer of the thin metallic films used in them. Within the thin film structures thermal transport properties often differ from bulk material and can be highly dependent on the manufacturing techniques. Thermal transport properties can be measured by the Transient ThermoReflectance technique (TTR). The TTR method uses an ultra-short laser pulse to generate a transient thermal response, and a weaker probe pulse to monitor the reflectivity response of the surface. The changes in reflectivity can be related to transient thermal effects in the film. With the use of an ultra short-pulsed laser, the extremely small time scale required to observe the microscale heat transfer phenomena is possible. The goal of this research is to observe and quantify the effect of differing substrates, film thicknesses, and deposition methods on thermal conductivity. This study uses the TTR technique to examine nickel films of varying thicknesses, fabricated on silicon, germanium, and gallium arsenide substrates, using electron beam evaporation and sputtering techniques. The thermal conductivities of the films were measured and compared.
Theoretical Modeling for Electroactive Polymer-Ceramic Hybrid Actuation Systems
Authors: Xu Tian-Bing , Su Ji
Published: April 2008

An electroactive polymer-ceramic hybrid actuation system (HYBAS) was recently developed. The HYBAS demonstrates significantly-enhanced electromechanical performance by utilizing advantages of cooperative contributions of the electromechanical responses of an electrostrictive copolymer and an electroactive single crystal. The hybrid actuation system provides not only a new type of device but also a concept to utilize different electroactive materials in a cooperative and efficient method for optimized electromechanical performance. In order to develop an effective procedure to optimize the performance of a hybrid actuation system (HYBAS), a theoretical model has been developed, based on the elastic and electromechanical properties of the materials utilized in the system and on the configuration of the device. The model also evaluates performance optimization as a function of geometric parameters, including the length of the HYBAS and the thickness ratios of the constituent components. The comparison between the model and the experimental results shows a good agreement and validates the model as an effective method for the further development of high performance actuating devices or systems for various applications.
Effects of Dwell-Time on the Thermal-Fatigue Life of SnAgCu Lead-Free Solder Joints
Authors: Lau John , Dauksher Walter
Published: April 2008

The effects of temperature-cycling dwell-time on the thermal-fatigue life of lead-free solder joints are investigated in this study. Emphasis is placed on the determination of creep responses and the creep strain energy density per cycle of a PBGA (plastic ball grid array) package’s lead-free solder joints subjected to various dwell times (namely, 15, 60, and 480 minutes) at peak cycling temperatures (0°C and 100°C).
Thermal Fatigue-Life Prediction of Lead-Free Solder Joints
Authors: Lau John , Lee Ricky , Shangguan Dongkai
Published: April 2008

Reliability of lead-free solder joints is investigated. Emphasis is placed on the design for reliability (DFR) of lead-free solder joints. In particular, the thermal-fatigue life of the lead-free solder joints of a plastic ball grid array (PBGA) package assembly is predicted and discussed.
Effect of Drop Impact on the Leakage of an Insulin Pump
Authors: Liu Sheng , Ma Bin , Zhang Honghai , Gan Zhiyin , Zhou Mingcai , Wang Xiaojun
Published: April 2008

Drop impact to portable medical electronics devices products may cause significant functional and physical damage. It is well known that insulin pumps are susceptible to drop impact which can cause various damage modes such as blending air in insulin liquid, interface breakage, battery separation, possible debonding between syringe and pipe, cracking on the LCD. This paper for the first time examines the insulin pump drop impact orientation and drop heights. A method by actual drop test using a sample pump as an example is proposed. Of interest is the measurement of the level of shock and the impact orientation experienced by the electronic components in the insulin pump during impact. The drop impact responses examined are the impact force and the strains and level of shock induced at the pump’s interfaces. A better understanding of the shock induced at the electronic components and packages in the products can assist manufacturers not only in designing better components and electronic package but also products which are more robust and reliable to handle shock and impact loading. Permanent deformation, stress, strain on the critical parts are examined and the effect on the possible leakage are examined.
Tool Selection and Path Control for Automated Posterior Teeth Coronal Canal Treatment Preparation
Authors: Dong J. , Hong S. Y. , Hasselgren G.
Published: April 2008

This paper represents a part of research plan of “Advanced Endodontic Technology Development.” In order to aid endodontic treatment a 3-D computer model of root canals has been created which shows the geometrical characteristics. The extent of work needed for root canal treatment is obtained from this 3-D model. The objective of this paper is to convert the geometrical characteristics into automatic treatment procedure planning. This computer-aided process planning for endodontic treatment determines tool selection and process method. It also calculates tool path and optimum tool movement distance. The output of this planning system is a numerical controlled program. Because of paper size limitation, only tool selection and path control during coronal canal treatment preparation for posterior teeth are discussed in the paper. The computer-aided treatment procedure planning system provides transformation from a 3-D canal model to a machine-controlled program that will yield a treated root canal ready for filling. It serves as a bridge between design (3-D canal model) and manufacturing (canal treatment). Unlike conventional methods for root canal treatment, the computer-aided treatment process planning system emphasizes a non-destructive internal tooth geometry examination and less invasive access preparation.
Flow Control Using Active Dimples
Authors: Udovidchik Natalie , Lambert Sebastien
Published: April 2008

In this study, a novel concept of using active dimples for flow control is introduced. It is widely known that dimples on a golf ball dramatically reduce its aerodynamic drag. They are much more effective than surface roughness since the hollow spherical shape produces cavity flow, thus the drag coefficient remains relatively constant at higher Reynolds numbers. It has also been shown by previous studies that by use of circular-arc grooves or dimples, the separation point on a cylinder could be regulated and drag reduced due to the re-circulation occurring in the dimpled surface. Another approach to flow separation that uses the concept of momentum-flux change by near-wall manipulation is an active one, such as synthetic jets or acoustical excitation. The long-term goal of this study is to merge these two approaches and create a continuous smart surface that would have active depressions, which would then be actuated at desired frequencies and conform to a desired shape for optimal results. Current investigation had only touched the tip of the iceberg of this new and unexplored field. In order to begin to comprehend the complexity of the fluid mechanics of the active dimples, a dual focus had been outlined in this study. The first focused on the investigation of a single active dimple on a flat plate, while the latter investigated the effect of a row consisting of such devices on a circular cylinder. The main factors of interest are optimal actuation frequency and dimple positioning relative to the freestream.
X-Rays for Fluid Flow Inspection
Authors: Chambellan D. , Gal O. , Legoupil S. , Vabre A.
Published: April 2008

X-rays techniques are widely used in the non-destructive evaluation field for mechanical inspection. However, development of new x-ray detectors and sources over the last decade has let to an intensive use of this technique in other fields. In this paper, we describe the use of X-rays techniques in the field of fluid flow engineering (fluidics and heat transfer). This technique is very attractive since measurements can be performed even if pressure, temperature require the use of opaque walls. In addition the X-ray technique is well suited to multiphase flows where optical technique can not be used if void fraction is larger than few percents. Specific gravity, mass or void fraction are the main accessible parameters.
Scroll Compressors for Heat Pumps: Application to Space Heating in New and Existing Residential Buildings
Authors: Ka¨mmer Norbert
Published: April 2008

Heat pumps for residential space heating has become an increasingly important alternative to the conventional European heating systems like gas or oil burners. They offer the opportunity to reduce CO2 emission associated with heating residential homes in central and northern Europe as well as energy savings. The operational parameters for compressors in heat pumps are derived from different heat pump configurations. High compressor efficiency and a wide operating map is required so that an economically viable heat pump is achieved which meets the required operational conditions. The modifications to the basic refrigeration scroll compressor design are demonstrated. These dedicated compressor designs make it possible to achieve the required high condensing temperatures. A performance comparison with the standard compressor designs is presented and the available product range for the design of heat pump systems is shown.
Aircraft Power System Options and Thermal Management for High, Pulsed Power Application
Authors: Shanmugasundaram V. , Ramalingam M. L. , Donovan Brian , Mahefkey T. , Hager B.
Published: April 2008

A general thermodynamic analytical evaluation tool was developed to investigate the impact of technological improvements on mission effectiveness and weapon power generation in an aircraft based pulsed power system. The power system investigated consists of six major components, the prime power source, the power generator, the power conditioner, the pulsed power source, the pulsed power processor and the thermal management with a total estimated payload restriction of 4600 kgs. based on a USAF cargo aircraft. The analysis was based on a 2.5 MW pulsed power source output and a notional mission profile with an engagement period of 60 minutes during which several duty cycle scenarios were considered. Six power system architectures were evaluated with a baseline power system model that incorporated current off-the-shelf technologies for each component. A helicopter engine was used as the primary power source because of its high power density but the engine performance is very sensitive to increasing altitude where the output power diminishes rapidly. As a result of this and the necessity to accommodate load-following during engagement, the investigations were extended to a hybrid power system architecture with turboalternator-battery and turboalternator-flywheel combinations. Preliminary analysis based on prorated values of specific power and power density for all the components revealed that the overall mass of the power system could be brought down from 13,330 kgs. for the baseline architecture to 4075 kgs. for the conceptual load-following turboalternator-battery hybrid power system. Coolant requirements for an open thermal management system ranged from 2007 kgs. of Ammonia or 1127 kgs. of water for a heat load of 2.9 Mwt corresponding to a 30% duty cycle pulsed power source operation.
An Environmental Analysis of Machining
Authors: Dahmus Jeffrey B. , Gutowski Timothy G.
Published: April 2008

This paper presents a system-level environmental analysis of machining. The analysis presented here considers not only the environmental impact of the material removal process itself, but also the impact of associated processes such as material preparation and cutting fluid preparation. This larger system view results in a more complete assessment of machining. Energy analyses show that the energy requirements of actual material removal can be quite small when compared to the total energy associated with machine tool operation. Also, depending on the energy intensity of the materials being machined, the energy of material production can, in some cases, far exceed the energy required for machine tool operation.
Measurement of Velocity Profiles of Laminar to Turbulent Water Flows in a Tube Using Particle Image Velocimetry
Authors: Martin Meredith R.
Published: April 2008

The transition from laminar to turbulent in-tube flow is studied in this paper. Water flow in a glass tube with an inside diameter of 21.7 mm was investigated by two methods. First, a dye visualization test using a setup similar to the 1883 experiment of Osborne Reynolds was conducted. For the dye visualization, Reynolds numbers ranging from approximately 1000 to 3500 were tested and the transition from laminar to turbulent flow was observed between Reynolds numbers of 2500 and 3500. For the second method, a particle image velocimetry (PIV) system was used to measure the velocity profiles of flow in the same glass tube at Reynolds numbers ranging from approximately 500 to 9000. The resulting velocity profiles were compared to theoretical laminar profiles and empirical turbulent power-law profiles. Good agreement was found between the lower Reynolds number flow and the laminar profile, and between the higher Reynolds number flow and turbulent power-law profile. In between the flow appeared to be in a transition region and deviated some between the two profiles.
Effect of Colloidal Chemistry on the Thermal Conductivity of Nanofluids
Authors: Prasher Ravi
Published: April 2008

Nanofluids have attracted tremendous attention lately due to their promise as high thermal conductivity liquid and also due the inability of researchers all across the world in explaining the enhancement in the thermal conductivity. Various models and physics have been proposed and some of them have been quite successful in explaining the data, however none of the models in the literature take colloidal chemistry into account. Experimental data, however have shown dependence of thermal conductivity on pH and surface chemistry. In this paper we introduce a model which captures all the anomalies reported in the data 1) Effect of pH 2) effect of aging i.e. time 3) maxima in the thermal conductivity with respect to the diameter of the nanoparticles 4) increase and decrease in the ratio of the thermal conductivity of the nanofluids and the base fluids with increasing temperature. The model is based on the combination of aggregation kinetics with the physics of thermal transport.
Parametric Experimental Study of Viscosity of Nanofluids
Authors: Prasher Ravi , Song David , Wang Jinlin , Phelan Patrick
Published: April 2008

There is a lot of interest in the research community about nanofluids due to their high thermal conductivity and potential applications as heat transfer fluids, however a systematic investigation on the viscosity of the nanofluids is still lacking from the literature. Any heat transfer enhancement due to force convention, also leads to increase in the pressure drop. Knowledge of the pressure drop is very important to understand the pumping requirements. Pressure drop is directly proportional to the viscosity of the liquid. Addition of nanoparticles will enhance the viscosity of the nanofluids. In this paper experimental results on the viscosity of propylene glycol based nanofluids are reported for various parameters such as nanoparticle size, temperature and volume fraction. Effect of Brownian motion on the viscosity of nanofluids is also explored.
Aspect Ratio Effect in a Thermally-Driven Rectangular Open Cavity With an Isothermal Shrouding Wall
Authors: Franco Admilson Texeira , Ganzarolli Marcelo M.
Published: April 2008

Natural convection in a rectangular open cavity with the presence of an isothermal shrouding wall is investigated. The horizontal walls of the cavity are adiabatic. One vertical wall is heated uniformly and another is open to a fluid reservoir. The isothermal shrouding wall is placed in front of this opening forming a vertical channel. Laminar and two-dimensional flow is assumed for a Rayleigh number ranging from 103 - 106. The numerical solution is carried out with the Finite Volume-SOLA method. The cavity aspect ratios considered are B = L/H = 0.5, 3.0 e 6.0, where L and H are the cavity width and cavity height, respectively. The distance b/H between the isothermal shrouding wall and the cavity opening is considered as 0.5 and 0.2. The effects of the Rayleigh number Ra, the cavity aspect ratios B and the ratio b/H, on the streamlines, isotherms and average Nusselt number are reported.
Approximate Analytical Model for a First Level Package With Non-Uniformly Powered Die
Authors: Kaisare Abhijit , Agonafer Dereje , Haji-Sheikh A. , Chrysler Greg , Mahajan Ravi
Published: April 2008

Microprocessors continue to grow in capabilities, complexity and performance. Microprocessors typically integrate functional components such as logic and level two (L2) cache memory in their architecture. This functional integration of logic and memory results in improved performance of the microprocessor as the clock speed increases and the instruction execution time has decreased. However, the integration also introduces a layer of complexity to the thermal design and management of microprocessors. As a direct result of function integration, the power map on a microprocessor is typically highly non-uniform and the assumption of a uniform heat flux across the chip surface is not valid. The active side of the die is divided into several functional blocks with distinct power assigned to each functional block. Previous work [1,2] has been done to minimize the thermal resistance of the package by optimizing the distribution of the non-uniform powered functional blocks with different power matrices. This study further gives design guideline and key pointers to minimized thermal resistance for any number of functional blocks for a given non-uniformly powered microprocessor. In this paper, initially (Part I) temperature distribution of a typical package consisting of a uniformly powered die, heat spreader, TIM 1 & 2 and the base of the heat sink is calculated using an approximate analytical model. The results are then compared with a detailed numerical model and the agreement is within 5%. This study follows (Part II) with a thermal investigation of non-uniform powered functional blocks with a different power matrices with focus on distribution of power over die surface with an application of maximum, minimum and average uniform junction temperature over a given die area. This will help to predict the trend of the calculated distribution of power that will lead to the least thermal gradient over a given die area. This trend will further help to come up with design correlations for minimizing thermal resistance for any number of functional blocks for a given non-uniformly powered microprocessor numerically as well as analytically. The commercial finite element code ANSYS® is used for this analysis as a numerical tool.
Directional Cooling of Adult Stem Cells
Authors: Fuller R. , Devireddy R. V.
Published: April 2008

The effect of directional cooling on the immediate post thaw membrane integrity of adipose tissue derived adult stem cells (ASCs) was investigated using a directional solidification stage (DSS). ASCs were cooled at either 1, 5, 20 or 40 °C/min to an end temperature of −80°C in the presence and absence of a cryoprotective agent (dimethylsulfoxide, DMSO). After freezing to -80°C, the samples were thawed at 200°C/min and the ability of the frozen/thawed ASCs to exclude fluorescent dyes was assessed. ASCs frozen using the DSS in the presence of 0.85M (or 10% v/v) DMSO were found to have a higher post-thaw cell membrane integrity (confidence level of 99%) when compared with the ASCs frozen in its absence. Intriguingly, a comparison with corresponding data for ASCs that were frozen using a commercially available controlled rate freezer (CRF) suggests that the directionally cooled ASCs (both in the absence and presence of DMSO) exhibit a significantly lower post-thaw cell membrane integrity (confidence level of 95%). This lowering of post-thaw cell membrane integrity for ASCs frozen using the DSS is postulated to be related to the differences in the nature, and the associated damaging effects, of ice crystals formed in the DSS vs. the commercial freezer.
Determining Aerosol Particle Size Distribution Using Time-Resolved Laser-Induced Incandescence
Authors: Daun K. J. , Stagg B. J. , Liu F. , Smallwood G. J. , Snelling D. R.
Published: April 2008

Time-resolved laser-induced incandescence is a powerful tool for determining the physical characteristics of aerosol dispersions of refractory nano-particles. In this procedure, particles within a small aerosol volume are heated with a nano-second laser pulse, and the temporal incandescence of the particles is then measured as they return to the ambient gas temperature. It is possible to infer particle size distribution from the temporal decay of the LII signal since the cooling rate of an individual particle depends on its area-to-volume ratio. This requires solving a mathematically ill-posed inverse problem, however, since the measured LII signal is due to the incandescence contributed by all particle sizes within the aerosol volume. This paper reviews techniques proposed in the literature for recovering particle size distributions from time-resolved LII data. The characteristics of this ill-posed problem are then discussed in detail, particularly the issues of solution stability and uniqueness. Finally, the accuracy and stability of each method is evaluated by performing a perturbation analysis, and the overall performance of the techniques is compared.
Thermal Analysis of Phase Change Materials With Embedded Graphite Nanofibers for Thermal Management of Electronics
Authors: Kopec Thomas C. , Weinstein Randy D. , Fleischer Amy S. , D'Addio Elizabeth , Bessel Carol A.
Published: April 2008

Phase change materials (PCMs) exhibit excellent thermal storage capacity due to their high latent heat of transformation and have been successfully utilized in small volumes for transient thermal management of electronics. However, their low thermal diffusivity makes it difficult to utilize large volumes of PCMs for transient thermal management of high power density systems. To improve the thermal performance of a paraffin PCM, high thermal conductivity graphite nanofibers are embedded into the paraffin PCM. The thermal effects of graphite fiber loading levels, measured in weight percent, are examined for a 131 cm3 volume cubic system with power loads of 3 and 7 W. It is found that the thermal response of the system improves with increased fiber loading levels.
An Experimental and Computational Sensitivity Analysis of Synthetic Jet Cooling Performance
Authors: Utturkar Yogen , Arik Mehmet , Gursoy Mustafa
Published: April 2008

Synthetic jets are meso or micro scale fluidic devices, which operate on the "zero-net-mass-flux" principle. However, they impart a positive net momentum flux to the external environment, and are able to produce the cooling effect of a fan sans its ducting, reliability issues, and oversized dimensions. The rate of heat removal from the thermal source is expected to depend on the location, orientation, strength, and shape of the jet. In the current study, we investigate the impact of jet location and orientation on the cooling performance via time-dependent numerical simulations, and verify the same with experimental results. We firstly present the experimental study along with the findings. Secondly, we present the numerical models/results, which are compared with the experiments to gain the confidence in the computational methodology. Finally, a sensitivity evaluation has been performed by altering the position and alignment of the jet with respect to the heated surface. Two prime orientations of the jet have been considered, namely, perpendicular and cross jet impingement on the heater. It is found that if jet is placed at an optimum location in either impingement or cross flow position, it can provide similar enhancement.
A Thermal Network Approach for Predicting Thermal Characteristics of Three-Dimensional Electronic Packages
Authors: Chen P. L. , Chang S. F. , Wu T. Y. , Hung Y. H.
Published: April 2008

In the present study, a numerical approach for characterizing three-dimensional (3-D) electronic packages is presented, based on the steady-state solution of the thermal network method for generalized rectangular geometries, where boundary conditions are uniformly specified over specific regions of the package. As we know, the thermal-network method is very powerful on thermal analysis of electronic packaging because of its feasibility and flexibility. Accordingly, the numerical approach with thermal-network method to simulate heat transfer characteristics for 3-D package geometries becomes important in the modem microelectronic applications. The thermal analyses are presented with a general overview of the thermal network method, boundary conditions and solution procedures. Furthermore, the application of boundary conditions at the fluid-solid, package-board and layer-layer interfaces provides a means for obtaining a unique numerical result for 3-D complex electronic packages. The complex geometries found in most 3-D electronic package configurations are modeled using numerical method through the careful use of simplifying assumptions. Comparisons of the present numerical results with the existing experimental data for 3-D electronic package of pin grid arrays and multi-chip modules are made with a satisfactory agreement. Thus, the present study demonstrates that the numerical thermal-network approach can offer an accurate and efficient solution procedure for evaluating the thermal characterization of 3-D electronic packages.
Thermal Imaging Studies and 3-D Thermal Finite Element Modeling of Laser Transmission Welding of a Lap-Joint
Authors: Mayboudi L. S. , Birk A. M. , Zak G. , Bates P. J.
Published: April 2008

Laser transmission welding (LTW) is a relatively new technology for joining plastic parts. This paper presents a three-dimensional (3-D) transient thermal model of LTW solved with the finite element method (FEM). A lap-joint geometry was modelled for unreinforced nylon 6 specimens. This thermal model addressed the heating and cooling stages in a laser welding process with a stationary laser beam. This paper compares the temperature distribution of a lap-joint geometry exposed to a stationary diode laser beam, obtained from 3-D thermal modelling with thermal imaging observations. It is shown that the thermal model is capable of accurately predicting the temperature distribution when laser beam scattering during transmission through the polymer is included in the model. The weld dimensions obtained from the model have been compared with the experimental data and are in good agreement.
Concurrent Electro-Thermal Design of VLSI Circuits
Authors: Choi J. , Bansal A. , Meterelliyoz M. , Roy K. , Murthy J. Y.
Published: April 2008

In recent years, self-heating has become a significant issue in the performance of emerging ultra-scaled microelectronics. This is a particular problem in emerging finFET designs because of the use of thick buried oxide layers which impede heat flow to the heat sink. Furthermore, leakage power is becoming an increasingly important contributor to total power dissipation in deep sub-micron technology. Sub-threshold leakage, a component of leakage power, scales exponentially with temperature, leading to the possibility of thermal runaway. In this paper, compact thermal models of logic gates are developed for use in floor-plan electrical models of VLSI circuits. Concurrent electro-thermal simulation of the floor plan temperature and total power is then used to evaluate the possibility of thermal runaway in benchmark circuits. It is found that thermal runaway can occur with finFETs using 28 nm technology at the ITRS specified sub-threshold leakage (150 nA/μm) at a principal input activity of 0.5.
Investigation of Cylindrical Pin-Fin Heat Sink Thermal Performance at High Altitude
Authors: Beauchemin Melanie , Rhee Jinny
Published: April 2008

Heat sinks with cross airflow are commonly used for enhancing the cooling of electronic components. When using heat sinks in avionics applications, the primary heat transfer challenges are due to low air densities, which occur when operating at high altitudes, and space and mass constraints. Because of the spatial constraints, heat sinks with a large surface area per unit volume are advantageous. In general, cylindrical pin-fin heat sinks offer such characteristics. The Nusselt number is used as an indication of the thermal performance of the heat sink for a given Reynolds number. At high altitude, we expect the Reynolds number (based on the fin diameter and maximum velocity, Red,max) to be smaller than 1000. Empirical correlations for the Nusselt number of cylindrical pin-fin heat sinks are available in the literature; however, these correlations were obtained for larger values of Red,max. The objective of this work is to correlate the Nusselt number and the friction factor of an in-line cylindrical pin-fin heat sink with its non-dimensional geometric parameters, and the airflow Reynolds number. The emphasis is on Red,max range between 25 and 1000, which allows the evaluation of the thermal performance of the heat sink for altitudes up to 70,000 feet. The results are obtained using three-dimensional numerical simulations with the commercial CFD software Flotherm. The numerical model is validated against experimental data. The results show that for a given Red,max, the average Nusselt number and friction factor are independent of the altitude for a given heat sink configuration. However, for a given air inlet velocity, an important drop in the average Nusselt number is observed as the altitude increases due to the reduction in air density. The effect of the variation of the fin span-wise and stream-wise pitches, as well as height is also studied.
Smoothed Particles Hydrodynamics Method for Interaction Between Multi-Droplets and Substrate
Authors: Zhang M. Y. , Zhang H. , Zheng L. L.
Published: April 2008

The smoothed particle hydrodynamics (SPH) method, one of meshfree methods, is developed to simulate the interaction between multi-droplets and substrate with solidification. However, the SPH method for this complicated problem needs a large amount of computing time, since it has to use a large number of the SPH particles to represent multidrops and substrate. All-pair search and linked list algorithms are popular in the neighbor search, which is the most time consuming part of the SPH calculation. Both algorithms are tested in this paper. For the solidification process, since the volume of the melt is decreased continuously, a new method is proposed to speed up the SPH calculation. The new treatment is used to handle the particles near the free surface and near the solidification interface Multi-droplets impinging on a smooth substrate in 2D and 3D are simulated to demonstrate the capability of current numerical method on simulating the spreading and solidification of multi-droplets.