bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase Stability and Sintering Behavior of 10mol%Sc2O3–1mol%CeO2–ZrO2 Ceramics
Authors: Yarmolenko Sergey , Sankar Jag , Bernier Nicholas , Klimov Michael , Kapat Jay , Orlovskaya Nina
Published: March 2009

The phase composition and sintering behavior of two commercially available 10mol%Sc2O3–1mol%CeO2–ZrO2 ceramics produced by Daiichi Kigenso Kagaku Kogyo (DKKK) and Praxair have been studied. DKKK powders have been manufactured using a wet coprecipitation chemical route, and Praxair powders have been produced by spray pyrolysis. The morphology of the powders, as studied by scanning electron microscopy, has been very different. DKKK powders were presented as soft (∼100μm) spherical agglomerates containing 60–100nm crystalline particles, whereas the Praxair powders were presented as sintered platelet agglomerates, up to 30μm long and 3–4μm thick, which consisted of smaller 100–200nm crystalline particles. X-ray diffraction analysis has shown that both DKKK and Praxair powders contained a mixture of cubic (c) and rhombohedral (r) phases: 79% cubic +21% rhombohedral for DKKK powders and 88% cubic +12% rhombohedral for Praxair powders. Higher quantities of the Si impurity level have been detected in Praxair powder as compared to DKKK powder by secondary ion mass spectroscopy. The morphological features, along with differences in composition and the impurity level of both powders, resulted in significantly different sintering behaviors. The DKKK powders showed a more active sintering behavior than of Praxair powders, reaching 93–95% of theoretical density when sintered at 1300°C for 2h. Comparatively, the Praxair powders required high sintering temperatures at 1500–1600°C. However, even at such high sintering temperatures, a significant amount of porosity was observed. Both DKKK and Praxair ceramics sintered at 1300°C or above exist in a cubic phase at room temperature. However, if sintered at 1100°C and 1200°C, the DKKK ceramics exist in a rhombohedral phase at room temperature. The DKKK ceramics sintered at 1300°C or above exhibit cubic to rhombohedral and back to cubic phase transitions upon heating at a 300–500°C temperature range, while Praxair ceramics exist in a pure cubic phase upon heating from room temperature to 900°C. However, if heated rather fast, the cubic to rhombohedral phase transformation could be avoided. Thus it is not expected that the observed phase transitions play a significant role in developing transformation stresses in ScCeZrO2 electrolyte upon heating and cooling down from the operation temperatures.
Conversion of Syngas From Biomass in Solid Oxide Fuel Cells
Authors: Karl Jurgen , Frank Nadine , Karellas Sotirios , Saule Mathilde , Hohenwarter Ulrich
Published: March 2009

Conversion of biomass in syngas by means of indirect gasification offers the option to improve the economic situation of any fuel cell system due to lower costs for feedstock and higher power revenues in many European countries. The coupling of an indirect gasification of biomass and residues with highly efficient solid oxide fuel cell (SOFC) systems is therefore a promising technology for reaching economic feasibility of small decentralized combined heat and power production (CHP).The predicted efficiency of common high temperature fuel cell systems with integrated gasification of solid feedstock is usually significantly lower than the efficiency of fuel cells operated with hydrogen or methane. Additional system components like the gasifier as well as the gas cleaning reduce this efficiency. Hence common fuel cell systems with integrated gasification of biomass will hardly reach electrical efficiencies above 30%. An extraordinary efficient combination is achieved in case that the fuel cells waste heat is used in an indirect gasification system. A simple combination of a SOFC and an allothermal gasifier enables then electrical efficiencies above 50%. However, this system requires an innovative cooling concept for the fuel cell stack. Another significant question is the influence of impurities on the fuel cell degradation. The European Research Project “BioCellus” focuses on both questions—the influence of the biogenous syngas on the fuel cells and an innovative cooling concept based on liquid metal heat pipes. First experiments showed that, in particular, higher hydrocarbons—the so-called tars—do not have any significant influence on the performance of SOFC membranes. The innovative concept of the TopCycle comprises to heat an indirect gasifier with the exhaust heat of the fuel cell by means of liquid metal heat-pipes. Internal cooling of the stack and the recirculation of waste heat increases the system efficiency significantly. This concept promises electrical efficiencies of above 50% even for small-scale systems without any combined processes.
Characterization of Process for Embedding SiC Fibers in Al 6061 O Matrix Through Ultrasonic Consolidation
Authors: Li Dezhi , Soar Rupert C.
Published: March 2009

In this paper, continuous SiC fibers were embedded in an Al 6061 O matrix through ultrasonic consolidation at room temperature. The optimum embedding parameters were determined through peel tests and metallographic analysis. The influence of the embedded fiber volume fraction and base metal thickness on the interface bond strength was studied, and the fiber/matrix bond strength was tested through fiber pullout test. The results showed that embedding ≥0.8% volume fraction of SiC fiber in a 6061 O matrix could significantly increase and even its interfacial strength, but there is a threshold for embedded fiber volume fraction at specific parameters, over which the plastic flow and friction may be insufficient to have a strong bond at foil/foil interfaces between fibers. The study also showed that base metal thickness did not have significant influence on the interfacial strength with an exception of samples with a base metal thickness of 500 μm. Based on the results, it was proposed that microfriction at consolidation interfaces plays an important role for joint formation, and localized plastic flow around fibers is important to have fibers fully and safely embedded.
The Patient-Specific Brace Design and Biomechanical Analysis of Adolescent Idiopathic Scoliosis
Authors: Nie Wen-Zhong , Ye Ming , Liu Zu-De , Wang Cheng-Tao
Published: March 2009

Brace application has been reported to be an effective approach in treating mild to moderate idiopathic adolescent scoliosis. However, little attention is focused on the biomechanical study of patient-specific brace treatment. The purpose of this study was to propose a design method of personalized brace and to analyze its biomechanical behavior and to compare the brace forces with the I-Scan measurement system. Based on a three-dimensional patient-specific finite element model of the spine, rib cage, pelvis, and abdomen, a parametric patient-specific model of a thoracolumbosacral orthosis was built. The interaction between the torso and the brace was modeled by surface-to-surface contact interface. Three standard strap tensions (20 N, 40 N, and 60 N) were loaded on the back of the brace to simulate the strap tension. The I-Scan distribution pressure measurement system was used to measure the different region pressures, and the equivalent forces in these regions were calculated. The spinal curve changes and the forces acted on the brace generated by the strap tension were evaluated and compared with the measurement. The reduction in the coronal curvature was about 60% for a strap tension of 60 N. The sacral slope and the lordosis were partially reduced in this case, but the kyphosis had no obvious change. The brace slightly modified the axial rotation at the apex of the scoliotic curve. The forces generated in finite element analysis were approximately in good agreement with the measurement. The design and biomechanical analysis methods of patient-specific brace should be useful in the design of more effective braces.
A Discrete Element Approach for Modeling Cage Flexibility in Ball Bearing Dynamics Simulations
Authors: Weinzapfel Nick , Sadeghi Farshid
Published: March 2009

A model for deep-groove and angular-contact ball bearings was developed to investigate the influence of a flexible cage on bearing dynamics. The cage model introduces flexibility by representing the cage as an ensemble of discrete elements that allow deformation of the fibers connecting the elements. A finite element model of the cage was developed to establish the relationships between the nominal cage properties and those used in the flexible discrete element model. In this investigation, the raceways and balls have six degrees of freedom. The discrete elements comprising the cage each have three degrees of freedom in a cage reference frame. The cage reference frame has five degrees of freedom, enabling three-dimensional motion of the cage ensemble. Newton’s laws are used to determine the accelerations of the bearing components, and a fourth-order Runge–Kutta algorithm with constant step size is used to integrate their equations of motion. Comparing results from the dynamic bearing model with flexible and rigid cages reveals the effects of cage flexibility on bearing performance. The cage experiences nearly the same motion and angular velocity in the loading conditions investigated regardless of the cage type. However, a significant reduction in ball-cage pocket forces occurs as a result of modeling the cage as a flexible body. Inclusion of cage flexibility in the model also reduces the time required for the bearing to reach steady-state operation.
Experimental Analysis of Rigid Body Motion. A Vector Method to Determine Finite and Infinitesimal Displacements From Point Coordinates
Authors: Page Álvaro , de Rosario Helios , Mata Vicente , Atienza Carlos
Published: March 2009

This paper presents a vector method for measuring rigid body motion from marker coordinates, including both finite and infinitesimal displacement analyses. The common approach to solving the finite displacement problem involves the determination of a rotation matrix, which leads to a nonlinear problem. On the contrary, infinitesimal displacement analysis is a linear problem that can be easily solved. In this paper we take advantage of the linearity of infinitesimal displacement analysis to formulate the equations of finite displacements as a generalization of Rodrigues’ formula when more than three points are used. First, for solving the velocity problem, we propose a simple method based on a mechanical analogy that uses the equations that relate linear and angular momenta to linear and angular velocities, respectively. This approach leads to explicit linear expressions for infinitesimal displacement analysis. These linear equations can be generalized for the study of finite displacements by using an intermediate body whose points are the midpoint of each pair of homologous points at the initial and final positions. This kind of transformation turns the field of finite displacements into a skew-symmetric field that satisfies the same equations obtained for the velocity analysis. Then, simple closed-form expressions for the angular displacement, translation, and position of finite screw axis are presented. Finally, we analyze the relationship between finite and infinitesimal displacements, and propose vector closed-form expressions based on derivatives or integrals, respectively. These equations allow us to make one of both analyses and to obtain the other by means of integration or differentiation. An experiment is presented in order to demonstrate the usefulness of this method. The results show that the use of a set of markers with redundant information (n>3) allows a good accuracy of measurement of kinematic variables.
Steady State Performance Characteristics of Single Pad Externally Adjustable Fluid Film Bearing in the Laminar and Turbulent Regimes
Authors: Shenoy B. S. , Pai R.
Published: March 2009

In an externally adjustable fluid film bearing, the hydrodynamic conditions can be changed as required in a controlled manner. The principal feature of the bearing is the facility to adjust its radial clearance and circumferential film thickness gradient. Unlike a tilting pad bearing, this bearing can have radial adjustments. The tilt adjustments are obtained by providing flexibility to the pad at one corner. This paper deals with the effect of turbulence on the steady state performance characteristics of a centrally loaded 120 deg single pad externally adjustable fluid film bearing. The bearing has an aspect ratio of 1 and operates over a wide range of eccentricity ratios with different radial and tilt adjustments. The Reynolds equation is solved numerically using the finite difference method. The linearized turbulence model of Ng and Pan (1965, “A Linearized Turbulent Lubrication Theory,” ASME J. Basic Eng., 87, pp. 675–688) as well as the simplified adiabatic model of Pinkus and Bupara (1979, “Adiabatic Solutions for Finite Journal Bearings,” ASME J. Lubr. Technol., 101, pp. 492–496) are incorporated in the solution scheme. The static performance characteristics calculated are presented in terms of load carrying capacity, attitude angle, friction variable, and Sommerfeld number. A comparative study with the combination of adjustments predicts that the static performance of the bearing is superior with negative radial and tilt adjustments.
Pneumatic Osteoarthritis Knee Brace
Authors: Stamenović Dimitrije , Kojić Miloš , Stojanović Boban , Hunter David
Published: March 2009

Knee osteoarthritis is a chronic disease that necessitates long term therapeutic intervention. Biomechanical studies have demonstrated an improvement in the external adduction moment with application of a valgus knee brace. Despite being both efficacious and safe, due to their rigid frame and bulkiness, current designs of knee braces create discomfort and difficulties to patients during prolonged periods of application. Here we propose a novel design of a light osteoarthritis knee brace, which is made of soft conforming materials. Our design relies on a pneumatic leverage system, which, when pressurized, reduces the excessive loads predominantly affecting the medial compartment of the knee and eventually reverses the malalignment. Using a finite-element analysis, we show that with a moderate level of applied pressure, this pneumatic brace can, in theory, counterbalance a greater fraction of external adduction moment than the currently existing braces.
The Pathogenesis of Syringomyelia: A Re-Evaluation of the Elastic-Jump Hypothesis
Authors: Elliott N. S. J. , Lockerby D. A. , Brodbelt A. R.
Published: March 2009

Syringomyelia is a disease in which fluid-filled cavities, called syrinxes, form in the spinal cord causing progressive loss of sensory and motor functions. Invasive monitoring of pressure waves in the spinal subarachnoid space implicates a hydrodynamic origin. Poor treatment outcomes have led to myriad hypotheses for its pathogenesis, which unfortunately are often based on small numbers of patients due to the relative rarity of the disease. However, only recently have models begun to appear based on the principles of mechanics. One such model is the mathematically rigorous work of Carpenter and colleagues (2003, “Pressure Wave Propagation in Fluid-Filled Co-Axial Elastic Tubes Part 1: Basic Theory,” ASME J. Biomech. Eng., 125(6), pp. 852–856; 2003, “Pressure Wave Propagation in Fluid-Filled Co-Axial Elastic Tubes Part 2: Mechanisms for the Pathogenesis of Syringomyelia,” ASME J. Biomech. Eng., 125(6), pp. 857–863). They suggested that a pressure wave due to a cough or sneeze could form a shocklike elastic jump, which when incident at a stenosis, such as a hindbrain tonsil, would generate a transient region of high pressure within the spinal cord and lead to fluid accumulation. The salient physiological parameters of this model were reviewed from the literature and the assumptions and predictions re-evaluated from a mechanical standpoint. It was found that, while the spinal geometry does allow for elastic jumps to occur, their effects are likely to be weak and subsumed by the small amount of viscous damping present in the subarachnoid space. Furthermore, the polarity of the pressure differential set up by cough-type impulses opposes the tenets of the elastic-jump hypothesis. The analysis presented here does not support the elastic-jump hypothesis or any theory reliant on cough-based pressure impulses as a mechanism for the pathogenesis of syringomyelia.
Nanoclay and Microballoons Wall Thickness Effect on Dynamic Properties of Syntactic Foam
Authors: Peter Sameer L. , Woldesenbet Eyassu
Published: March 2009

The effect of nanoclay on the high strain rate mechanical properties of syntactic foams is studied. Two types of microballoons with different wall thicknesses are used in fabrication of plain and nanoclay syntactic foams. Plain syntactic foams are fabricated with 60% volume fraction of glass microballoons. 1%, 2%, and 5% volume fractions of Nanomer I.30E nanoclay are incorporated to produce nanoclay syntactic foams. High strain rate test using split Hopkinson pressure bar (SHPB) apparatus is performed on all types of plain and nanoclay syntactic foams. Dynamic modulus, strength, and corresponding strain are calculated using the SHPB data. Quasistatic test is also performed and results are compared with the dynamic SHPB results. The results demonstrate the importance of nanoclay and microballoon wall thickness in determination of syntactic foam dynamic properties. It is found that at a high strain rate, the strength and modulus of composite foams having K46 microballoons increase due to addition of 1% volume fraction of nanoclay. However, in composite foams having S22 microballoons, the increase in strength is not significant at a high strain rate. Further increase in nanoclay volume fraction to 2% and 5% reduces the strength and modulus of composite foams having S22 microballoons. Difference in wall thickness of microballoons is found to affect the strength, modulus, strain energy, and deformation of composite foams. Composite foams fabricated with thicker walled microballoons (K46) show comparatively higher values of strength, modulus, and strain energy compared with thin walled (S22) microballoons. Scanning electron microscopy shows that crack propagation behavior is distinct at different strain rates.
Control of 3003-H18 Aluminum Ultrasonic Consolidation
Authors: Kulakov M. , Rack H. J.
Published: March 2009

Ultrasonic consolidation of 150 μm thick 3003-H18 aluminum foil to a 3003-H18 build plate has been investigated. The effects of the normal load, vibrational amplitude, and sonotrode velocity on consolidation quality as characterized by the total linear weld density (LWD) including both edge and edge defects were examined utilizing two sequential three-level full factorial design of experiments at a constant build plate temperature of 150°C. These showed that the normal load and the vibrational amplitude have a significant influence on LWD, while the sonotrode rotational velocity has only a marginal effect. The formation of edge defects at the foil-build plate interface has been attributed to the nonuniform strain state across the foil width, while the central defects were related to sonotrode-foil contact pressure variations following the sonotrode pattern. In addition, LWD variations in ±10–20% were attributed to a nonuniform sonotrode-foil contact pressure distribution. Finally, representation of the total LWD as a function of the control parameters indicates that high total linear weld densities can be achieved with a control space bounded by high normal loads and intermediate to high vibrational amplitudes. Consideration of only central defects expands this control space allowing utilization of lower normal loads and vibrational amplitudes to achieve similar linear weld densities.
Characterization of the Residual Stresses and Strength of Ceramic-Metal Braze Joints
Authors: Galli Matteo , Botsis John , Janczak-Rusch Jolanta , Maier Gerd , Welzel Udo
Published: March 2009

Residual stress relief in ceramic-metal joints produced by active brazing depends primarily on the plastic response of the filler metal. A procedure for the production and mechanical characterization of bulk active filler alloy specimens is developed. In parallel ceramic-metal joints are produced and tested. Residual stresses are measured by X-ray diffraction while the joint strength is assessed by four-point bend tests. The obtained elastoplastic properties of the filler are introduced into finite element models to predict the residual stresses in the joints and their behavior in bending. The results of the simulations show good agreement both with the residual stress measurements and with the results of four-point bend tests.
An Exact Fourier Series Method for the Vibration Analysis of Multispan Beam Systems
Authors: Li Wen L. , Xu Hongan
Published: March 2009

An exact Fourier series method is developed for the vibration analysis of multispan beam systems. In this method, the displacement on each beam is expressed as a Fourier series expansion plus an auxiliary closed-form function such as polynomials. The auxiliary function is used to deal with all the possible discontinuities, at the end points, with the original displacement function and its derivatives when they are periodically extended over the entire x-axis as implied by a Fourier series representation. As a result, not only is it always possible to expand the beam displacements into Fourier series under any boundary conditions, but also the series solution will be substantially improved in terms of its accuracy and convergence. Mathematically, the current Fourier series expansion represents an exact solution to a class of beam problems in the sense that both the governing equations and the boundary/coupling conditions are simultaneously satisfied to any specified degree of accuracy. In the multispan beam system model, any two adjacent beams are generally connected together via a pair of linear and rotational springs, allowing a better modeling of many real-world joints. Each beam in the system can also be independently and elastically restrained at its ends so that all boundary conditions including the classical homogeneous boundary conditions at the end and intermediate supports can be universally dealt with by simply varying the stiffnesses of the restraining springs accordingly, which does not involve any modification of basis functions, formulations, or solution procedures. The excellent accuracy and convergence of this series solution is demonstrated through numerical examples.
An Efficient Hybrid Method for Multibody Dynamics Simulation Based on Absolute Nodal Coordinate Formulation
Authors: Tian Qiang , Chen Li Ping , Zhang Yun Qing , Yang Jingzhou
Published: March 2009

This paper presents an efficient hybrid method for dynamic analysis of a flexible multibody system. This hybrid method is the combination of a penalty and augmented Lagrangian formulation with the mass-orthogonal projections method based on the absolute nodal coordinate formulation (ANCF). The characteristic of the ANCF that the mass matrix is constant and both Coriolis and centrifugal terms vanish in the equations of motion make the proposed method computationally efficient. Within the proposed method, no additional unknowns, such as the Lagrange multipliers in the Newmark method, are introduced, and the number of equations does not depend on the number of constraint conditions. Furthermore, conventional integration stabilization methods, such as Baumgarte’s method. are unnecessary. Therefore, the proposed method is particularly suitable for systems with redundant constraints, singular configurations, or topology changes. Comparing results from different methods in terms of efficiency and accuracy has shown that the proposed hybrid method is efficient and has good convergence characteristics for both stiff and flexible multibody systems.
Scaling of Constraints and Augmented Lagrangian Formulations in Multibody Dynamics Simulations
Authors: Bauchau Olivier A. , Epple Alexander , Bottasso Carlo L.
Published: March 2009

This paper addresses practical issues associated with the numerical enforcement of constraints in flexible multibody systems, which are characterized by index-3 differential algebraic equations (DAEs). The need to scale the equations of motion is emphasized; in the proposed approach, they are scaled based on simple physical arguments, and an augmented Lagrangian term is added to the formulation. Time discretization followed by a linearization of the resulting equations leads to a Jacobian matrix that is independent of the time step size, h; hence, the condition number of the Jacobian and error propagation are both O(h0): the numerical solution of index-3 DAEs behaves as in the case of regular ordinary differential equations (ODEs). Since the scaling factor depends on the physical properties of the system, the proposed scaling decreases the dependency of this Jacobian on physical properties, further improving the numerical conditioning of the resulting linearized equations. Because the scaling of the equations is performed before the time and space discretizations, its benefits are reaped for all time integration schemes. The augmented Lagrangian term is shown to be indispensable if the solution of the linearized system of equations is to be performed without pivoting, a requirement for the efficient solution of the sparse system of linear equations. Finally, a number of numerical examples demonstrate the efficiency of the proposed approach to scaling.
A New Concept for High Temperature Fuel Cell Hybrid Systems Using Supercritical Carbon Dioxide
Authors: Sánchez D. , Chacartegui R. , Jiménez-Espadafor F. , Sánchez T.
Published: March 2009

Hybrid power systems based on high temperature fuel cells are a promising technology for the forthcoming distributed power generation market. For the most extended configuration, these systems comprise a fuel cell and a conventional recuperative gas turbine engine bottoming cycle, which recovers waste heat from the cell exhaust and converts it into useful work. The ability of these gas turbines to produce useful work relies strongly on a high fuel cell operating temperature. Thus, if molten carbonate fuel cells or the new generation intermediate temperature solid oxide fuel cells are used, the efficiency and power capacity of the hybrid system decrease dramatically. In this work, carbon dioxide is proposed as the working fluid for a closed supercritical bottoming cycle, which is expected to perform better for intermediate temperature heat recovery applications than the air cycle. Elementary fuel cell lumped-volume models for both solid oxide and molten carbonate are used in conjunction with a Brayton cycle thermodynamic simulator capable of working with open/closed and air/carbon dioxide systems. This paper shows that, even though the new cycle is coupled with an atmospheric fuel cell, it is still able to achieve the same overall system efficiency and rated power than the best conventional cycles being currently considered. Furthermore, under certain operating conditions, the performance of the new hybrid systems beats that of existing pressurized fuel cell hybrid systems with conventional gas turbines. From the results, it is concluded that the supercritical carbon dioxide bottoming cycle holds a very high potential as an efficient power generator for hybrid systems. However, costs and balance of plant analysis will have to be carried out in the future to check its feasibility.
Polymer Electrolyte Fuel Cell Design Based on Three-Dimensional Computational Fluid Dynamics Modeling
Authors: Cordiner Stefano , Lanzani Simon Pietro , Mulone Vincenzo , Chiapparini Marco , D’Anzi Angelo , Orsi Donatella
Published: March 2009

An entirely numerical design procedure, based on computational fluid dynamics, is introduced to evaluate the performance of different polymer electrolyte fuel cell layouts and sets of operating conditions for assigned target parameters in terms of performance. The design procedure has been applied to a coflow design, characterized by large active area (500 cm2), moderate temperature (70°C), liquid cooling, and metal supporting. The role of heat transfer between the cell and the cooling system is analyzed to properly address the influence of operating conditions on power density and flooding via a comprehensive parametric analysis.
Three-Dimensional Semiperiodic Line Contact–Periodic in Contact Length Direction
Authors: Liu Shuangbiao , Hua Diann Y.
Published: March 2009

Line contact problems, such as those seen in spur gears and cam-roller follower systems, are often simplified with the plane-strain assumption and thus modeled by two-dimensional equations. However, in order to address the effects of roughness and textured surfaces, three-dimensional modeling is necessary. The challenge arises when the contact domain is several orders of magnitude greater than the grid size needed to properly describe the surface roughness or texture. Considering the surface geometry of a so-called “line contact,” the contact domain is nonperiodic in contact width direction, but it can be treated as periodic in the contact length direction–semiperiodic line contact problem. Thus, only a section of the entire contact domain is used as the computational domain with a much-reduced size. Based on an in-depth investigation of available algorithms, DC-FFTS and DC-CC-FFT algorithms are proposed. The DC-FFTS algorithm is a modified discrete convolution and fast Fourier transform algorithm with superposition of influence coefficients. The DC-CC-FFT algorithm is a hybrid fast Fourier transform based algorithm, which combines the discrete convolution–FFT and the continuous convolution–FFT methods. The proposed algorithms are used to solve three-dimensional displacement, contact pressure, and stresses for line contact problems. The results are compared with the other available algorithms from literature. The accuracy and efficiency of different algorithms are discussed.
Bi2Te3 Nanowire Array/Epoxy Composites for Thermoelectric Power Generators and Microcoolers
Authors: Biswas Kalapi G. , Rawat Vijay , DaSilva Manuel , Sands Timothy D.
Published: March 2009

The thermal conductivity of a nanowire array composite is controlled by the matrix thermal conductivity and volume fraction, thus the effective thermoelectric figure-of-merit (ZT) of the composite will be reduced relative to that of the thermoelectric nanowire material alone. In this report, we demonstrate a process for fabricating nanowire array /epoxy composite with high structural integrity and low effective thermal conductivity required for thermoelectric power generation applications. Using galvanostatic electrodeposition into sacrificial porous anodic alumina (PAA) templates of 50 micron thickness, we synthesized dense (∼75% volume fraction) self-supporting nanowire arrays of textured Bi2Te3. X-ray diffraction and transmission electron microscopy analysis showed that the nanowires have <11.0> texture, the orientation that yields the highest ZT in single crystals. The nanowire array was infiltrated with SU-8 epoxy resin, a low thermal conductivity material (0.2 W/m-K) with a thermal conductivity that is about an order of magnitude lower than that of PAA (1.7 W/m-K). Scanning electron micrographs of fractured composites confirm nearly complete infiltration of SU-8 epoxy in nanowire array with good adhesion and high structural integrity.
The Effects of Particle Size, Morphology, and Mass Loading on the Reactivity of Nanocatalytic Particles
Authors: Fellon Brian , Ricciuti Christopher , Ma Yi , Miller Tiffany , Pearlman Howard
Published: March 2009

Nanocatalytic particles of Gold (Au), Platinum (Pt), and Palladium (Pd) are highly reactive at room-temperature and can be used to generate heat in micro-scale devices for portable power generation. No pre-heating is required for light-off and high steady-state operating temperatures can be sustained with high density alcohol-air premixtures. Preliminary experiments conducted in our lab and those reported by Hu and co-workers at Oak Ridge National Lab have measured peak operating temperatures ∼ 300–500 degrees Celsius using near-stoichiometric methanol/air and ethanol/air premixtures at ambient initial temperature and atmospheric pressure. The effect of particle size, morphology, mass loading, and flow residence time are reported for different mixture stoichiometries. Temperature measurements and gas species analyses are also tabulated. Interestingly, smaller particles were observed to be less reactive than larger particles for the same mass loadings for select conditions. Materials characterization of the particles has also been conducted to characterize the specific surface area of the catalyst and evaluate the importance of particle sintering, morphology changes, and particle distribution.
Combinatorial Development of Ternary Electrocatalysts for Methanol Oxidation
Authors: Strasser Peter
Published: March 2009

We report a combinatorial and high throughput catalyst optimization of ternary Pt-Co-Ru alloy electrocatalysts for the oxidation of methanol in Direct Methanol Fuel Cell anodes. A densely sampled ternary Pt alloy catalyst library was prepared and electrochemically tested in parallel for catalytic activity. A composition-activity map was obtained from which suitable catalyst candidates with improved activity were identified. Then, high throughput methods for evaluating corrosion stability of the alloy catalysts were developed based on structural and compositional criteria. Finally, combining stability-composition and activity-composition maps resulted in consensus maps which pointed to a new optimized ternary alloy electrocatalyst with overall composition Pt18Co62Ru20.
Study of the Vortex Breakdown in a Conical Swirler Using LDV, LES and POD
Authors: Duwig Christophe , Fuchs Laszlo , Lacarelle Arnaud , Beutke Matthias , Paschereit Christian Oliver
Published: March 2009

Modeling and understanding the vortex breakdown is a key issue of modern Lean Premixed Combustors. The main difficulty of the problem is the unsteady behavior of this type of flow: Large structures resulting from vortex breakdown and the swirling shear-layers, affect directly the flame stabilization leading to heat-release fluctuations and combustion instabilities. Consequently, one needs to capture and understand turbulent coherent structures dynamics for designing efficient burners. This task is particularly challenging since it deals with capturing coherent motions within a chaotic system and should be done using state-of-the art numerical and experimental techniques. The present work focuses on the experimental and numerical study of iso-thermal vortex breakdown in a conical swirler. Experimental investigations were performed with 2D Laser Doppler Velocimetry (LDV) and Hotwire Anemometry at the outlet of the combustor model. Averaged velocity fields and RMS values are showing a strong central recirculation zone. In addition, characteristic frequencies of the flow have been exhibited showing the strong influence of large scale turbulent fluctuation on the flow pattern. These measurements showed also the impact of different outlet geometries on the strength and position of the coherent structures of the flow. Further, Large Eddy Simulation (LES) has been used to obtain a 4D description of the flow. Comparison with LDV profiles showed a good agreement, indicating that the LES tool captures accurately the flow. The LES results were then processed for capturing and identifying the coherent structures. Firstly, characteristic frequencies were analyzed. Here also a good agreement with the experimental data was achieved. Secondly the cores of the vortices were visualized providing a good insight into the unsteady flow pattern. Finally, Proper Orthogonal Decomposition (POD) was applied to the 4D field in order to identify the contribution of different large scale fluctuation modes. The presence of the Precessing Vortex Core (PVC) corresponding to a pair of helical structures was captured.
Numerical Modeling of Heat Transfer and Pressure Losses for Uncooled Gas Turbine Blade Tip: Effect of Tip Clearance and Tip Geometry
Authors: El-Gabry Lamyaa A.
Published: March 2009

A computational study has been performed to predict the heat transfer distribution on the blade tip surface for a representative gas turbine first stage blade. CFD predictions of blade tip heat transfer are compared to test measurements taken in a linear cascade, when available. The blade geometry has an inlet Mach number of 0.3 and an exit Mach number of 0.75, pressure ratio of 1.5, exit Reynolds number based on axial chord of 2.57×106, and total turning of 110 deg. Three blade tip configurations were considered; they are flat tip, a full perimeter squealer, and an offset squealer where the rim is offset to the interior of the tip perimeter. These three tip geometries were modeled at three tip clearances of 1.25, 2.0, and 2.75% of blade span. The tip heat transfer results of the numerical models agree fairly well with the data and are comparable to other CFD predictions in the open literature.
Acoustic and Vibrational Analyses on a Multi-Stage Compressor for Unstable Behavior Precursor Identification
Authors: Morini M. , Pinelli M. , Venturini M.
Published: March 2009

The analysis of compressor behavior in critical working conditions, such as rotating stall and surge, represents a key aspect in the turbomachinery research field. In fact, both phenomena can lead to severe damage to the compression system and so the identification of unstable behaviour precursors is needed. The paper shows the test results of an experimental campaign conducted on a small size compressor with six axial stages and one centrifugal stage. In particular, the paper focuses on the analysis of acoustic and vibration measurements for both stationary conditions and for varying-over-time (i.e. gradually increasing) inlet mass flow rate. The correlation between compressor behavior in the operating region close to the surge line and vibration and acoustic measurements is investigated.
Correlations Adaptation for Optimal Emissions Prediction
Authors: Tsalavoutas A. , Kelaidis M. , Thoma N. , Mathioudakis K.
Published: March 2009

An approach for estimating the pollutants emitted from a gas turbine using semi-empirical correlations is described. An extensive literature review has been carried out, in order to obtain information already available in the public domain, on the subject of pollutants emitted from turbine engines and on the effect of different parameters on them. It is shown that application of correlations in their original form does not provide a reliable estimation of emissions. Such estimation requires adaptation to the particular case studies. The possibility of adapting the considered semi-empirical correlations to available emissions measurements, through the use of optimization method is further studied. Multivariate analysis, for the establishment of generic correlations had been also applied. Results are presented and compared to the test data that derive from the dry performance of an industrial turbine and a turbojet military engine. It is demonstrated that a good predictive ability can be established.
A New Slip Factor Model for Axial and Radial Impellers
Authors: Qiu Xuwen , Mallikarachchi Chanaka , Anderson Mark
Published: March 2009

This paper proposes a unified slip model for axial, radial, and mixed flow impellers. For many years, engineers designing axial and radial turbomachines have applied completely different deviation or slip factor models. For axial applications, the most commonly used deviation model has been Carter’s rule or its derivatives. For centrifugal impellers, Wiesner’s correlation has been the most popular choice. Is there a common thread linking these seemingly unrelated models? This question becomes particularly important when designing a mixed flow impeller where one has to choose between axial or radial slip models. The proposed model in this paper is based on blade loading, i.e., the velocity difference between the pressure and suction surfaces, near the discharge of the impeller. The loading function includes the effect of blade rotation, blade turning, and the passage area variation. This velocity difference is then used to calculate the slip velocity using Stodola’s assumption. The final slip model can then be related to Carter’s rule for axial impellers and Stodola’s slip model for radial impellers. This new slip model suggests that the flow coefficient at the impeller exit is an important variable for the slip factor when there is blade turning at the impeller discharge. This may explain the interesting slip factor trend observed from experiments, such as the rise of the slip factor with flow coefficient in Eckardt A impeller. Some validation results of this new model are presented for a variety of applications, such as radial compressors, axial compressors, pumps, and blowers.
Effect of Inlet Flow Angle on Gas Turbine Blade Tip Film Cooling
Authors: Gao Zhihong , Narzary Diganta , Mhetras Shantanu , Han Je-Chin
Published: March 2009

The influence of incidence angle on film cooling effectiveness is studied for a cutback squealer blade tip. Three incidence angles are investigated — 0° at design condition and ± 5° at off-design conditions. Based on mass transfer analogy, the film cooling effectiveness is measured with pressure sensitive paint (PSP) techniques. The film cooling effectiveness distribution on pressure side near tip region, squealer cavity floor and squealer rim tip is presented for the three incidence angles at varying blowing ratios. The average blowing ratio is controlled to be 0.5, 1.0, 1.5 and 2.0. One row of shaped holes are provide along the pressure side just below the tip; two rows of cylindrical film cooling holes are arranged on the blade tip in such a way that one row offset to the suction side profile and the other row is along the camber line. The pressure side squealer rim wall is cut near the trailing edge to allow the accumulated coolant in the cavity to escape and cool the tip trailing edge. The internal coolant-supply passages of the squealer tipped blade are modeled similar to those in the GE-E3 rotor blade. Test is done in a five-blade linear cascade in a blow down facility with a tip gap clearance of 1.5% of the blade span. The Mach number and turbulence intensity level at the cascade inlet was 0.23 and 9.7%, respectively. It is observed that the incidence angle affects the coolant jet direction on the pressure side near tip region and blade tip. The film cooling effectiveness distribution is altered. The peak of laterally averaged effectiveness is shifted to upstream or downstream depending on the off-design incidence angles. The film cooling effective inside the tip cavity can be increased up to 25% with the positive incidence angle. However, in general, the overall area averaged film cooling effectiveness is not significantly changed by the incidence angles in the range of study.
Heat Transfer in a Two-Pass Rectangular Channel (AR=1:4) Under High Rotation Numbers
Authors: Liu Yao-Hsien , Huh Michael , Han Je-Chin , Chopra Sanjay
Published: March 2009

This paper experimentally investigated the rotational effects on heat transfer in a two-pass rectangular channel (AR=1:4), which is applicable to the channel near the leading edge of the gas turbine blade. The test channel has radially outward flow in the first passage through a re-directed sharp bend entrance and radially inward flow in the second passage after a 180° sharp turn. In the first passage, rotation effects on heat transfer are reduced by the re-directed sharp bend entrance. In the second passage, under rotating conditions, both leading and trailing surfaces experienced heat transfer enhancements above the stationary case. Rotation greatly increased heat transfer enhancement in the tip region up to a maximum Nu ratio (Nu/Nus) of 2.4. The objective of the current study is to perform an extended parameter study of the low rotation number (0–0.3) and low buoyancy parameter (0–0.2) achieved previously. By varying the Reynolds numbers (10000–40000) and the rotational speeds (0–400 rpm), the increased range of the rotation number and buoyancy parameter reached in this study are 0–0.67 and 0–1.9, respectively. The higher rotation number and buoyancy parameter have been correlated very well to predict the rotational heat transfer in the two-pass, 1:4 aspect ratio flow channel.
Prediction of Film Cooling and Heat Transfer on a Rotating Blade Platform With Stator-Rotor Purge and Discrete Film-Hole Flows in a 1-1/2 Turbine Stage
Authors: Yang H. , Gao Z. , Chen H. C. , Han J. C. , Schobeiri M. T.
Published: March 2009

Numerical simulations were performed to predict the film cooling effectiveness and heat transfer coefficient distributions on a rotating blade platform with stator-rotor purge flow and downstream discrete film-hole flows in a 1-1/2 turbine stage using a Reynolds stress turbulence model together with a non-equilibrium wall function. Simulations were carried out with sliding mesh for the rotor under three rotating speeds (2000, 2550, and 3000 rpm) to investigate the effects of rotation and stator-rotor interaction on the rotor blade platform purge flow cooling and discrete-hole film cooling and heat transfer. The adiabatic film cooling effectiveness and heat transfer coefficients were calculated using the adiabatic wall temperatures with and without coolant to examine the true coolant protection excluding the effect of turbine work process. The stator-rotor interaction strongly impacts the purge slot film cooling and heat transfer at the platform leading portion, while only slightly affects the downstream discrete-hole film cooling near the platform trailing portion. In addition, the effect of turbine work process on the film cooling effectiveness and the associated heat transfer coefficients have been reported.
Effect of Unsteady Wake on Film-Cooling Effectiveness Distribution on a Gas Turbine Blade With Compound Shaped Holes
Authors: Narzary Diganta P. , Gao Zhihong , Mhetras Shantanu , Han Je-Chin
Published: March 2009

The effect of fan-shaped, laid-back compound angled cooling holes placed along the span of a fully-cooled high pressure turbine blade in a 5-blade linear cascade on film cooling effectiveness is studied using the Pressure Sensitive Paint (PSP) technique. Four rows of shaped film cooling holes are provided on the pressure side while two such rows are provided on the suction side of the blade. Three rows of cylindrical holes are drilled at 30° to the surface on the leading edge to capture the effect of showerhead film coolant injection. The coolant is injected at four different average blowing ratios of 0.3, 0.6, 0.9 and 1.2. Presence of wake due to upstream vanes is studied by placing a periodic set of rods upstream of the test blade. The wake is generated using 4.8mm diameter rods. The wake rods can be clocked by changing their stationary positions in front of the test blade to simulate a progressing wake. Effect of wake is recorded at four phase locations with equal intervals. The free stream Reynolds number, based on the axial chord length and the exit velocity, is 750,000 and the inlet and the exit Mach numbers are 0.27 and 0.44, respectively resulting in a blade pressure ratio of 1.14. Turbulence intensity level at the cascade inlet is 6% with an integral length scale of around 5cm. Results show that the fan-shaped, laid-back compound angled holes produce uniform and wide coolant coverage on the suction side except for those regions affected by the passage and tip leakage vortices. The advantage of compound shaped hole design is seen from the higher effectiveness values on the suction side compared to that of the compound cylindrical holes. The presence of a stationary upstream wake can result in lower film cooling effectiveness on the blade surface. Variation of blowing ratio from 0.3 to 1.2 show more or less uniform increment in effectiveness increase on the pressure side, whereas on the suction side, the increment shows signs of saturation beyond M = 0.6.