bluuebunny/crossref_metadata_2025 huggingface dataset:
A New Implicit Numerical Treatment for Non-Linear Turbulence Models and Its Application to Channels With Spatially Periodic Corrugated Walls
Authors: Assato Marcelo , de Lemos Marcelo J. S.
Published: May 2008

This work examines the performance of linear and nonlinear eddy-viscosity models when used to predict the turbulent flow in periodically sinusoidal-wave channels. Two geometries are investigated, namely a converging-diverging channel and a channel with concave-convex walls. The numerical method employed for the discretization of the equations is the control-volume method in a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm is used for correcting the pressure field. The classical wall function and a low Reynolds model are used to describe the flow near the wall. Comparisons between those two approaches using linear and non-linear turbulence models are done. Here, an implicit numerical treatment was proposed for the non-linear diffusion terms of the momentum equations in order to increase the robustness of the solution method.
A Study of the Effect of Surface Metalization on the Thermal Conductivity Measurements by the Transient Thermo-Reflectance Method
Authors: Burzo Mihai G. , Komarov Pavel L. , Raad Peter E.
Published: May 2008

This work is a numerical and experimental investigation of the effect of the use of a metallic absorption layer on the laser-based measurements of the thermal conductivity of dielectric, semiconductor, and highly-conductive materials. The numerical and supporting experimental results reveal the presence of behaviors associated with thermally-thin and thermally-thick absorption layers, depending on the ratio between the thickness of the absorption layer and the heat penetration depth. It is concluded that the TTR method performs optimally when the thickness of the metalization layer falls in the transition range between the identified thermally-thin and thermally-thick layers.
Thermoelectric Mapping of Nanostructures
Authors: Lyeo Ho-Ki , Shih C. K. Ken , Ghoshal Uttam , Shi Li
Published: May 2008

There is intense interest to develop nanowires [1] and superlattices [2] that may offer superior thermoelectric figure of merit for efficient energy conversion. Meanwhile, the advance of semiconductor processing techniques has yielded impurity-doped semiconductor nanostructures with a doped region as small as a few nanometers. These include shallow junction Si field-effect transistors, strained Si/SiGe/Ge heterostructures and quantum dots, III-V heterostructures, and doped nanowires and nanotubes. Due to various size confinement effects, these doped semiconductor nanostructures often have unique electrical, optoelectronic, or thermoelectric properties that may lead to a wide range of applications. In contrast to the progress made in synthesizing thermoelectric nanostructures and in fabricating doped semiconductor nanostructures, the ability to quantify thermoelectric property and carrier concentration in comparable length scale has been lagging behind. For example, the 1997 U.S. Roadmap of Semiconductors from the Semiconductor Industry Association (SIA) defines the need for nanometer-scale measurements of carrier concentration profiles [3]. Though progress has been made, currently no technique can satisfy the requirements posted by the SIA roadmap due to the lack of either spatial resolution or accuracy.
Fundamental Effects of Boundary Structure on Nano-Scale Conductive Heat Transfer
Authors: Aitcheson Nicole , Fickes Dan , Lloyd John R.
Published: May 2008

Modeling of energy transport in nanostructures is very different from that in larger scaled materials. Researchers have reported considerably lower “thermal conductivity” in nano-scaled materials compared to the corresponding bulk material property. This observation is most often attributed to phonon-boundary interactions. However, in-depth explanations of this phenomenon are lacking. This study defines the fundamentals of low-dimensional phonon transport by relating the structural boundary characteristics of a material with the mechanisms of phonon transport. The structural boundary of a material controls phonon transport, creating a “boundary region” rather than a boundary surface, resulting in apparent differences in energy transport efficiency between the bulk material and the nano thin materials.
Radiative Properties of Multilayer Thin Films With Positive and Negative Refractive Indexes
Authors: Fu Ceji , Zhang Zhuomin M. , Tanner David B.
Published: May 2008

In 1968, Veselago [1] predicted that there could still be propagating waves in a medium that had simultaneously negative permittivity ε and permeability μ, because the product of ε and μ would be positive. However, to ensure energy conservation, he concluded that the refractive index must use the negative square root of the product of ε and μ (i.e., n = εμ). A consequence is certain unusual optical features in negative-index media. The electric field vector E, magnetic field vector H and wave vector k are a left-handed triplet, the basis for calling materials with simultaneously negative ε and μ “left-handed materials” (LHMs). LHMs would have novel optical properties. Light at non-normal incidence would bend to the side opposite that in a normal RHM; positive lenses would become negative; a flat slab could focus. The phase velocity of an electromagnetic wave would be opposite to the direction of energy flux, resulting in a reversed Doppler effect. Photons would have negative momentum and apply tension to the interface upon reflection. Recently, this kind of material has been demonstrated experimentally to exist. Shelby et al. [2] measured the scattering angle of the transmitted beam through a prism manufactured from a composite material consisting of a two-dimensional array of copper wires and split ring resonators and showed that the effective refractive index of the material is negative at microwave frequencies. Recent theoretical studies also showed that some photonic crystals might have negative-refraction properties in the near infrared spectral region [3].
Thermal Diffusivity Characterization of Bi2Te3 Nanowires Array Inside Amorphous Alumina Template
Authors: Borca-Tasciuc Diana-Andra , Chen Gang , Martin-Gonzales Marisol S. , Prieto Amy L. , Stacy Angelica , Sands Timothy
Published: May 2008

In the recent years there has been an increasing interest in low-dimensional thermoelectric materials such as superlattices, quantum dots and nanowires [1,2,3]. An enhancement in thermoelectric figure-of-merit, Z=σα2/k, where α is the Seebeck coefficient, σ is electrical conductivity, and k is the thermal conductivity, is predicted due to increased density of states near Fermi level [4]. A large contribution to the enhancement of thermoelectric figure-of-merit in low-dimensional systems may come from thermal conductivity reduction due to increased scattering of heat carriers from interfaces. In this light, we are interested in thermal properties of Bi2Te3 nanowires and carried out thermal diffusivity characterization of these nanowires embedded in an amorphous alumina template.
Bubble Nucleation and Growth on Surface of Rapidly Heated Micro Heaters
Authors: Jung Jung-Yeul , Park Hong-Chul , Kwak Ho-Young
Published: May 2008

Time dependent surface temperatures on a micro heater were measured when a voltage pulse of 25~200 μs was applied to the heater. The dimensions of the micro polysilicon heaters fabricated on a silicon substrate are 50×5×0.523 μm3 for a micro line heater and 65×65×0.523 μm3 for a micro square heater. With the minimum voltage needed for bubble growth, the temperature for bubble nucleation on the heater immersed in FC-72 was measured to be 420.7 K for the micro line heater, which is close to the superheat limit of the working fluid. On the other hand a bubble nucleates at temperature of 388 K on the square heater. Bubble nucleation, growth and subsequent collapse process were visualized by a microscope-35 mm camera with a μs flash unit.
Thermal Property Measurements of Giant Magnetoresistive (GMR) Layers
Authors: Yang Yizhang , Liu W. , Shojaeizadeh S. , Zhang S. , Asheghi M.
Published: May 2008

Advanced thin-film growth technology has made it possible to arrange different materials at the atomic level and to fabricate thin-film structures (e.g., GMR) with strong quantum effects. GMR devices appear in two important areas of storage technology: GMR sensors for magnetic read heads, and Magnetic Random Access Memory (MRAM). The former is a well-established commercial technology, in which nanoscale thermal transport is related to reliability, through electrostatic discharge (ESD) events as well as performance. In MRAM, the nanoscale thermal transport limits device ultimate performance, but may also present an opportunity for assisted switching. The thermal transport properties of these thin periodic layers play an important role in design, performance and reliability of these devices, which are fundamentally different from their bulk counterparts [1,2].
Thermal Property Measurements of Nanotubes, Nanowires, and Nanobelts
Authors: Li Deyu , Majumdar Arun , Jang Wanyoung , Yao Zhen , Kim Philip , Kim Dohyung , Yu Choongho , Hao Qing , Shi Li
Published: May 2008

As materials are confined to low dimensions with a size comparable to the scattering mean free paths, the thermal conductivity is often reduced due to increased boundary scattering. The reduced thermal conductivity is desirable in some applications such as thermoelectric cooling, but is often unwanted for others especially for nanoelectronic devices. An exception of this scaling trend is carbon nanotubes (CNTs). Due to the unique crystalline structure, boundary scattering is nearly absent in CNTs, giving rise to super high thermal and electrical conductivity that makes the CNT an ideal candidate for replacing Cu in future VLSI interconnects. The potential electronic applications have inspired several groups to employ a variety of techniques for measuring the Seebeck coefficient [1], specific heat [2–3], and thermal conductivity [4] of CNT bundles and mats. Estimated thermal conductivity from these measurements is significantly reduced by numerous tube-tube junctions in the sample and is much lower than the theoretical expectation [5–7].
Three-Dimensional Microfluidic Measurements Using Optical Sectioning by Confocal Microscopy: Flow Around a Moving Air Bubble in a Micro-Channel
Authors: Park J. S. , Kihm K. D. , Allen J. S.
Published: May 2008

Microscopic visualization of the transport phenomena associated with an air bubble entrained in liquid flowing through a microchannel has revealed a transition in the bubble shape and proximity to the channel walls with increasing flow rates. Experiments have been conducted to investigate the effect of channel geometry (three different microchannels with square cross-sections, 1.0-mm, 500-μm and 100-μm), and flow conditions (Capillary numbers ranging from 0 to 0.23) on the transition in bubble shape. In addition, micro-Particle Image Velocimetry (μ-PIV) has been used to map the detailed flow field around the bubble and an innovative attempt has been made to provide optically-sectioned image recordings using a state-of-the-art high-speed Confocal Laser Scanning Microscope (CLSM) technique. The research presented herein is directed towards characterizing transport phenomena in two-phase (gas-liquid) flows in microchannels. Of particular interest to this research is the determination of entrance and exit length regions in two-phase microchannel flows. The experimental techniques being utilized towards the objective of characterizing two-phase flow transport phenomena in microchannels are the focus of the following discussion.
Measurements of a Microchannel Flow Using Micro-PIV
Authors: Lee Inwon , Lee In-seop
Published: May 2008

A micro-PIV (particle image velocimetry) measurement has been conducted to investigate flow fields in such microfluidic devices as microchannels and micronozzle. The present study employs a state-of-art micro-PIV system which consists of epifluorescence microscope, 620nm diameter fluorescent seed particles and an 8-bit megapixel CCD camera. Velocity vector fields with a resolution of 6.8 × 6.8μm has been obtained, and the attention has been paid on the effect of varying measurement conditions of particle diameter and particle concentration on the resulting PIV results. In this study, the microfluidic elements were fabricated on plastic chips by means of MEMS processes and a subsequent molding process. Flow fields in a variety of microchannels as well as micronozzle have been investigated.
Phonon Heat Conduction in Micro- and Nano-Cylindrical and Spherical Media
Authors: Zeng Taofang
Published: May 2008

Phonon heat conduction in thin films and superlattices of dielectric materials has attracted extensive attentions in the past decade. Nonetheless, a wide range of micro- and nanoscale thermal problems is associated with non-planar geometries such as spherical and cylindrical media. With the rapid growth of research and development in nano-materials and nanostructures, understanding of heat transfer in micro- and nano-cylindrical and spherical media becomes important. Examples include cladding and coating for optic fibers and nanowires.
A PIV Study of Natural Convection Induced by Oscillating Thermal Gradients
Authors: Shu Y. , Higgins M. , Li B. Q. , Ramaprian B. R.
Published: May 2008

Microscale fluid flow and mass transfer are of both fundamental and practical importance to the design of solidification systems for melt processing of materials. These microscale fluid flow phenomena are affected by the macroscopic bulk flow motion and heat transfer away from the solidification front. As a first step towards a systematic understanding of the interactions of the micro- and macro-scale phenomena, a miniature cavity of a few millimeters in size is considered, where an oscillating temperature gradient is established to simulate the driving force under perturbed solidification conditions typical of microgravity environments. Flow visualization and velocity measurements of the transient oscillating fluid motion under two sets of thermal conditions are conducted using the Particle Image Velocimetry (PIV) technique. These experimental results are used to validate numerical simulations carried out using a finite element based model, developed by the authors for the prediction of flows in microgravity environments. The visualized flow pattern and velocity measurements in the two test cases compare very well with the numerical simulations. The numerical model is now ready to be used as a reliable tool for understanding and predicting the structure of fluid flow and heat transfer in microgravity environments.
Analysis of Thermal Conductivity and Convective Heat Transfer in Nanotube Suspensions
Authors: Yu Wenhua , Choi Stephen U.-S.
Published: May 2008

Nanofluids, which are mixtures of nanosize solids and liquids, show considerable promise as an efficient heat transfer medium. Our preliminary experimental studies have shown improved thermal conductivity in nanotube-in-oil suspensions. The experimental data were much higher than predicted values from the Hamilton-Crosser correlation. The experimental results were used to theoretically analyze the convective heat transfer in nanotube-in-oil suspensions. The theoretical results show that the convective heat transfer of nanotube-in-oil suspensions can be increased 30% at very low nanotube concentration (0.6 vol%).
Microscopic Characterization of Frost Surface During Liquid-Ice Phase Change Period
Authors: Hao Y. L. , Iragorry J. , Castro D. , Tao Y.-X. , Jia S.
Published: May 2008

In response to the need for developing a better model to predict frost formation and defrosting processes in freezer and evaporator applications, a microscopic analysis of frost growth on a flat surface is conducted to determine the microscopic characteristics of a frost layer during the early growth period when sub-cooled droplets are formed and changed to the ice. The surface characterization is performed by employing the holographic interferometry technique to determine the air-frost interface temperature, and the video microscope to determine the mean droplet size and ice particle fractions. Typical experimental results are presented to demonstrate the test technique. Preliminary experimentally determined frost thickness and air-frost interface temperature are compared with simulation results.
Bifurcation and Chaos in Friction-Induced Vibration
Authors: Li Yong , Feng Z. C.
Published: May 2008

Friction-induced vibration is a phenomenon that has received extensive study by the dynamics community. This is because of the important industrial relevance and the evere-volving development of new friction models. In this paper, we report the result of bifurcation study of a single-degree-of-freedom mechanical oscillator sliding over a surface. The friction model we use is that developed by Canudas de Wit et al, a model that is receiving increasing acceptance from the mechanics community. Using this model, we find a stable limit cycle at intermediate sliding speed for a single-degree-of-freedom mechanical oscillator. Moreover, the mechanical oscillator can exhibit chaotic motions. For certain parameters, numerical simulation suggests the existence of a Silnikov homoclinic orbit. This is not expected in a single-degree-of-freedom system. The occurrence of chaos becomes possible because the friction model contains one internal variable. This demonstrates a unique characteristic of the friction model. Unlike most friction models, the present model is capable of simultaneously modeling self-excitation and predicting stick-slip at very low sliding speed as well.
Parallel Computing: New Power Aids Understanding of Turbulent Flows
Authors: Pletcher Richard H. , Lee Joon S. , Meng Ning , Avancha Ravikanth
Published: May 2008

Accurate predictions of turbulent flows occurring in many engineering and environmental applications remain a significant challenge. However, increasing computer power, particularly high performance parallel systems, is enabling more and more complex flows to be solved from first or nearly first principles through direct numerical and large eddy simulation. Such technologies can contribute greatly not only by enabling more accurate predictions but also by revealing details of physics that can contribute to a more complete understanding of turbulence. Several examples where large eddy simulations have provided new details including how rotation and buoyancy alter the structure of turbulence are described.
High-Performance Computing for Fluid Flow and Heat Transfer
Authors: Pepper Darrell W. , Lombardo Joseph M.
Published: May 2008

The use of computers in heat transfer and fluid flow has become so commonplace today that no one would consider working in either field without some knowledge of computing. Problems are now being solved on a daily basis that even a few years ago were considered intractable. While we once thought that a problem with a few million nodes was huge a few years ago, researchers are now addressing problems with over 100 million nodes. At such levels of detail, one can begin to model processes at the micro level of physics. When researchers are able to quickly analyze these gigantic data sets and can generate insightful graphical displays, the understanding of fundamental processes and governing relations will escalate tremendously.
Design of Experiments in Ball Grid Array Thermal Fatigue Life Tests
Authors: Wong T. E. , Lau C. Y. , Kachatorian L. A. , Fenger H. S. , Chen I. C.
Published: May 2008

The objective of the present study is to evaluate the impact of electronic packaging design/manufacturing process parameters on the thermal fatigue life of ball grid array (BGA) solder joints. The four selected parameters are BGA under-fill materials, conformal coating, solder pad sizes on printed wiring board, and BGA rework, with each having either two or three levels of variation. A test vehicle (TV), on which various sizes of BGA daisy-chained packages are soldered, is first designed and fabricated, and then subjected to temperature cycling (−55°C to +125°C) with continuous monitoring of solder joint integrity. The total of 15 experimental cases is used in the present study. Based on monitored results, a destructive physical analysis is conducted to further isolate the failure locations and determine the failure mechanisms of the solder joints. Test results indicate that the influence of these design parameters on fatigue life is dependent on the particular package, in some instances improving the fatigue life tenfold.
Development of Large-Eddy Simulation for Vehicle Aerodynamics
Authors: Krajnovic S. , Davidson L.
Published: May 2008

The feasibility of use of large-eddy simulation (LES) in external vehicle aerodynamics is investigated. The computational cost needed for LES of the full size car at road conditions is beyond the capability of the computers in the near future (Krajnovic´ (2002)). Since LES cannot be used for quantitative prediction of this flow, i.e. obtaining the aerodynamic forces and moments, an alternative use of this technique is suggested that can enhance the understanding of the flow around a car. It is found that making LES of the flow around simplified car-like shapes at lower Reynolds number can increase our knowledge of the flow around a car. Two simulations are made, one of the flow around a cube and the other of the flow around a simplified bus. The former simulation proved that LES with relatively coarse resolution and simple inlet boundary condition can provide accurate results. The latter simulation resulted in flow in agreement with experimental observations and displayed some flow features that were not observed in experiments or steady simulations of such flows. This simulation gave us possibility to study the transient mechanisms that are responsible for the aerodynamic properties of a car. The knowledge gained from this simulation can be used by the stylist to tune the aerodynamics of the car’s design but also by the CFD specialists to improve the turbulence models.
Simulation of Particle-Solidification Front Interactions in Metal-Matrix Composite Manufacture
Authors: Garvin J. W. , Udaykumar H. S.
Published: May 2008

Interactions of a solidification front with an embedded particle is studied by numerical simulations of the thermal transport and phase change. A sharp solid-fluid interface method is used to track the interface. The results of a phase change analysis coupled with a dynamic force model for particle pushing is compared to that of previous models which have used a model for the solidification process as well, as opposed to directly calculating the solidification problem. Our results show that solution of the phase boundary evolution and coupling with a dynamic front-particle interaction model leads to pushing/engulfment interactions that differ from more simplistic models. The method employed in this paper allows for inclusion of the effects of thermal property differences between the particle and the matrix, thus enabling detailed analysis of the role of thermal transport in particle engulfment phenomena.
Interaction of Solidification Fronts With Embedded Particles and Biological Cells
Authors: Garvin J. W. , Mao L. , Udaykumar H. S.
Published: May 2008

The interaction of solidification fronts with embedded heterogeneities, such as particles and living cells, is an important physical ingredient underlying the successful solidification processing of some materials. An example in the first category is the processing of metal-matrix composites (MMCs) where solidifying metal dendrites interact with ceramic reinforcements [1–3]. Processing of biological material, such as cells and tissue for cryopreservation [4], falls into the second category. In each case, the evolving solidification microstructure interacts in complex ways with the embedded particle.
Analytical Target Cascading for the Design of an Advanced Technology Heavy Truck
Authors: Louca L. S. , Kokkolaras M. , Delagrammatikas G. J. , Michelena N. F. , Filipi Z. S. , Papalambros P. Y. , Assanis D. N.
Published: May 2008

Analytical target cascading (ATC) is a methodology that can be used during the early development stages of large and complex systems for propagating desirable overall product targets to appropriate individual specifications for the various subsystems and components. The ATC process is applied to the design of an advanced technology heavy truck. A series hybrid-electric propulsion system, in-hub motors, and variable height suspensions are introduced with the intent to improve both commercial and military design attributes according to a dual-use design philosophy. Emphasis is given to fuel economy, ride, and mobility characteristics. These vehicle responses are predicted by appropriately developed analytical and simulation models. This article is an extension to previous work: the engine is now included at the bottom level, several battery types are considered to study their effect on fuel economy, and a more demanding driving schedule is used to assess regenerative braking benefits and ride quality. Results are presented for target values associated with a 100% improvement on fuel economy while maintaining performance attributes relative to existing designs.
Microstructural Damage and Evolution of a PbSn Solder Alloy Under Isothermal Cyclic Loading
Authors: Lim T. Jesse , Lu Wei-Yang
Published: May 2008

In our recent experiments, torsional specimens of PbSn Solder Alloy were cyclically loaded under different loading levels at both room temperature and high temperature (1008C). In these experiments, the Pb-rich phase size and micro-crack damage were also observed using scanning electron microscopy in-between loading cycles. At the moderate strain rate (10−4), the growth of Pb-rich phase does not differ much for both temperatures. At room temperature, the damage in the form of micro-cracks along the Pb- and Sn-rich phases are formed during early cycling, and the damage accumulates as the cycling proceeds. At high temperature, the damage does not accumulates as fast as for the similar strain level in room temperature, due to the lowering of stress level in high temperature and dynamic restructuring of Pb-rich phase.
Multiphase Flow Modeling of Equiaxed Dendritic Solidification With Melt Convection
Authors: Laux Harald , Nielsen O̸yvind
Published: May 2008

A Computational Fluid Dynamics model for simulation of equiaxed dendritic solidification with melt convection and crystal transport is developed. This multi-scale multiphase model accounts for transport phenomena on the macro- and the microscales of a solidifying alloy. It predicts nucleation and growth of equiaxed dendrites from a melt, and their transport in the melt. Thermo-solutal convection and remelting of crystals are also included in the multiphase model that is based on the work of Wang and Beckermann (Metall. Mater. Trans. 27A, 1996, p. 2754). To verify the modeling results comparison to published experimental data were made (nucleation, growth and transport of equiaxed crystals from a cooled NH4Cl-70wt%H2O solution). It is shown that the predicted evolution of temperature and average bed height (due to crystal sedimentation) are in good agreement with the experimental results.
Effects of Grain Boundary Sliding on Microstructural Evolution and Damage Accumulation in Tin-Lead Alloy
Authors: Shen Y.-L. , Abell K. C. R. , Garrett S. E.
Published: May 2008

Experiments on the eutectic tin-lead alloy were conducted to study the effects of grain boundary sliding on the deformation and damage processes at the microscopic level. The primary objective is to gain mechanistic understandings of solder joint reliability in microelectronic packaging. Bulk specimens were subject to relatively fast deformations of tension, compression and bending, for the purposes of examining the pure mechanical effect without the influence of diffusion related phenomena. Grain realignment and phase redistribution were characterized by microscopy and microhardness indentation. A micromechanical model is proposed to elucidate the observed microstructural changes and progressive damage. This study illustrates the significance of damage in the form of microscopic heterogeneity caused by grain boundary sliding. It also illustrates the possibility of mechanically induced phase coarsening in actual solder joints. High-frequency cyclic shear tests on tin-lead solder joints showed damage along the coarsened band after only a short time, in accord with the proposed effects. Boundary sliding without the influence of atomic diffusion plays an essential role in fatigue damage in solder.
An Anisotropic Damage Model for Ductile Metals
Authors: Hammi Youssef , Horstemeyer Mark F. , Bammann Doug J.
Published: May 2008

An anisotropic ductile damage description is motivated from fracture mechanisms and physical observations in Al-Si-Mg aluminum alloys with second phases. Ductile damage is induced by the classical process of nucleation of voids at inclusions, followed by their growth and coalescence. These mechanisms are related to different microstructural and length scale parameters like the fracture toughness, the void size, the intervoid ligament distance, etc. The classical thermodynamic constraints of irreversible processes with material state variables are used to model the tensorial damage evolution coupled to the Bammann-Chiesa-Johnson (BCJ) rate-dependent plasticity. The damage-plasticity coupling is based on the effective stress concept, assuming the total energy equivalence, and written through a deviatoric damage effect tensor on the deviatoric part and through the trace of the second rank damage tensor on the hydrostatic part. The damage rate tensor is additively decomposed into a nucleation rate tensor, a void growth rate scalar, and a coalescence rate tensor. The induced damage anisotropy is mainly driven by the nucleation, which evolves as a function of the absolute value of the plastic strain rate tensor. Finally, some experimental data of cast A356 aluminum alloy are correlated with predictive void-crack evolution to illustrate the applicability of the anisotropic damage model.
Numerical Analysis of Laminar Flow and Heat Transfer for an Abrupt Circular Contraction
Authors: Piva S. , Sambinello D. , Collins M. W.
Published: May 2008

Detailed predictions have been made of laminar thermo-fluid entry effects for the important practical case of an abrupt circular contraction. The Computational Fluid Dynamics code CFDS-FLOW3D (now CFX) was used, and special care was taken to achieve accurate data, by attention to issues of algorithm choice and grid fineness. Assuming constant properties, detailed local heat transfer predictions have been obtained, for a range of Prandtl and Reynolds numbers and contraction ratio β. The near-entry heat transfer is consistent with the flow behaviour, and gives a clear understanding of the process involved. Qualitatively the results are comparable with experimental and predictive data available in the literature, where however substantial effects due to property changes with temperature were evidenced. This study confirms the presence of flow separation and recirculation found and inferred by other authors, and investigates the phenomena in considerably more detail.
Modeling of Macrosegregation and Secondary Phase Precipitation During Solidification of Binary Alloys
Authors: Nielsen O̸yvind , Laux Harald , Dons Anne Lise
Published: May 2008

A multiphase volume-averaged model describing macrosegregation in an alloy that solidifies by the formation and growth of a primary and a secondary solid phase has been formulated. The model is based on the work presented by Ni and Beckermann (Metall. Trans. 22B, 1991, p. 349), but is extended to account for secondary phase precipitation. A CFD model has been developed by implementation of the mathematical model in a commercial CFD code. Macrosegregation due to thermo-solutal convection in binary alloys with a stationary solid phase (primary and secondary solids) has been simulated. The predictions compare fairly well to experimental results and simulations previously reported in the literature.
Optimal Placement Depth for Ground-Source Heat Engines
Authors: Stevens James W.
Published: May 2008

A recent research project explored the design and performance of a ground-source heat engine. This device used a thermoelectric module sandwiched between two heat exchangers to produce a very small amount of electricity from the daily temperature difference between the air and the ground. For a given device, the power produced is proportional to the square of the temperature difference across the module and it is desirable to maximize the power. For a sinusoidal (in time) temperature boundary condition, the solution to the heat equation for a semi-infinite body yields an attenuated sinusoid shifted in phase from the boundary condition. The phase shift and attenuation both increase with depth, so while the phase shift may be exploited to increase the temperature difference, the attenuation tends to reduce this effect. In this paper, an optimal depth for the ground side heat exchange is derived which maximizes the square of the difference between the surface temperature and the ground temperature. The maximum temperature difference in the optimized case is seven percent higher than what would be obtainable from an infinitely deep heat exchanger. The range of depths in which the temperature difference is an improvement over the infinitely deep case is derived and presented. The improvement is asymmetric so that away from the optimum depth, it declines rapidly with decreasing depth while it declines slowly with increasing depth. An alternative optimization that reflects the temperature drop through the heat engine device is derived and presented. The optimal depth decreases by slightly more than half in this case. The effect of a convection boundary condition at the free surface is explored in terms of a Biot number for the ground. The Biot number affects both the attenuation and the phase shift. The attenuation is approximately doubled for Biot of order unity, and becomes inversely proportional to Biot number if it is much less than unity. The phase shift varies from an additional shift of forty-five degrees for Biot much less than unity to an additional shift of twenty-seven degrees for Biot of order unity. For Biot much greater than unity, the temperature boundary condition applies.
Investigation and Development of Condensation Heat Transfer Correlations for Straight and Helically Coiled Tubes
Authors: Bowman Anthony , Park Hyunjae , Hayes Benjamin Z. , Rinehart Mark , Raether Scott E. , Farrell Mike D.
Published: May 2008

This paper introduces a set of developed condensation heat transfer correlations used in both straight and helically coiled tube systems. A literature search found that there are no applicable heat transfer correlations for two-phase flow in a helically coiled tube. As such, a study was first undertaken on condensation heat transfer in a straight tube. The investigation into condensation heat transfer for a fluid flowing in a straight tube began with a review of existing correlations. It is assumed that the annular flow is the dominant flow pattern over most of the condensing length of the tube. Depending upon the experimental methods and the theoretical and/or numerical approach, the results obtained by using the (existing) correlations show some degree of inconsistency over the ranges of parameters. Thus, it is not a simple matter to select a proper condensation heat transfer correlation for a straight tube system. This resulted in the development of generalized correlations for specific fluids such as water and R134a by employing least-squared property curves in terms of parameters such as Reynolds number based on the liquid, vapor quality and reduced pressure for each fluid considered. As a consequence, a number of plausible condensation heat transfer correlations in a coiled tube for specific fluids mentioned above are proposed employing the two-phase fluid flow Coiling Influence Factor (CIFTP) for heat transfer obtained by using the Coiling Influence Factor for liquid and vapor (CIFl and CIFV) respectively along with a vapor quality as a weighting parameter.
Numerical Investigation of Heat Transfer From a Surface Under the Influence of Two Impinging Pulsating Slot Jets
Authors: Chaniotis A. K. , Poulikakos D. , Ventikos Y.
Published: May 2008

The problem of gas jet impingement on a surface has many applications in the processing and manufacturing industries, ranging from convective heat exchange in a variety of practical implementations to surface coating. The present basic research work considers two air jets, exiting from two slots and impinging on a target solid surface, heated with constant heat flux. The jet impingement arrangement is attractive due to its simplicity and to the high convective heat transfer coefficients it yields, corresponding to high heat fluxes. The simulations of this process are conducted using an improved Smooth Particle Hydrodynamics (SPH) methodology. The simulation of an array of two slot jets impingement focuses on the investigation of the flow field and the energy exchange between jets and surface, with an emphasis on the oscillatory characteristics of the two jets relative to one another. The effect of the main problem parameters is examined within the paper. The strong aerodynamic and thermal interaction that exists between the gaseous jets and the impingement surface leads to non-linear system responses; their dynamic behavior and heat transfer implications are also discussed in the paper. The numerical results indicate that within the investigated parametric domain, the phase angle difference between the two jets and the amplitude of the pulsation have a significant effect on the surface temperature and overall heat transfer.
Numerical Simulation of Microstructure of Aluminum Alloy Casting Using Macro-Micro Coupled Method
Authors: Zhang Guangyue , Jing Tao , Liu Baicheng , Zhao Daiping
Published: May 2008

Numerical simulation of solidification microstructure of aluminum alloy casting is studied by using a macro-micro coupled method. Different mesh sizes and time steps are used to calculate the macro temperature field and the crystal growth. The microstructure simulation is conducted by selecting a macro cell in the central region of the aluminum casting, and then the initial distribution of the temperature in the micro domain is obtained by interpolating the temperature of the macro cells near the selected cell. The crystal growth is controlled by the phase field equation, and the mathematical model also includes mix solution conservation equation and energy conservation equation to compute the distribution of the solution and the temperature. Noise is introduced to simulate the side branches. To describe the heterogeneous nucleation the Gaussian distribution is applied. The interface undercooling is considered to be the sum of thermal, solute and curvature effects. The 2-D dendritic growth morphology, the 3-D dendritic growth morphology and the crystal sizes in the region are simulated. The simulation results are compared with those obtained by experiments.
Hydrodynamics and Heat Transfer of an Oblique Plane Jet Impinging Onto a Substrate
Authors: Tong Albert Y.
Published: May 2008

The objective of the present study is to understand the hydrodynamics and heat transfer of the impingement process, particularly the complexities attributable to the asymmetric geometry of an oblique free liquid jet. The Navier-Stokes equations are solved using a finite-volume formulation with a two-step projection method on a fixed non-uniform rectangular grid. The free surface of the jet is tracked by the volume-of-fluid (VOF) method with a second order accurate piecewise-linear scheme. The energy equation is modeled by using an enthalpy-based formulation. The method provides a state-of-the-art comprehensive model of the dynamic and thermal aspects of the impinging process. Nusselt number plots and pressure distributions on the substrate are obtained. The locations of the maximum Nusselt number as well as maximum pressure on the surface are identified and compared with the geometric jet impingement point. Results for normal impingement are also obtained and are used as reference. The effects of several parameters are examined. These include jet Reynolds number, jet impingement angle and jet inlet velocity profile. Experimental and analytical data from the literature are also included for comparison.
Analysis of Undercooling, Nucleation, Rapid Solidification, and Microstructure Formation in Thermal Spraying
Authors: Zhang H. , Wang X. Y. , Sampath S.
Published: May 2008

A splat formation model including undercooling, nucleation, and non-equilibrium solidification has been developed to study the thermal characteristics of molten droplets impacting on a flat substrate. The nucleation sub-model is based on the classic nucleation theory accounting for heterogeneous nucleation kinetic and crystal growth. The effects of contact angle on nucleation temperature and grain density have been investigated. The grain size distributions have been predicted and compared with experimental results for molybdenum (Mo) splats on different substrates. Based on scaling analysis, time scales of various sub-processes, e.g., the nucleation delay, recalesence, and total solidification time, have been derived, and compared with simulation results.
Multi-Domain Topology Optimization for Vehicle Substructure Design
Authors: Ma Zheng-Dong , Kikuchi Noboru , Pierre Christophe , Raju Basavaraju
Published: May 2008

Topology optimization of a structure is generally considered as a problem of optimum material distribution (OMD) within a given structural domain, subject to a given amount of material and to boundary conditions and loading conditions applied to the structure. The effective design of a complex engineering structure, however, may require controllability over how the material should be distributed among the various subdomains of the structure, as well as even the use of different materials for the different subdomains. A multi-domain topology optimization technique is therefore proposed in this paper, which is based upon the homogenization method, and which employs a generalized Sequential Approximation Optimization (GSAO) algorithm. This algorithm enhances the capabilities of current topology optimization methods by using advanced updating rules and providing additional flexibility in the optimization process, thus resulting in improved convergence and higher computational efficiency. A Component Mode Synthesis method is also employed, which can significantly reduce the number of degrees of freedom associated with the subdomains whose designs are fixed at the current stage. Several example design problems are considered, including a “structure-fixture simultaneous design” problem, a “functionally graded material design” problem, a “crush energy management” problem, and a “truck frame design” problem that illustrates how the technique developed can be applied to real vehicle substructure design problems.
Control of Convective Heat Transfer in a Confined Laminar Impinging Jet by Low Amplitude Forcing
Authors: Chiriac Victor Adrian , Ortega Alfonso
Published: May 2008

A numerical finite-difference model, derived using a control-volume approach, was used to compute the flow and heat transfer characteristics in a two-dimensional confined laminar air jet impinging on an isothermal surface. Several cases were considered with Re=650, 750, and nozzle to plate spacing, H/W=5. The behavior of the jet and the attendant heat transfer from the target wall were investigated when the jet was forced by fluidic excitation at the nozzle exit. At Re between 585 and 610, the unforced jet exhibits a transition to an unsteady regime leading to asymmetric vortex shedding and jet flapping [1, 2]. Investigation of the velocity spectra indicate three distinct dominant modes; the lowest frequency is associated with the jet flapping while the highest frequency is associated with the asymmetric vortex formation which causes buckling of the jet column. As a result of the two combined modes, the peak heat transfer is enhanced and the lateral cooling extent is broadened. The jet was subjected to forcing by introduction of numerical excitation at each side of the jet that modeled fluidic excitation. The jet was forced on both opposing sides at its exit, both with in-phase and out-of-phase modes. Under some conditions, out of phase forcing at Re=650 at the highest frequency leads to stabilization of the normally separated flow on one side only. This unusual asymmetric flow field is unsteady but repeatable, and results in an enhancement of the heat transfer. At Reynolds number of 750, forcing with an out of phase mode at the highest frequency leads to a complete stabilization of the jet. The forcing suppresses the high-amplitude low frequency flapping mode leaving only a high frequency vortex formation mode. The suppression of the jet flapping leads to a decrease in the peak heat transfer, but because separation is suppressed, the average wall heat transfer is enhanced.
On the Optimisation of a Double Cone Synchroniser for Improved Manual Transmission Shiftability
Authors: Gobbi Massimiliano , Mastinu Gianpiero , D’Orazio Augusto , Caudano Massimo , Faustini Giorgio
Published: May 2008

The paper presents a method to optimise the synchroniser of a road vehicle gearbox in order to improve shiftability and driver comfort. A multi-body physical model of the synchroniser has been developed and validated experimentally. The optimisation method is based on a Multi-objective Programming approach, and it allows to tune the thirty-two parameters of the synchroniser in order to achieve the desired dynamic behaviour of the system during a reference shift action, defined by seven performance indices. A Global Approximation procedure has been followed to solve numerically the optimisation problem. A special study has been performed and implemented in order to explore all of the feasible design solutions within the design variables domain. A global sensitivity method has been applied in order to analyse the relationships among the thirty-two design variables and the seven performance indices. Pareto-optimal design solutions have been computed in a very short time. These Pareto-optimal solutions have been checked for robustness by applying the minimum sensitivity method. The optimisation method has been applied with successful results. A number of optimised synchronisers have been defined, all of them featuring relevant improvements in the dynamic behaviour (shiftability) with respect to the reference synchroniser, aleady effective and under production.
Irreversible Thermodynamics for Damage Mechanics of Solid Materials
Authors: Basaran Cemal , Nie Shihua
Published: May 2008

In this paper a thermodynamic framework has been presented for damage mechanics of solids materials. Traditional damage mechanics theory uses damage potential function to trace damage evolution. In this framework entropy production is used as a measure of damage in the system. As a result there is no need for physically meaningless empirical material parameters to define a damage potential function. It is assumed that entropy production is non-negative for solids.
An Experimental Investigation for Flow Past a Circular Cylinder With Rectangular Strips
Authors: He Chuan , Long Tianyu , Xin Mingdao , Chung Benjamin T. F.
Published: May 2008

This paper reports an experimental investigation for fluid flow past a circular cylinder with two small rectangular strips and single sharp-edge strips on its surface. The experimental results reflected that different arrangements or dimensions of the strips produced significantly different effects on the flow. The forward step caused a stronger disturbance with a small increase in drag. The backward step arrangement softened the disturbance but reduced the drag coefficient by 33%.