bluuebunny/crossref_metadata_2025 huggingface dataset:
Ignition Delay Time and Laminar Flame Speed Calculations for Natural Gas/Hydrogen Blends at Elevated Pressures
Authors: Brower Marissa , Petersen Eric , Metcalfe Wayne , Curran Henry J. , Füri Marc , Bourque Gilles , Aluri Naresh , Güthe Felix
Published: July 2013

Applications of natural gas and hydrogen co-firing have received increased attention in the gas turbine market, which aims at higher flexibility due to concerns over the availability of fuels. While much work has been done in the development of a fuels database and corresponding chemical kinetics mechanism for natural gas mixtures, there are nonetheless few if any data for mixtures with high levels of hydrogen at conditions of interest to gas turbines. The focus of the present paper is on gas turbine engines with primary and secondary reaction zones as represented in the Alstom and Rolls Royce product portfolio.

The present effort includes a parametric study, a gas turbine model study, and turbulent flame speed predictions. Using a highly optimized chemical kinetics mechanism, ignition delay times and laminar burning velocities were calculated for fuels from pure methane to pure hydrogen and with natural gas/hydrogen mixtures. A wide range of engine-relevant conditions were studied: pressures from 1 to 30 atm, flame temperatures from 1600 to 2200 K, primary combustor inlet temperature from 300 to 900 K, and secondary combustor inlet temperatures from 900 to 1400 K. Hydrogen addition was found to increase the reactivity of hydrocarbon fuels at all conditions by increasing the laminar flame speed and decreasing the ignition delay time. Predictions of turbulent flame speeds from the laminar flame speeds show that hydrogen addition affects the reactivity more when turbulence is considered. This combined effort of industrial and university partners brings together the know-how of applied, as well as experimental and theoretical disciplines.
Simulation of Spray Autoignition Introducing “Spraylets”
Authors: Klinkov Konstantin , Eigenbrod Christian , Giese Frank , Rickmers Peter , Reimert Manfredo
Published: July 2013

Spray ignition is a complex combination of physical and chemical processes. Therefore there is no way to simulate all processes without significant simplifications. Dynamic and thermal effects of droplets in a two phase flow are respected through averaged properties of mass, momentum and energy. When modeling mixing of fuel vapor with the surrounding gas, the differences between diffusion rates of components as well as the finite penetration of the diffusion fluxes are not taken into account. Last seen in the fact that fuel vapor is uniformly mixed with the gas filling within the calculation cell, whose dimensions are several orders of magnitude larger than the droplet. As a consequence, the chemical reactions in the two-phase medium are represented by volume processes in the computational cell. Meanwhile, it is well known that the surrounding of an individual droplet or a group of droplets is characterized through significant inhomogeneity. Under such circumstances, self-ignition of fuel vapor cannot be considered as a process in a well stirred reactor of a size of the computational cell.

In this work a new approach for spray ignition simulation is presented. The simulation is split into two tasks. A CFD package is used to calculate spray injection and subsequent two-phase flow in the given conditions. From this two-phase flow the droplet track data and the gas phase parameters are extracted to serve as variable boundary conditions for the second task, the one dimensional single droplet ignition simulations, named “Spraylet”, which are fully transient and based on comprehensive chemistry. The obtained ignition delay times can be transformed into spatial distributions of ignition probability. While the CFD part is handling spray formation, turbulence, temperature, pressure and global vaporization, the spraylet calculations take care of droplet related physics and chemistry.

The spraylet model is based on the transient differential energy and mass conservation equations in the liquid and gas phases with variable physical properties. Also the concept of multi-component diffusion is applied. The effects of the presence of neighboring droplets in the flow, usually referred to as “spray” effects, is approximated by taking modifications of the conditions at the outer boundary of the computational domain into account. Thus the model considers the finite rates of diffusion and chemical reactions, as well as spray effects, and allows for spray ignition simulations to predict the most probable instants of ignition of the individually calculated droplet trajectories.

The validation of the spraylet-based simulation is performed by comparison with experimental data obtained in the hot-wind-tunnel. N-heptane as liquid fuel was injected into cross flow of air with a pressure of 5 bar and a temperature of 800 K.
Spray and Combustion Modeling in High Pressure Cryogenic Jet Flames
Authors: De Giorgi Maria Grazia , Sciolti Aldebara , Ficarella Antonio
Published: July 2013

The aim of the present work is the investigation of the combustion phenomenon in liquid-propellant rocket engines. The combustion of liquid oxygen and gaseous methane in a shear coaxial injector under supercritical pressure was analyzed. To realize an efficient numerical description of the phenomena, it is important to treat the LOx jet in a manner which takes into account its real behavior. In the present work different kinetics, combustion models and thermodynamics approaches were used in association with the description of the jet as a discrete phase. For all the approaches used, a comparison with experimental data from literature was performed.
Effects of the Mean Flow Field on the Thermo-Acoustic Stability of Aero-Engine Combustion Chambers
Authors: Gikadi Jannis , Sattelmayer Thomas , Peschiulli Antonio
Published: July 2013

This paper presents a finite element methodology to predict the thermoacoustic eigenmodes of combustion chambers using the linearized Navier-Stokes equations (LNSE) in frequency space. The effect of the mean flow on the acoustics is accounted for. Besides scattering and refraction of acoustic waves in shear layers, this set of equation describes two main damping mechanisms. One is related to the generation of entropy waves, so called hot-spots, in flame regions. The other is related to the transformation of acoustic energy into vorticity waves at sharp leading or trailing edges. Both fluctuation types, i.e. entropy and vorticity, are convected by the mean flow, leading to significant damping when the fluid discharges into an open outlet. In combustion chamber environments these waves are accelerated in the downstream high pressure distributor and are partially transformed back into acoustic waves constituting to the feedback loop of thermo-acoustic instabilities. Accurate prediction of the eigenmodes and eigenfrequencies of instability require therefore to take these interaction effects into account.

First, the accuracy of the LNSE approach, to capture the damping generated by the first mechanism of entropy generation and convection, is investigated for a generic premixed flame configuration. Solutions of the LNSE are compared to the analytic solutions as well as eigenvalues determined by an Helmholtz ansatz. Later methodology assumes a quiescent medium and neglects all interactions of acoustics with the mean flow. It is shown that large errors are introduced with increasing Mach-number.

To illustrate errors assuming a quiescent medium for realistic combustion chambers, the LNSE are used to assess the eigenmodes of a two-dimensional aero-engine combustor including strong shear regions, in the next step. The non-isothermal mean flow field is obtained performing an incompressible RANS simulation. It features an expanding jet with inner and outer recirculation zones. The acoustic computations using LNSE reveal a set of unstable and neutral hydrodynamic modes in addition to acoustic modes. Both damping mechanisms are present and contribute to the overall system stability. Again the obtained solution is compared to the solution of an Helmholtz code and differences are discussed.
Dynamics Suppression in Liquid-Fueled Combustors Using Fuel Modulation
Authors: Hong Zekai , Haynes Joel M. , Herbon John T. , McManus Keith R.
Published: July 2013

In the present work, an atmospheric pressure combustor using a modern aviation gas turbine fuel nozzle was used to demonstrate active combustion control. The combustor exhibited a low-frequency dynamics mode under fuel-rich conditions. A fast-response fuel valve was adapted as an in-line valve upstream of the nozzle for fuel modulation. Large fuel modulation amplitudes were achieved with the combination of the valve and the engine nozzle at frequencies exceeding 200 Hz. Open-loop flame response to fuel modulation was first examined when the instability mode was absent; for a range of inlet air temperatures, fuel flow rates, and combustor pressure drops. Simple open-loop control at discrete off-resonance frequencies was found ineffective in suppressing the instability mode. An advanced, fast algorithm was developed to enable closed-loop control. In this scheme, the entire fuel supply to the combustor was modulated with the control valve and injected through the fuel nozzle. The control algorithm commanded the fuel injector to produce a steady fuel flow, when the combustion was stable, or to modulate the fuel when the level of pressure oscillations in the combustion chamber became unacceptable. With an optimized control algorithm, an 88% reduction in the amplitude of the low-frequency dynamics mode was achieved.
Numerical Studies on the Influence of Periodical Flow Forcing on Mixing Quality and Flow Structure of a Swirl Burner
Authors: Schrödinger C. , Moeck J. P. , Paschereit C. O. , Oevermann M.
Published: July 2013

Helical coherent structures and strong axial mass flow fluctuations related to acoustic instabilities are common features in swirl-stabilized gas turbine combustors. The investigation of these dynamical phenomena, in particular their interaction, is therefore important. In the present work, the periodical excitation of the flow field of a swirl burner and its impact on coherent flow structures and mixing properties is examined. The effect of axial mass flow excitation with the dominant frequency of the coherent structure is studied. The investigations are conducted with unsteady RANS (URANS) and Large Eddy simulations (LES) for isothermal conditions to examine their capability of capturing the impact of forcing on the mixing and flow characteristics. Mean and turbulent flow fields as well as the concentration fields at the burner outlet are studied. The mixing quality is characterized by the degree of unmixedness, and the flow structures of the unforced and forced cases are analyzed by proper orthogonal decomposition (POD). With external forcing of the mean flow, the concentration field at the burner outlet becomes more homogeneous, i.e. the mixing quality increases. The unforced flow field exhibits a helical structure, which changes its characteristics when forcing is applied. The simulations (both URANS and LES) are able to reproduce the main features. URANS, however, strongly underpredicts the turbulence intensity in the recirculation zone and shear layers, and as a result, the kinetic energy distribution captured by the POD modes is not in agreement with experimental the findings. The results from LES are qualitatively and also quantitatively in good agreement with the experimental data.
Measurements and LES of a SGT-800 Burner in a Combustion Rig
Authors: Lörstad Daniel , Lindholm Annika , Barhaghi Darioush Gohari , Bonaldo Alessio , Fedina Ekaterina , Fureby Christer , Lantz Andreas , Collin Robert , Aldén Marcus
Published: July 2013

The Siemens gas turbine SGT-800 is the largest industrial gas turbine produced by Siemens Industrial Turbomachinery (SIT) offering a dry low emission (DLE) capability below 15 ppm NOx. It has a very high reliability using an annular combustor system with passive damping and 30 DLE burners. To obtain a greater understanding of the mixing process and the flame dynamics and in order to further reduce the emission levels, single burner rig tests have been performed. The laboratory measurements are complemented by Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes (RANS) simulations to further investigate the transient fuel distribution and subsequent flame behavior.

The measurements were performed jointly by SIT and Lund University using the SIT single burner combustion rig, where the square chamber allows great optical access in the flame region. The experimental data includes wall temperature, pressure fluctuations, light intensity variation and simultaneous Planar Laser Induced Fluorescence of OH and acetone. This investigation is complemented using fuel concentration field laser measurements of the fuel distribution upstream of the flame region in SIT water rig, using a burner partly made of Plexiglas to allow for optical access.

The LES model was developed jointly by SIT and FOI. The LES computations were performed using a combustion code developed from the OpenFOAM library utilizing the mixed subgrid flow model, complemented with a subgrid wall model. The reacting flow was simulated using a Finite Rate Chemistry (FRC) combustion model based on the Partially Stirred Reactor (PaSR) model. For this study, a two-step global/reduced methane-air reaction mechanism was employed to describe the combustion chemistry. The RANS simulations were performed with ANSYS Fluent, using the k-ε Realizable eddy viscosity turbulence model in combination with the Fluent partially premixed combustion model. This model is a combination of the Zimont flamelet progress variable model and a Probability Density Function based non-premixed combustion model.

The investigation includes a detailed evaluation of the numerical results compared to the measurement data. The numerical model includes the upstream air supply and fuel line systems up to well-defined constrictions to ensure appropriate acoustic inlet conditions. The measurements reveal large fluctuations in the flame region, which has been investigated using LES.
Influence of the Air Preheat Temperature and the Fuel Preheat Temperature in the Adiabatic Flame Temperature for Gaseous Fuels of Low Heating Value
Authors: Tourlidakis A. , Malkogianni A.
Published: July 2013

Adiabatic flame temperature is of significant importance for the design of a GT combustor, as it is the temperature under the condition of no heat loss takes place from the combustion system. This importance arises from the fact that it plays an important role in the pollutants emitted from the system, such as carbon oxides and nitrogen oxides. Additionally, the temperature also affect the thermal stresses set up in the combustion system, such stress may lead to the deterioration of the chamber if not well controlled. Consequently, it is essential before the construction of the combustion chamber, a simulation process for the temperature distribution within the combustion system to be carried out, in order to avoid local thermal stresses and to minimize nitrogen oxides and carbon dioxides emissions, pollutants of great concern, that are very dependent on the flame temperature. The factors that predominantly affect the adiabatic flame temperature are the fuel heating value, the type of oxidant, FAR, the temperature of the reactants, the amount of oxygen in the air, as well as the dissociation phenomena. In this study, a code in FORTRAN programming language is developed for the calculation of the adiabatic flame temperature. The simulation is performed for different gaseous fuels of low calorific value, for air preheat, for fuel preheat, as well as for various Φ.

From the simulation resulted that Tad and Φ for each fuel are totally dependent on the fuel’s calorific value. Both for the case of the air preheat, and the fuel preheat temperature it was observed increase of the Tad. Preheating of combustible mixture by recycling heat from flue gases has been considered an effective technology not only for combustion of low calorific fuels but also for fuel conservation.
Analysis and Scalings of Blowoff Limits of 2D and Axisymmetric Bluff Body Stabilized Flames
Authors: Foley Christopher W. , Seitzman Jerry , Lieuwen Tim
Published: July 2013

This paper considers shear layer flame stabilization with a particular focus on velocity scaling of blowoff limits. Analysis of the expression for hydrodynamic flame stretch, κ, in a shear layer shows that it consists of two contributions, associated with normal and shearing flow strain. These two contributors lead to flame stretch scalings of SL/δ and U/L, respectively, where δ and L denote shear layer thickness and characteristic geometric length scale. These two flame stretch terms have different velocity and length scalings (roughly U1/2 and U1, respectively) and so different blowoff trends can be expected depending upon which term dominates. These scalings are used to interpret a variety of bluff body blowoff data in the literature by analyzing the velocity and length scale dependence of extinction stretch rates calculated at the measured blowoff conditions. We also show that the measured velocity sensitivities to chemical time at blowoff range from U−0.3 to U−1.6. A key point of this study is that blowoff boundaries do not necessarily follow a U−1 scaling suggested by classical Damköhler number scalings and that more work is needed to understand the controlling extinction processes of near-blowoff flames.
Thermal Characteristics of Graphite Foam Thermosyphon for Electronics Cooling
Authors: Roh Hongkoo , Kim Jungho , Boudreaux Paul J.
Published: July 2013

Graphite foams consist of a network of interconnected graphite ligaments and are beginning to be applied to thermal management of electronics. The thermal conductivity of the bulk graphite foam is similar to aluminum, but graphite foam has one-fifth the density of aluminum. This combination of high thermal conductivity and low density results in a specific thermal conductivity about five times higher than that of aluminum, allowing heat to rapidly propagate into the foam. This heat is spread out over the very large surface area within the foam, enabling large amounts of energy to be transferred with relatively low temperature difference. For the purpose of graphite foam thermosyphon design in electronics cooling, various effects such as graphite foam geometry, sub-cooling, working fluid effect, and liquid level were investigated in this study. The best thermal performance was achieved with the large graphite foam, working fluid with the lowest boiling point, a liquid level with the exact height of the graphite foam, and at the lowest sub-cooling temperature.
Antenna Miniaturization Using Flexible PBG Materials
Authors: Salonen Pekka , Syda¨nheimo Lauri , Keskilammi Mikko
Published: July 2013

Antennas play a paramount role today’s communication centered market place. Recently the demands for miniaturization of electronic devices have increased rapidly in which a miniaturization of integrated antennas has confronted the same development. However, antennas and especially the performance parameters of antennas obey physical laws in which the electrical dimensions of an antenna have a major effect on these parameters such as voltage standing wave ratio (SWR) and radiation efficiency. Recently, a new, multidisciplinary field of study called “Electromagnetic BandGap” (EBG) structures have been developed. An EBG structure forms a lattice whose period determines its resonant frequency i.e. the range of frequencies where the stop band exists for transmission of microwave signals. Antennas physical dimensions can be made noticeably smaller applying EBG materials. These advantages of EBG structures allow us to design smaller antennas with high radiation efficiency on high-dielectric substrates such as ceramics. This paper presents how conventional microstrip antennas can be miniaturized using EBG materials with improved performance parameters. In addition, a novel flexible antenna is presented which can be rolled up during e.g. transportation.
Design and Reliability Analysis of Wafer Level Package With Bubble-Like Buffer Layer
Authors: Lee Chang-Chun , Chiang Kuo-Ning
Published: July 2013

In order to enhance the wafer level package (WLP, Figure 1) reliability for larger chip size, many different kinds of WLP have been adopted, all have a compliant layer under the pads have to relieve the thermal stress of the solder joint. Usually, the solder joint reliability is enhanced with the increase of the thickness of the compliant layer. However, the fabrication processes of the WLP restrict the thickness of the compliant layer. With that in mind this research proposed a novel WLP package with bubble-like buffer layer (Figure 2) which is composed of a bubble-like plate and a buffer layer between the chip and the solder joint. The main goal of this research was to study the effects of the geometric dimensions and material properties of the bubble-like layer on the reliability of the WLP. For the parametric analysis purpose, a 2-D nonlinear finite element analysis for the proposed WLP was conducted. The results revealed that both the bubble-like plate and the buffer layer provide excellent compliant effects. However, the buffer layer has a more significant effect on enhancing the solder joint reliability. Also, for a WLP with buffer structure, the effect of the chip thickness on the reliability could be significantly reduced. In addition, the difference between the filled and non-filled buffer layers also affected the reliability of the solder joint. The results revealed that the WLP with the buffer layer and the no-fill bubble-like plate had the better reliability.
Comparing Random Vibration Inputs: Power Spectral Density (PSD) Versus Root Mean-Square Acceleration (Grms)
Authors: Straznicky Ivan
Published: July 2013

Many defense programs have vibration requirements for electronics which are often specified as random vibration input. Often, this input is based on measurements taken at the locations of interest for the spectrum of vehicle operating environments. The resulting specification is typically several power spectral density, or PSD, curves with associated durations. The root mean square acceleration, or Grms, can be readily calculated for each PSD curve. Grms values are sometimes used to compare different PSD curves for severity. However, this can be misleading. The impacts of two different random vibration inputs, with the same Grms value, can be very different. By calculating fatigue damage values for various components on a circuit card assembly subjected to these inputs, it can be shown that equal Grms values do not result in equal damage. In fact, there can be two orders of magnitude difference in component damage values. This means that Grms values are very poor indicators of random vibration effect, and should not be used for comparison purposes.
Humidity Control Simulation for Electronics Packaging
Authors: Myers David G.
Published: July 2013

An important aspect of environmental control is the protection of equipment from the harmful effects of humidity. It is often desirable to desiccate a sealed volume in order that it satisfy a dryness requirement over the duration of its service life. Desiccant sizing, along with proper sealing, become important design elements in ensuring that such a requirement is achieved. To this end, Raytheon has developed a simulation tool to analyze and predict the humidity within sealed volumes. The pertinent physical principles that are modeled are permeation of water vapor by diffusion, pneumatic leakage, the hygroscopic properties of sealed materials, desiccant characteristics, and the kinetic model of ideal gas mixtures. The simulation has the capability of imposing any external environmental condition, consisting of diurnal cycles of temperature and humidity. The physical principles which underlie the simulation are described. An example of simulation results, design decisions and lessons-learned are presented.
Smart Cooling of Data Centers
Authors: Patel Chandrakant D. , Bash Cullen E. , Sharma Ratnesh , Beitelmal Monem , Friedrich Rich
Published: July 2013

The data center of tomorrow is characterized as one containing a dense aggregation of commodity computing, networking and storage hardware mounted in industry standard racks. In fact, the data center is a computer. The walls of the data center are akin to the walls of the chassis in today’s computer system. The new slim rack mounted systems and blade servers enable reduction in the footprint of today’s data center by 66%. While maximizing computing per unit area, this compaction leads to extremely high power density and high cost associated with removal of the dissipated heat. Today’s approach of cooling the entire data center to a constant temperature sampled at a single location, irrespective of the distributed utilization, is too energy inefficient. We propose a smart cooling system that provides localized cooling when and where needed and works in conjunction with a compute workload allocator to distribute compute workloads in the most energy efficient state. This paper shows a vision and construction of this intelligent data center that uses a combination of modeling, metrology and control to provision the air conditioning resources and workload distribution. A variable cooling system comprising variable capacity computer room air conditioning units, variable air moving devices, adjustable vents, etc. are used to dynamically allocate air conditioning resources where and when needed. A distributed metrology layer is used to sense environment variables like temperature and pressure, and power. The data center energy manager redistributes the compute workloads based on the most energy efficient availability of cooling resources and vice versa. The distributed control layer is no longer associated with any single localized temperature measurement but based on parameters calculated from an aggregation of sensors. The compute resources not in use are put on “standby” thereby providing added savings.
Silicone Phase Change Thermal Interface Materials: Properties and Applications
Authors: Zhang S. Mark , Swarthout Diane , Noll Thomas , Gelderbloom Susan , Houtman Douglas , Wall Kelly
Published: July 2013

Thermal interface materials (TIM) play a very important role in effectively dissipating unwanted heat generated in electronic devices. This requires that the TIM should have a high bulk thermal conductivity, intimate contact with the substrate surfaces, and the capability to form a thin bond line. In designing new TIMs to meet these industry needs, alkyl methyl siloxane (AMS) waxes have been studied as phase change matrices. AMS waxes are synthesized by grafting long chain alpha-olefins on siloxane polymers. The melting point range of the silicone wax is determined by the hydrocarbon chain length and the siloxane structure. When the AMS wax is mixed with thermally conductive fillers such as alumina, a phase change compound is created. The bulk thermal conductivities of the phase change material (PCM) are reduced as they go through the phase change transition from solid to liquid. By coating the PCM onto an aluminum mesh, both the mechanical strength and the thermal conductivity are drastically improved. The thermal conductivity increases from 4.5 W/mK for the PCM without aluminum support to 7.5 W/mK with the supporting mesh. The thermal resistance of the aluminum-supported sheet at a bond line thickness of 115 microns has been found to be ∼0.24 cm2-C/W. Applying pressure at the time of application has a positive effect on the thermal performance of the PCM. Between contact pressures of 5–80 psi, the thermal resistance decreases as the pressure increases. The weak mechanical strength of the phase change material turns out to be a benefit when ease of rework and the effects of shock and vibration during shipping and handling are considered. A stud pull test of the aluminum mesh-supported PCM shows an average of 13 psi stress at the peak of the break.
Micro Heat Pipes for Stacked 3D Microelectronic Modules
Authors: Khandekar Sameer , Groll Manfred , Luckchoura Vivak , Findl Walter , Zhuang Jun
Published: July 2013

In context of a European Commission funded project, development of a standardized multifunctional stacked 3D package was envisioned for potential applications in aviation, space and telecommunication sectors. The standardization and modularity was aimed to integrate packages from different technologies and to allow mutual slice inter-changeability. Thermal management solutions to the proposed new stacked 3D package as per the project specifications (a total of three stacked substrate slices, each slice of size 55 × 55 × 1 mm3 and total package height not exceeding 10.5 mm) are reported here. Three potential options were studied i.e. (a) module liquid cooling, (b) integration of miniature copper-water cylindrical heat pipes (OD 3.0 mm) with the 1.0 mm substrate slice and (c) development of flat plate heat pipes of 0.9 mm thickness. For options (a) and (b), initial tests have been performed taking aluminum as a representative material for AlSiC metal matrix composites which were to be employed in the final design. Further, copper based flat plate micro-structure conventional heat pipes have been developed and performance tested. Thermal interactions have been investigated with thermocouples coupled with infrared thermography. For safe operation up to 30W heating power (10W/slice), while thermal diffusion through the bare metallic substrate is sufficient for heat transfer from chip to the substrate, micro heat pipes should be employed to cool the substrate and transfer heat from it to an external cold plate. Flat plate heat pipes are advantageous for higher power levels per slice. Interlayer thermal interactions also affect the response of stacked 3D packages.
Advanced Thermal Interface Materials Using Natural Graphite
Authors: Smalc Martin , Norley Julian , Reynolds R. Andy , Pachuta Richard , Krassowski Dan W.
Published: July 2013

Expanded flexible graphite sheet materials have become attractive as Thermal Interface Materials (TIM’s). Flexible graphite sheet materials were originally developed as gaskets for fluid sealing applications. Properties that make flexible graphite sheets of interest as TIM’s include their relatively high thermal conductivities and their ability to conform well to surfaces. Specific grades of flexible graphite have been developed for TIM applications. Because flexible graphite sheets are porous, with open interconnected pores, it is possible to impregnate these sheets with various materials to improve properties. In particular, various polymers, such as mineral oils, synthetic oils, etc can be added to flexible graphite to improve its performance in thermal interface applications. This paper will review the thermal properties of TIM’s made by adding two different polymeric materials to a grade of flexible graphite specifically developed as a thermal interface material.
Heat Transfer in Triangular Microchannels
Authors: Dickey J. Todd , Lam Tung T.
Published: July 2013

A numerical heat transfer solution is compared with an analytical solution for a microchannel flow. The analytical derivation is based upon the porous material assumption as put forth by various investigators. While extensive work exists for the rectangular microchannel cross sectional area, other cross sections have not received the same attention. It is the intent of this paper to investigate the applicability of the porous material assumption to a triangular “saw tooth” cross section microchannel with respect to heat transfer and fluid flow characteristics. The results are presented in nondimensionalized form applicable to any fluid and geometric aspect ratio combination presented herein.
Characterization of A6 LTCC Systems for Microwave and Millimeter-Wave Applications
Authors: Chai Liang , Shaikh Aziz , Stygar Vern
Published: July 2013

Low Temperature Co-fired Ceramic (LTCC) materials are widely used in ceramic packaging and interconnect applications, such as RF receiving modules for next generation handsets and T/R modules for satellite communications. The application frequency spans from low GHz to 100+ GHz. A variety of techniques must be used to characterize dielectric properties (loss tangent and dielectric constant) of LTCC in the wide frequency range. These techniques include the parallel plate capacitor method from DC to several MHz, cavity methods in x-band, and open resonator techniques for frequencies up to 100+ GHz. LTCC is a multi-layer ceramic technology, which offers capability for low cost manufacturing, realizing 3D structures, and embedding passive components. Both strip line and microstrip types of transmission line structures can be conveniently manufactured using LTCC material systems. As an important performance parameter, microstrip loss can be characterized using the ring resonator technique. Both LTCC and metal conductors contribute to the microstrip loss. At microwave and mm-wave frequencies, metal losses become significant, although low resistance silver conductor can be used. The metal loss may be divided into two categories: one is the resistive loss and other is surface roughness loss. A photo imaging technique has been introduced which patterns the microstrip, reducing the surface loss. In this paper, the techniques for measuring both dielectric properties of A6 LTCC and microstrip properties of A6 LTCC with conductor systems will be reviewed. Characterization results of the dielectric constant and loss tangent of A6 from near DC to 100 GHz will be presented. Microstrip losses, measured for A6 LTCC with both gold and silver metallization up to 40 GHz, are compared to those computed using the closed form equations and experimentally determined dielectric and conductor properties. The calculated effective microstrip permitivity and attenuation compare well to those from measurements.
Performance Comparison of Mesoscale Refrigeration Technologies for Electronics Packaging
Authors: Phelan Patrick E. , Chiriac Victor , Lee Tien-Yu Tom
Published: July 2013

Utilizing refrigeration is one alternative to improve the operating performance of microelectronic devices. Unless large-scale refrigeration systems are acceptable, it is likely that miniature, or mesoscale, refrigerators will be required that can be directly incorporated into electronic packaging. The present study builds on the authors’ previous investigation of mesoscale refrigerators, which emphasized how only thermoelectric coolers (TEC’s) are capable of being miniaturized in the near future. Many other refrigeration systems, however, are being examined for their potential to be miniaturized, including the vapor-compression, Stirling, and pulse-tube cycles. Here a simple analysis is presented that shows the feasibility of utilizing these systems in electronics packaging. One finding is that, provided a suitable mesoscale compressor can be constructed, a vapor-compression refrigerator may yield the best performance of the investigated systems.
Numerical Study and Experimental Validation of the Thermal Performance for a Parallel Channel Optical Module
Authors: Shi Z. F. , Lu Albert C. W. , Tan Eric , Tan Ronson
Published: July 2013

To meet the insatiable demand for data bandwidth in VSR (very short reach up to 300m) applications including server and routers, parallel optical interconnection offers a promising solution in terms of performance and cost effectiveness. A 12-channel pluggable paralle optical transmitter module has been developed to achieve a data rate of 2.5 Gb/s per channel. To maintain the robustness of the optical signal integrity under different environmental conditions, the thermal management is crucial. In this paper the thermal performance evaluation of the optical module was carried out through both numerical simulation and experimental verification. The optical module mainly consists of a VCSEL (vertical cavity surface emitting laser) array, a driver IC and a heat sink. Three types of heat sinks were integrated into the transmitter module separately. The thermal environments used for this evaluation include the normal and high ambient temperature, and both still-air and forced-air conditions. The ambient temperature and the wind speed were controlled by using a Wind Tunnel. The simulation was performed by using a CFD (computational fluid dynamics) program. In all the three modules, the simulation and experimental results of the junction temperature have shown good agreements. For Module 1 under the high ambient temperature, a forced-air condition was required to keep the junction temperature below 70°C. For Module 2 and Module 3, the junction temperature can be controlled below 70°C even under the high ambient temperature without using a fan.
Flip Chip Thermal Test Vehicle Design: Requirements, Evaluations, and Validations
Authors: Goh Teck Joo , Chiu Chia-Pin , Seetharamu K. N. , Quadir G. A. , Zainal Z. A.
Published: July 2013

This paper reviews the design of a flip chip thermal test vehicle. Design requirements for different applications such as thermal characterization, assembly process optimization, and product burn-in simulation are outlined. The design processes of different thermal test chip structures including the temperature sensor and passive heaters are described in detail. In addition, the design of fireball heater, a novel test chip structure used for evaluating the effectiveness of heat spreading of advanced thermal solutions, is also illustrated. The design considerations and processes of the package substrate and printed circuit board with special emphasis on the physical routing of the thermal test chip structures are described. These design processes are supported with thermal data from various finite-element analyses (FEA) carried out to evaluate the capability and limitations of thermal test vehicle design. Design optimization as the outcome of these analyses is also elaborated. Lastly, the validation and calibration procedures of the thermal test vehicle are presented in this paper.
Constructing a Trade-Off Surface for Extruded Heat Sinks Exposed to Forced Convection
Authors: de Kock D. J. , Visser J. A.
Published: July 2013

In modern electronic components power densities are being increased continuously while the size and weight decrease. The effective dissipating of the heat produced by these components has now become a major design problem. Ordinary heat sinks often used to dissipate this heat, can in many instances no longer be used. Heat sinks therefore need to be designed and optimized for specific applications. The design of these heat sinks requires a difficult trade-off between conflicting parameters, e.g. mass or material cost, maximum temperature and pressure drop. Since these parameters influence one another, optimum designs require the use of mathematical optimization techniques. In the case of heat sinks, the thermal engineer would typically like to optimize the design simultaneously for three design parameters. The parameters are maximum heat sink temperature, mass and pressure drop. In the formulation of such an optimization problem, where more than one design criterion is important, the engineer currently has to assign the relative importance of each design criteria before starting the optimization. A better approach is to perform a range of optimization problems where the relative importance of the design criteria is varied systematically to obtain a trade-off surface of optimum heat sinks. This surface can then be used to investigate the influence of the different design criteria on each other and to select the optimum heat sink for a specific application. In this study such a trade-off surface is created for an extruded heat sink exposed to forced convection. The constructing of this surface is obtained by combining a semi-empirical simulation program, QFin 3.0 with the DYNAMIC-Q optimization method.
A Novel Microelectronics Cooling Technique Using a Pair of Unsteady Confined Impinging Air Jets
Authors: Rosales Jorge L. , Chiriac Victor A.
Published: July 2013

The unsteady laminar flow and heat transfer characteristics for a pair of confined impinging air jets centered in a channel were studied numerically. The time-averaged heat transfer coefficient for a pair of heat sources centered in the channel and aligned with the jets was determined as well as the oscillating jet frequency for the unsteady cases. The present study continues the authors’ previous investigation, which emphasized how single confined jets will remain steady at Reynolds numbers that make side-by-side jets highly unsteady. The nature of this unsteadiness depends on the proximity of the jet inlets, the channel dimensions and the jet Reynolds number. The jet unsteadiness causes the stagnation point locations to sweep back and forth over the impingement region, and the jets “wash” a larger surface area on the target wall. The results indicate that the dual jets become unsteady between a Reynolds number of 200 and 300. Also, in the range of Reynolds numbers studied, a fixed stagnation “bubble” was formed on the target wall between the two jets, which reduced the heat transfer removal from that region, leading in fact to a quasi-independence of the local heat transfer on flow conditions. The stagnant region contains slow moving warm air that forces the cool impinging air jets to flow to the sides of this target wall area. The oscillating frequency of the flow increases with Reynolds number for the unsteady cases. Also, the time-averaged heat transfer coefficient on the heat sources rises as the Reynolds number increases for the steady cases but there is a slight decrease when it transitions to unsteady flow, indicating again that the stagnation “bubble” occurring between the two heat sources affects the local heat transfer.
Boundary Effect on Temperature Gradients in Microchannels
Authors: Cabral Patrick S. , Lam Tung T.
Published: July 2013

The effect of boundary conditions on heat transfer in rectangular microchannels is investigated. Of primary interest in this study is the influence of the boundary conditions on the temperature gradients in such geometries. The top of the microchannel under consideration is exposed to a uniform heat flux, which simulates the energy dissipated from electronic chips. Cooling fluid is allowed to flow through channels underneath the heat source. The physical configuration is subjected to various boundary conditions (e.g., first, second, and third kinds) and any combination of convection and radiation energy exchanges between the exposed surfaces and the external environment. The current configuration covers typical situations as found in most electronic cooling systems. The influences of the heat source strength, the external environment, and the aspect ratio are studied. The effects of varying these factors in a range typical of electronics thermal management applications are investigated. Temperature gradients along the fluid flow direction and their effect on the fluid resistance are presented.
System-Level Transient Thermal Analysis for Performance Optimization of High Power Microelectronics
Authors: Chiriac Victor Adrian , Lee Tien-Yu Tom , Chen H. S.
Published: July 2013

The increasing trend in power levels and densities leads to the need of design thermal optimization, at either module or system level. A numerical study using finite-volume software was conducted to model the transient thermal behavior of a system including a package dissipating large amounts of power over short time durations. The system is evaluated by choosing the appropriate heat sink for the efficient operation of the device under 100W of constant powering, also to enhance the thermal performance of the enclosure/box containing the test stack-up. The intent of the study is to provide a meaningful understanding and prediction of the high transient powering scenarios. The study focuses on several powering and system design scenarios, identifying the main issues encountered during a normal device operation. The power source dissipates 100W for 2 seconds then is cooled for another 2 seconds. This thermal cycle is likely to occur several times during a normal test-up, and it is the main concern of the manufacturers not to exceed a limit temperature during the device testing operation. The transient trend is further extrapolated analytically to extract the steady state peak temperature values, in order to maintain the device peak temperatures below 120°C. The benefit of the study is related to the possibility to extract the maximum/minimum temperatures for a real test involving a large number of heating-cooling cycles, yet maintaining the initial and peak temperatures within a certain range, for the optimal operation of the device. The flow and heat transfer fields are thoroughly investigated. By using a combination of numerical and analytical study, the thermal performance of the device undergoing infinity of periodic thermal cycles is further predicted.
System and Package Level Thermal Optimization of Power Amplifier Modules With Application in Wireless Communication
Authors: Chiriac Victor Adrian , Lee Tien-Yu Tom , Lutz David
Published: July 2013

Increased functionality of microelectronic packages for commercial applications leads to the necessity of identifying packaging solutions with high standards for thermal performance, during its functioning lifetime as well as during various test conditions. A detailed numerical analysis examines the thermal characteristics of a power amplifier module for time division multiple access (TDMA), using commercially available software. The increasing trend in power levels and densities leads to the need of design thermal optimization, either at module level or system level. Under specific test conditions, the thermal performance of the module degrades gradually; therefore, alternative test designs are investigated for thermal performance optimization. Initial study focuses on assessing the thermal performance of a baseline design. The peak temperature reaches 144°C, about 60°C temperature increase over the reference temperature. The peak temperature value is below the limit of 150°C. Further investigation focuses on several systems level designs, by incorporating individual test contactors between the DUT and load board or with conductive elastomers or pedestal solid ground slug for thermal performance enhancement. The peak temperatures are calculated in this case for the system being exposed to the ambient at 85°C. The results indicate that the test design with solid ground slug provides the best thermal performance, ∼ 5% better than the other designs. The small difference between the first two designs (with individual contactors and separate solid ground slug with conductive elastomer) resides in the fact that the elastomer has a small thickness (0.25mm), thus a low thermal resistance (based on thermal conductivity greater than 1W/mK), with minimal impact on the overall thermal performance of the TDMA under current test conditions. The temperature difference between the top section of the contactor designs with the CBC pin/copper block/pedestal is small; in spite of this, the high temperature reached by the individual CBC pins induces possible failures in the elastomer. The designs with pedestal and solid ground slugs have a notable advantage over the design with individual contactors, due to no moving parts within the elastomer, being more robust. The peak temperature reached by the module under the best/worst testing scenarios varies by ∼ 4–5%.