bluuebunny/crossref_metadata_2025 huggingface dataset:
Variation in Fractal Properties and Non-Gaussian Distributions of Microcontact Between Elastic-Plastic Rough Surfaces With Mean Surface Separation
Authors: Chung Jung Ching , Lin Jen Fin
Published: January 2006

The fractal parameters (fractal dimension and topothesy), describing the contact behavior of rough surface, were considered as constant in the earlier models. However, their results are often significantly different from the experimental results. In the present study, these two roughness parameters have been derived analytically as a function of the mean separation first, then they are found with the aid of the experimental results. By equating the structure functions developed in two different ways, the relationship among the scaling coefficient in the power spectrum function, the fractal dimension, and topothesy of asperity heights can be established. The variation of topothesy can be determined when the fractal dimension and the scaling coefficient have been obtained from the experimental results of the number of contact spots and the power spectrum function at different mean separations. The probability density function of asperity heights, achieved at a different mean separation, was obtained from experimental results as a non-Gaussian distribution; it is expressed as a function of the skewness and the kurtosis. The relationship between skewness and mean separation can be established through the fitting of experimental results by this non-Gaussian distribution. For a sufficiently small mean separation, either the total load or the real contact area predicted by variable fractal parameters, as well as non-Gaussian distribution, is greater than that predicted by constant fractal parameters, as well as Gaussian distribution. The difference between these two models is significantly enhanced as the mean separation becomes small.
Surfactant Use for Slug Flow Pattern Suppression and New Flow Pattern Types in a Horizontal Pipe
Authors: Wilkens R. J. , Thomas D. K. , Glassmeyer S. R.
Published: January 2006

A set of experiments was performed to study flow pattern suppression in horizontal air-water pipe flow by means of surfactant additive. Results suggest that addition of the surfactant to the gas-liquid flow significantly reduces the occurrence of slug flow. In addition, previously unreported flow patterns were observed to exist between slug and dispersed bubble flows. It is concluded that new mechanisms for slug flow transition need to be considered.
On the Uniqueness of Solutions for the Identification of Linear Structural Systems
Authors: Franco Guillermo , Betti Raimondo , Longman Richard W.
Published: January 2006

This work tackles the problem of global identifiability of an undamped, shear-type, N degrees of freedom linear structural system under forced excitation without any prior knowledge of its mass or stiffness distributions. Three actuator/sensor schemes are presented, which guarantee the existence of only one solution for the mass and stiffness identification problem while requiring a minimum amount of instrumentation (only 1 actuator and 1 or 2 sensors). Through a counterexample for a 3DOF system it is also shown that fewer measurements than those suggested result invariably in non-unique solutions.
Mass Transfer From a Rotating Disk to a Bingham Fluid
Authors: Rashaida A. A. , Bergstrom D. J. , Sumner R. J.
Published: January 2006

In the present investigation, an analytical numerical solution is presented for the mass transfer from a rotating disk to a Bingham fluid for the case of laminar boundary layer flow. The analytical approach includes the coupled effects of steady disk rotation and non-Newtonian fluid properties on the mass transfer rate. A dimensionless expression for the wall mass transfer rate based on the Sherwood number, Sh, is obtained in terms of the system parameters (Reynolds number, Rep, and Schmidt number, Scp) which depend on the dimensionless yield stress or Bingham number, By. The analytical relation indicates that an increase in By (up to the limit By⩽1) leads to a slight increase in the wall mass transfer rate, and thereafter, for By>1, the mass transfer rate is reduced.
On the Mechanism Responsible for Turbulent Drag Reduction by Dilute Addition of High Polymers: Theory, Experiments, Simulations, and Predictions
Authors: Jovanović J. , Pashtrapanska M. , Frohnapfel B. , Durst F. , Koskinen J. , Koskinen K.
Published: January 2006

Turbulent drag reduction by dilute addition of high polymers is studied by considering local stretching of the molecular structure of a polymer by small-scale turbulent motions in the region very close to the wall. The stretching process is assumed to restructure turbulence at small scales by forcing these to satisfy local axisymmetry with invariance under rotation about the axis aligned with the main flow. It can be shown analytically that kinematic constraints imposed by local axisymmetry force turbulence near the wall to tend towards the one-component state and when turbulence reaches this limiting state it must be entirely suppressed across the viscous sublayer. For the limiting state of wall turbulence, the statistical dynamics of the turbulent stresses, constructed by combining the two-point correlation technique and invariant theory, suggest that turbulent drag reduction by homogeneously distributed high polymers, cast into the functional space which emphasizes the anisotropy of turbulence, resembles the process of reverse transition from the turbulent state towards the laminar flow state. These findings are supported by results of direct numerical simulations of wall-bounded turbulent flows of Newtonian and non-Newtonian fluids and by experiments carried out, under well-controlled laboratory conditions, in a refractive index-matched pipe flow facility using state-of-the art laser-Doppler anemometry. Theoretical considerations based on the elastic behavior of a polymer and spatial intermittency of turbulence at small scales enabled quantitative estimates to be made for the relaxation time of a polymer and its concentration that ensure maximum drag reduction in turbulent pipe flows, and it is shown that predictions based on these are in very good agreement with available experimental data.
A Unified Extended Thermodynamic Description of Diffusion, Thermo-Diffusion, Suspensions, and Porous Media
Authors: Lebon Georgy , Desaive Thomas , Dauby Pierre
Published: January 2006

It is shown that extended irreversible thermodynamics (EIT) provides a unified description of a great variety of processes, including matter diffusion, thermo-diffusion, suspensions, and fluid flows in porous media. This is achieved by enlarging the set of classical variables, as mass, momentum and temperature by the corresponding fluxes of mass, momentum and heat. For simplicity, we consider only Newtonian fluids and restrict ourselves to a linear analysis: quadratic and higher order terms in the fluxes are neglected. In the case of diffusion in a binary mixture, the extra flux variable is the diffusion flux of one the constituents, say the solute. In thermo-diffusion, one adds the heat flux to the set of variables. The main result of the present approach is that the traditional equations of Fick, Fourier, Soret, and Dufour are replaced by time-evolution equations for the matter and heat fluxes, such generalizations are useful in high-frequency processes. It is also shown that the analysis can be easily extended to the study of particle suspensions in fluids and to flows in porous media, when such systems can be viewed as binary mixtures with a solid and a fluid component.
In Situ Optical Diagnostics for Measurements of Water Vapor Partial Pressure in a PEM Fuel Cell
Authors: Basu Saptarshi , Xu Hang , Renfro Michael W. , Cetegen Baki M.
Published: January 2006

A fiber optic coupled diode laser sensor has been constructed for in situ measurements of water vapor partial pressure in active proton-exchange membrane (PEM) fuel cell systems. The bipolar plate of a prototypical PEM fuel cell was modified to allow for transmission of a near infrared laser beam through the flow channels on either the fuel or oxidizer side of its membrane-electrode assembly. The laser wavelength was scanned over several water rotational and vibrational transitions and the light absorption was detected by measuring the transmitted laser power through the device. The intensity and line shape of the measured transition was used to extract path-averaged values for the water vapor partial pressure. Measurements were initially taken in a non-operating cell with known temperature and humidity input gas streams to calibrate and test the optical device. A technique for rapid determination of the water partial pressure was developed. The optical technique is applicable over a significant temperature and humidity operating range of a PEM fuel cell. The measurement technique was applied to an operating PEM fuel cell system to examine the effects of incoming gas humidity and load on the water vapor partial pressure variation in one of the flow channels.
Design and Performance Analysis of Innovative Bipolar Direct Methanol Fuel Cell Stacks with 2D Planar Configuration
Authors: Jung Guo-Bin , Weng Fang-Bor , Su Ay , Chung Chen-Chung , Tu Cheng-Hsin , Chan Shih-Hung
Published: January 2006

In this study, an innovative planar stack design employing graphite plate with bipolar structure on the same side to replace current stainless steel is investigated. The design, fabrication, and performance evaluation of a 25cm2, air-breathing, room-temperature, direct methanol fuel cell are described. The cell is completely passive with external pumps for controlling the methanol flow rate. This planar design consists of an open cathode side which allows a completely passive, self-breathing operation of the stack. Single cells with active area of 25cm2 showed a maximum power density of 25-40mW. A three-cell stack was constructed in an innovative planar configuration and it produced a power output of 110mW at 3M of methanol concentration at room temperature. In addition, the performance gives rise to 280mW at 3M under air-forced mode. The more concentrated methanol solution attains higher power due to instant take away of the methanol crossover through the membrane while the oxygen within the air is ready to react with the proton from the membrane with an appropriate rate and current output.
Optical Measurement Technique of Water Contents in Polymer Membrane for PEFCs
Authors: Fushinobu Kazuyoshi , Shimizu Kenji , Miki Nariaki , Okazaki Ken
Published: January 2006

The feasibility of an optical technique is examined for the measurement of the membrane water content in polymer electrolyte fuel cells (PEFCs). Transmission of the infrared light of 1.92 μm wavelength is used to measure the water content in the polymer electrolyte membrane. A calibration procedure is examined, and the technique is applied for the transient measurement of a Nafion membrane that gives the value of water diffusion coefficient, consistent with previous reports. The technique is then applied to an operating PEFC to show its applicability for in situ measurement.
Dynamic Simulation of a Pressurized 220kW Solid Oxide Fuel-Cell–Gas-Turbine Hybrid System: Modeled Performance Compared to Measured Results
Authors: Roberts R. A. , Brouwer J.
Published: January 2006

Hybrid fuel-cell–gas-turbine (FC/GT) systems are technologically advanced systems that are promising for electric power generation with ultralow emissions and high efficiency for a large range of power plant sizes. A good understanding of the steady-state and dynamic performance of a FC/GT system is needed in order to develop and advance this hybrid technology. In this work, a detailed dynamic model of a solid oxide fuel cell/gas turbine (SOFC/GT) system has been developed. The system that is simulated represents the 220kW SOFC/GT hybrid system developed by Siemens Westinghouse. Results of the dynamic model and experimental data gathered during the operation and testing of the 220kW SOFC/GT at the National Fuel Cell Research Center are compared and presented.
Discussion and Analysis of Flue Gas Utilization in a Phosphoric Acid Fuel Cell Engine During Idle Operation
Authors: Betts Daniel A. , Roan Vernon P. , Fletcher James H.
Published: January 2006

Fuel cell technology has in recent years won the favor of all major car manufacturers as a likely future replacement of the internal combustion engine. This has been driven by the potential for high efficiency and low emissions. Still, fuel cell engines must overcome major hurdles before being introduced into the market. One such hurdle is systems integration; in particular, with fuel cell engines that do not use hydrogen as their primary fuel. In these engines, fuels, such as methanol, are employed instead of hydrogen because of their high-energy density and ease of storage. However, these benefits are counterbalanced by the need to reform these fuels on-board the fuel cell vehicle, thus substantially increasing the complexity of the fuel cell engine. Through the course of operating a 30ft methanol-fueled phosphoric acid fuel cell bus, repeated overheating of the steam-reformer catalyst bed at low engine power outputs was noted. For the purpose of better understanding these overheating events, relevant data was obtained from the bus. Bus operation at low-power outputs was found to have low hydrogen consumption (∼40% to ∼60% of the stack’s incoming hydrogen) due to low electrical demands and reformate flow rate constraints. In the bus, the anode flue gas is burned in the reformer burner, which provides the heat required for the endothermic methanol steam reforming reaction (see Fig. 1 later). At low engine power, the flue gas is the reformer burner’s only fuel, but the energy carried by the flue gas is much greater than the energy required for the reforming reaction and consequently causes reformer overheating at low power. This case serves as an example of how appropriate fuel cell system integration is crucial in order to harness fuel cell benefits. To study such system integration problems, a steady-state model of the fuel cell bus engine was developed. Using this model the bus reformer overheating problem was understood and several possible improvements for utilization of the flue gas were conceited.
Comprehensive Numerical Modeling and Analysis of a Cell-Based Indirect Internal Reforming Tubular SOFC
Authors: Nishino Takafumi , Iwai Hiroshi , Suzuki Kenjiro
Published: January 2006

A comprehensive numerical model of an indirect internal reforming tubular Solid Oxide Fuel Cell (IIR-T-SOFC) has been developed. Two-dimensional axisymmetry of the velocity, temperature, and mass transfer fields was assumed in the model, but accommodating the peripheral nonuniformity of electric potential and electric current fields in the tubular cell for the case with internal reforming and electrochemical reactions. By using the developed model, it was examined how the thermal field and power generation characteristics of the cell are affected by gas inlet conditions and filling pattern of the reforming catalyst inside the fuel feed tube. In particular, optimization of the catalyst distribution pattern was demonstrated to be effective in the reduction of the maximum temperature and temperature gradient, in the mitigation of the possible appearance of a hot spot and therefore in making the life of a fuel cell longer with little loss of the power generation performance of the cell.
Effects of Inlet Mass Flow Distribution and Magnitude on Reactant Distribution for PEM Fuel Cells
Authors: McGarry M. , Grega L.
Published: January 2006

The mass flow distribution and local flow structures that lead to areas of reactant starvation are explored for a small power large active area PEM fuel cell. A numerical model was created to examine the flow distribution for three different inlet profiles; blunt, partially developed, and fully developed. The different inlet profiles represent the various distances between the blower and the inlet to the fuel cell and the state of flow development. The partially and fully developed inlet profiles were found to have the largest percentage of cells that are deficient, 20% at a flow rate of 6.05 g/s. Three different inlet mass flow rates (stoichs) were also examined for each inlet profile. The largest percent of cells deficient in reactants is 27% and occurs at the highest flow rate of 9.1 g/s (3 stoichs) for the partially and fully developed turbulent profiles. In addition to the uneven flow distribution, flow separation occurs in the front four channels for the blunt inlet profile at all flow rates examined. These areas of flow separation lead to localized reactant deficient areas within a channel.
Multi-Resolution PEM Fuel Cell Model Validation and Accuracy Analysis
Authors: Liu Qingyun , Wu Junxiao
Published: January 2006

A multi-resolution simulation method was developed for the polymer electrolyte membrane (PEM) fuel cell simulation: a full 3D model was employed for the membrane and diffusion layer; a 1D+2D model was applied to the catalyst layer, that is, at each location of the fuel cell plate, the governing equations were integrated only in the direction perpendicular to the fuel cell plate; and a quasi-1D model with high numerical efficiency and reasonable accuracy was employed for the flow channels. The simulation accuracy was assessed in terms of the fuel cell polarization curves and membrane Ohmic overpotential. Overall, good agreements between the simulated results and the experimental data were obtained. However, at large current densities, with high relative humidity reactant inputs, the simulation under-predicted the fuel cell performance due to the single-phase assumption; the simulation slightly over-predicted the fuel cell performance for a dry cathode input, possibly due to the nonlinearity of the membrane properties in dehydration case. Further, a parameter study was performed under both fully humidified and relatively dry conditions for the parameters related to the cathode catalyst layer and the gas diffusion layer (GDL). It is found that the effects of liquid water in both the GDL and catalyst layer on the cell performance, and the accurate identification of the cathode catalyst layer parameters such as the cathodic transfer coefficient should be focused for future studies in order to further improve the model accuracy.
Modeling Radiation Heat Transfer With Participating Media in Solid Oxide Fuel Cells
Authors: VanderSteen J. D. J. , Pharoah J. G.
Published: January 2006

Solid oxide fuel cell (SOFC) technology has been shown to be viable, but its profitability has not yet been seen. To achieve a high net efficiency at a low net cost, a detailed understanding of the transport processes both inside and outside of the SOFC stack is required. Of particular significance is an accurate determination of the temperature distribution because material properties, chemical kinetics, and transport properties depend heavily on the temperature. Effective utilization of the heat can lead to a substantial increase in overall system efficiency and decrease in operating cost. Despite the extreme importance in accurately predicting temperature, the SOFC modeling community appears to be uncertain about the importance of incorporating radiation into their models. Although some models have included it, the majority of models ignore radiative heat transfer. SOFCs operate at temperatures around or above 1200 K, where radiation effects can be significant. In order to correctly predict the radiation heat transfer, participating gases must also be included. Water vapor and carbon dioxide can absorb, emit, and scatter radiation, and are present at the anode in high concentrations. This paper presents a simple thermal transport model for analyzing heat transfer and improving thermal management within planar SOFCs. The model was implemented using a commercial computational fluid dynamic code and includes conduction, convection, and radiation in a participating media. It is clear from this study that radiation must be considered when modeling solid oxide fuel cells. The effect of participating media radiation was shown to be minimal in this geometry, but it is likely to be more important in tubular geometries.
Changes of Internal Stress in Solid-Oxide Fuel Cell During Red-Ox Cycle Evaluated by In Situ Measurement With Synchrotron Radiation
Authors: Sumi Hirofumi , Ukai Kenji , Yokoyama Misuzu , Mizutani Yasunobu , Doi Yoshihisa , Machiya Shutaro , Akiniwa Yoshiaki , Tanaka Keisuke
Published: January 2006

The internal stress in anode-supported solid-oxide fuel cells (SOFCs) was evaluated by in situ measurement using high-energy x-ray synchrotron radiation. The oxidized cell had a compression of ∼400MPa in the c-ScSZ electrolyte thin film and a tension of 50–100 MPa in the NiO-YSZ anode substrate at room temperature. The internal stress decreased with increasing temperature, becoming approximately zero at 1000 K. Although the internal stress returned to its initial value after the thermal cycle, the stress decreased to ∼200MPa in the electrolyte after the reduction cycle because of the decrease of the coefficient of thermal expansion mismatch between the electrolyte and anode. The red-ox cycle would be detrimental for anode-supported SOFC.
A Technological Solution for an Everywhere Energy Supply With Sun, Hydrogen, and Fuel Cells
Authors: Orecchini F. , Santiangeli A. , Dell’Era A.
Published: January 2006

The hydrogen economy is still at the beginning, but society, innovation, and the market push inexorably toward hydrogen, inspiring the idea to build an energy-integrated system that can satisfy, in an independent way, the energy needs of small-sized consumers. The technologies used for the system design are already available in the market and, at least for the standard solutions, sufficiently mature. The innovation consists of an integration, optimization, and industrialization of this modular system, which is an electric zero-emissions generator, giving 3.5kWp as an output power. This is the only system able to produce its own fuel, guaranteeing renewable and clean energy, available where and when you want. This system is constituted by a polymer membrane electrolyzer, a metal hydrides tank (which absorbs and desorbs hydrogen), and a polymer fuel cell (PEM). The system modularity can also satisfy higher energy requirements, and the low-pressure hydrogen storage system through metal hydrides guarantees the system safety.
Mixed-Fuels Fuel Cell Running on Methane-Air Mixture
Authors: Yoon Sung Pil , Kim Hyun Jae , Park Byung-Tak , Nam Suk Woo , Han Jonghee , Lim Tae-Hoon , Hong Seong-Ahn
Published: January 2006

In order to develop solid oxide fuel cells (SOFCs) running on hydrocarbon fuels, we have focused on a new method of improving electrode performance and reducing carbon deposition by coating thin films of samaria-doped ceria (SDC) within the pores of electrode by a sol-gel coating technique. The SDC coating on the pores of anode made it possible to have a good stability for long-term operation due to low carbon deposition and nickel sintering. In this study, we demonstrated a new method of improving electrode performance and reducing carbon deposition by coating thin films of samaria-doped ceria and applied the modification technique to two different types of fuel cell structures, anode-supported SOFC and comb-shaped SOFC. From our results, the maximum power density of an anode-supported cell (electrolyte; 8 mol% YSZ and thickness of 30μm, and cathode; La0.85Sr0.15MnO3) with the modified anode was ∼300mW∕cm2 at 700°C in the mixture of methane (25%) and air (75%) as the fuel, and air as the oxidant. The cell was operated for 500hr without significant degradation of cell performance. For the comb-shaped SOFCs operated in the mixed-fuels fuel cell conditions, the cell performance was 40mW∕cm2 at 700°C in the CH4∕O2 ratio of 1.
Thermal Characterization of Cu∕CoFe Multilayer for Giant Magnetoresistive Head Applications
Authors: Yang Y. , White R. M. , Asheghi M.
Published: January 2006

Giant magnetoresistance (GMR) head technology is one of the latest advancements in the hard disk drive (HDD) storage industry. The GMR head multilayer structure consists of alternating layers of extremely thin metallic ferromagnetic and nonmagnetic films. A large decrease in the electrical resistivity from antiparallel to parallel alignment of the film magnetizations is observed, known as the GMR effect. The present work characterizes the in-plane electrical and thermal conductivities of Cu∕CoFe GMR multilayer structures in the temperature range of 50K to 340K using Joule-heating and electrical resistance thermometry on suspended bridges. The thermal conductivity of the GMR layer monotonically increases from 25Wm−1K−1 (at 55K) to nearly 50Wm−1K−1 (at room temperature). We also report a GMR ratio of 17% and a large magnetothermal resistance effect (GMTR) of 25% in the Cu∕CoFe multilayer structure.
Investigation of Lightpipe Volumetric Radiation Effects in RTP Thermometry
Authors: Frankman David J. , Webb Brent W. , Jones Matthew R.
Published: January 2006

A major obstacle to the widespread implementation of rapid thermal processing (RTP) is the challenge of wafer temperature measurement. Frequently, lightpipe radiation thermometers are used to measure wafer temperatures in RTP reactors. While the lightpipe distorts the wafer temperature profile less than temperature measurement techniques which require physical contact, the presence of the lightpipe influences the wafer temperature profile. This paper presents the results of a theoretical study exploring that influence for an idealized RTP reactor in which the wafer is treated as a nonconducting, opaque, constant-heat-flux surface imaged by the lightpipe. The coupled radiation/conduction transport in the lightpipe measurement enclosure is solved numerically. Radiation transfer in the system is modeled with varying levels of rigor, ranging from a simple volumetrically nonparticipating treatment to a full spectral solution of the radiative transfer equation. The results reveal a rather significant effect of the lightpipe on the wafer temperature, which depends on the separation between the lightpipe tip and the wafer. The study illustrates clearly the need to model the lightpipe as a volumetrically participating, semitransparent medium, and further, the importance of accounting for spectral variation of the lightpipe properties in the prediction of the radiative transfer. Finally, two primary mechanisms are identified by which the lightpipe affects the wafer temperature distribution.
Magnetohydrodynamic Convective Flow of a Micropolar Fluid Past a Continuously Moving Vertical Porous Plate in the Presence of Heat Generation/Absorption
Authors: Rahman M. M. , Sattar M. A.
Published: January 2006

Magnetohydrodynamic convective flow and heat transfer of a micropolar fluid past a continuously moving vertical porous plate in the presence of heat generation/absorption with constant suction has been analyzed numerically. With appropriate transformations the boundary layer equations are transformed into nonlinear ordinary differential equations. The local similarity solutions of the transformed dimensionless equations for the flow, microrotation, and heat transfer characteristics are evaluated using Nachtsheim-Swigert shooting iteration technique. Numerical results are presented in the form of velocity, microrotation, and temperature profiles within the boundary layer for different parameters entering into the analysis. Also the effects of the pertinent parameters on the local skin friction coefficient and rate of the heat transfer in terms of the local Nusselt number are also discussed.
Functional or Operator Representation of Numerical Heat and Mass
Transport Models
Authors: Danko G.
Published: January 2006

A numerical-computational procedure is described to determine a multidimensional functional or an operator for the representation of the computational results of a numerical transport code. The procedure is called numerical transport code functionalization (NTCF). Numerical transport codes represent a family of engineering software to solve, for example, heat conduction problems in solids using ANSYS (a multiphysics software package by ANSYS, Inc.), heat and moisture transport problems in porous media using NUFT (Non-equilibrium, Unsaturated-saturated Flows and Transport—porous-media transport code, developed by John Nitao at the Lawrence Livermore National Laboratory), or laminar or turbulent flow and transport problems using FLUENT (a software package by Fluent, Inc.), a computational fluid dynamic (CFD) model. The NTCF procedure is developed to determine a model for the representation of the code for a variety of time-dependent input functions. Coupled solution of multiphysics problems often require repeated, iterative calculations for the same model domain and with the same code, but with different boundary condition functions. The NTCF technique allows for reducing the number of runs with the original numerical code to the number of runs necessary for NTCF model identification. The NTCF procedure is applied for the solution of coupled heat and moisture transport problems at Yucca Mountain, NV. The NTCF method and the supporting software is a key element of MULTIFLUX (by University of Nevada, Reno), a coupled thermohydrologic-ventilation model and software. Numerical tests as well as applications for Yucca Mountain, NV are presented using both linear and nonlinear NTCF models. The performance of the NTCF method is demonstrated both in accuracy and modeling acceleration.
Heat Transfer Around a Cylindrical Protuberance Mounted in a Plane Turbulent Boundary Layer
Authors: Tsutsui Takayuki , Kawahara Masafumi
Published: January 2006

Heat transfer characteristics around a low aspect ratio cylindrical protuberance placed in a turbulent boundary layer were investigated. The diameters of the protuberance, D, were 40 and 80mm, and the height to diameter aspect ratio H∕D ranged from 0.125 to 1.0. The Reynolds numbers based on D ranged from 1.1×104 to 1.1×105 and the thickness of the turbulent boundary layer at the protuberance location, δ, ranged from 26 to 120mm for these experiments. In this paper we detail the effects of the boundary layer thickness and the protuberance aspect ratio on heat transfer. The results revealed that the overall heat transfer for the cylindrical protuberance reaches a maximum value when H∕δ=0.24.
Calculation of the Die Entry Flow of a Concentrated Polymer Solution Using Micro-Macro Simulations
Authors: Feigl Kathleen , Senaratne Deepthika C.
Published: January 2006

A micro-macro simulation algorithm for the calculation of polymeric flow is developed and implemented. The algorithm couples standard finite element techniques to compute velocity and pressure fields with stochastic simulation techniques to compute polymer stress from simulated polymer dynamics. The polymer stress is computed using a microscopic-based rheological model that combines aspects of network and reptation theory with aspects of continuum mechanics. The model dynamics include two Gaussian stochastic processes, each of which is destroyed and regenerated according to a survival time randomly generated from the material’s relaxation spectrum. The Eulerian form of the evolution equations for the polymer configurations is spatially discretized using the discontinuous Galerkin method. The algorithm is tested on benchmark contraction domains for a polyisobutylene solution. In particular, the flow in the abrupt die entry domain is simulated and the simulation results are compared to experimental data. The results exhibit the correct qualitative behavior of the polymer and agree well with the experimental data.
Experimental Study on the Helical Flow in a Concentric Annulus With Rotating Inner Cylinder
Authors: Woo Nam-Sub , Kim Young-Ju , Hwang Young-Kyu
Published: January 2006

This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed laminar flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose, respectively, when the inner cylinder rotates at the speed of 0-600rpm. The results of the present study show the effect of the bulk flow Reynolds number Re and Rossby number Ro on the skin friction coefficients. They also point to the existence of a flow instability mechanism. The effect of rotation on the skin friction coefficient depends significantly on the flow regime. In all flow regimes, the skin friction coefficient is increased by the inner cylinder rotation. The change in skin friction coefficient, which corresponds to a variation of the rotational speed, is large for the laminar flow regime, whereas it becomes smaller as Re increases for transitional flow regime and, then, it gradually approaches to zero for turbulent flow regime. Consequently, the critical bulk flow Reynolds number Rec decreases as the rotational speed increases. The rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.
Surface Micromachined Dielectrophoretic Gates for the Front-End Device of a Biodetection System
Authors: James Conrad D. , Okandan Murat , Galambos Paul , Mani Seethambal S. , Bennett Dawn , Khusid Boris , Acrivos Andreas
Published: January 2006

We present a novel separation device for the front-end of a biodetection system to discriminate between biological and non-biological analytes captured in air samples. By combining AC dielectrophoresis along the flow streamlines and a field-induced phase-separation, the device utilizes “dielectrophoretic gating”to separate analytes suspended in a flowing fluid based on their intrinsic polarizability properties. The gates are integrated into batch fabricated self-sealed surface-micromachined fluid channels. We demonstrate that setting the gate to a moderate voltage in the radio frequency range removed bacteria cells from a mixture containing non-biological particles without the need for fluorescent labeling or antibody-antigen hybridization, and also validate experimentally basic relations for estimating the gate performance.
Swirling Flow of a Viscoelastic Fluid in a Cylindrical Casing
Authors: Itoh Motoyuki , Suzuki Masahiro , Moroi Takahiro
Published: January 2006

The swirling flow of a viscoelastic fluid in a cylindrical casing is investigated experimentally, using aqueous solutions of 0.05–1.0wt.% polyacrylamide as the working fluid. The velocity measurements are made using laser Doppler anemometer. The aspect ratios H∕R (H: axial length of cylindrical casing; R: radius of rotating disk) investigated are 2.0, 1.0, and 0.3. The Reynolds numbers Re0 based on the zero shear viscosity and the disk-tip velocity are between 0.36 and 50. The velocity measurements are mainly conducted for the circumferential velocity component. The experimental velocity data are compared to the velocity profiles obtained by numerical simulations using Giesekus model and power-law model. It is revealed that at any aspect ratios tested the dimensionless circumferential velocity component Vθ′ decreases with increasing Weissenberg number We. Both the Giesekus and power-law models could predict the retardation of circumferential velocity fairly well at small We. The extent of the inverse flow region, where the fluid rotates in the direction opposite to the rotating disk, is clarified in detail.
A Numerical Study of Dean Instability in Non-Newtonian Fluids
Authors: Fellouah H. , Castelain C. , Ould El Moctar A. , Peerhossaini H.
Published: January 2006

We present a numerical study of Dean instability for non-Newtonian fluids in a laminar 180deg curved-channel flow of rectangular cross section. A methodology based on the Papanastasiou model (Papanastasiou, T. C., 1987, J. Rheol., 31(5), pp. 385–404) was developed to take into account the Bingham-type rheological behavior. After validation of the numerical methodology, simulations were carried out (using FLUENT CFD code) for Newtonian and non-Newtonian fluids in curved channels of square or rectangular cross section and for a large aspect and curvature ratios. A criterion based on the axial velocity gradient was defined to detect the instability threshold. This criterion was used to optimize the grid geometry. The effects of curvature and aspect ratio on the Dean instability are studied for all fluids, Newtonian and non-Newtonian. In particular, we show that the critical value of the Dean number decreases with increasing curvature ratio. The variation of the critical Dean number with aspect ratio is less regular. The results are compared to those for Newtonian fluids to emphasize the effect of the power-law index and the Bingham number. The onset of Dean instability is delayed with increasing power-law index. The same delay is observed in Bingham fluids when the Bingham number is increased.
Vortex Motion in a Swirling Flow of Surfactant Solution with Drag Reduction
Authors: Munekata Mizue , Matsuzaki Kazuyoshi , Ohba Hideki
Published: January 2006

Surfactants are well known as additives which induce drag reduction in the straight (nonswirling) pipe flow. However, in industrial applications of the drag-reducing effect, many flow fields besides the straight pipe flow need to be considered. The purpose of this study is to investigate the flow characteristics of the surfactant solution in swirling pipe flow. The drag-reducing effect is estimated from the measurement of wall pressure drop and velocity profiles on various pipe sections by two-dimensional LDV (Laser Doppler Velocimeter). Since the surfactant solution has viscoelasticity, interesting flow characteristics are obtained. The decay of swirl, the vortex type and the turbulence intensity are discussed, compared with the swirling flow of the water. As the results, it is concluded that the change from Rankin’s combined vortex to the forced vortex at a more upstream section by suppressing progress of free vortex and stretch of forced vortex introduces considerable drag reduction. Oscillation of the vortex core is also investigated, and it is found that the oscillation is independent of swirl number.
Swirling Flow of a Viscoelastic Fluid With Free Surface—Part I: Experimental Analysis of Vortex Motion by PIV
Authors: Wei Jinjia , Li Fengchen , Yu Bo , Kawaguchi Yasuo
Published: January 2006

The swirling flows of water and CTAC (cetyltrimethyl ammonium chloride) surfactant solutions (50-1000ppm) in an open cylindrical container with a rotating disc at the bottom were experimentally investigated by use of a double-pulsed PIV (particle image velocimetry) system. The flow pattern in the meridional plane for water at the present high Reynolds number of 4.3×104 differed greatly from that at low Reynolds numbers, and an inertia-driven vortex was pushed to the corner between the free surface and the cylindrical wall by a counter-rotating vortex caused by vortex breakdown. For the 1000ppm surfactant solution flow, the inertia-driven vortex located at the corner between the bottom and the cylindrical wall whereas an elasticity-driven reverse vortex governed the majority of the flow field. The rotation of the fluid caused a deformation of the free surface with a dip at the center. The dip was largest for the water case and decreased with increasing surfactant concentration. The value of the dip was related to determining the solution viscoelasticity for the onset of drag reduction.
Swirling Flow of a Viscoelastic Fluid With Free Surface—Part II: Numerical Analysis With Extended Marker-and-Cell Method
Authors: Yu Bo , Wei Jinjia , Kawaguchi Yasuo
Published: January 2006

In Part I [Wei et al., 2004, 2004 ASME Int. Mech. Eng. Conference], we presented the experimental results for swirling flows of water and cetyltrimethyl ammonium chloride (CTAC) surfactant solution in a cylindrical vessel with a rotating disk located at the bottom for a Reynolds number of around 4.3×104 based on the viscosity of solvent. For the large Reynolds number, violent irregular instantaneous secondary flows at the meridional plane were observed by use of a particle image velocimetry system. Because of the limitations of our computer resources, we did not carry out direct numerical simulation for such a large Reynolds number. The LES and turbulence model are alternative methods, but a viscoelastic LES/turbulence model has not yet been developed for the surfactant solution. In this study, therefore, we limited our simulations to a laminar flow. The marker-and-cell method proposed for Newtonian flow was extended to the viscoelastic flow to track the free surface, and the effects of Weissenberg number and Froude number on the flow pattern and surface shape were studied. Although the Reynolds number is much smaller than that of the experiment, the major experimental observations, such as the inhibition of primary and secondary flows and the decrease of the dip of the free surface by the elasticity of the solution, were qualitatively reproduced in the numerical simulations.
Taylor-Couette Instabilities in Flows of Newtonian and Power-Law Liquids in the Presence of Partial Annulus Obstruction
Authors: Loureiro B. V. , de Souza Mendes P. R. , Azevedo L. F. A.
Published: January 2006

The flow inside a horizontal annulus due to the inner cylinder rotation is studied. The bottom of the annular space is partially blocked by a plate parallel to the axis of rotation, thereby destroying the circumferential symmetry of the annular space geometry. This flow configuration is encountered in the drilling process of horizontal petroleum wells, where a bed of cuttings is deposited at the bottom part of the annulus. The velocity field for this flow was obtained both numerically and experimentally. In the numerical work, the equations which govern the three-dimensional, laminar flow of both Newtonian and power-law liquids were solved via a finite-volume technique. In the experimental research, the instantaneous and time-averaged flow fields over two-dimensional meridional sections of the annular space were measured employing the particle image velocimetry (PIV) technique, also both for Newtonian and power-law liquids. Attention was focused on the determination of the onset of secondary flow in the form of distorted Taylor vortices. The results showed that the critical rotational Reynolds number is directly influenced by the degree of obstruction of the flow. The influence of the obstruction is more perceptible for Newtonian than for non-Newtonian liquids. The more severe is the obstruction, the larger is the critical Taylor number. The height of the obstruction also controls the width of the vortices. The calculated steady-state axial velocity profiles agreed well with the corresponding measurements. Transition values of the rotational Reynolds number are also well predicted by the computations. However, the measured and predicted values for the vortex size do not agree as well. Transverse flow maps revealed a complex interaction between the Taylor vortices and the zones of recirculating flow, for moderate to high degrees of flow obstruction.
Plane Poiseuille Flow of Two Compatible Polymers: Influence of the Interphase on the Flow Stability
Authors: Rousset François , Bourgin Patrick , Palade Liviu-Iulian
Published: January 2006

This paper deals with coextrusion flows of two compatible polymers which are known to be generally more stable than the same flows of incompatible systems. We show that the weak response to disturbance of such flows can be predicted by considering an interphase of nonzero thickness (corresponding to an interdiffusion zone) instead of a purely geometrical interface between the two layers. As a first step we try to explain the weak sensibility to disturbance of compatible systems by the sole presence of this intermediate layer. For that purpose we study the linear stability response to very long waves of a three-layer phase Poiseuille flow with an inner thin layer which represents the interphase. Although this fact is an approximation, it nevertheless takes into account the diffusion phenomena which are generated in the interphase. This first approach (corresponding to a reduction in the effective viscosity ratio) is shown to explain the diminished growth rates but not the reduction in the size of the unstable region. As a second step, we formulate an energetic approach of the problem. We evaluate the energy dissipated during the interdiffusion process and the variation of kinetic energy of the global system. A modified growth rate is then determined by taking into account the energy dissipated by the interdiffusion process. This lower growth rate enables us to explain the increase of the stable domain in the case of compatible polymeric systems.
Controlled Nanoassembly and Construction of Nanofluidic Devices
Authors: Riegelman M. , Liu H. , Bau H. H.
Published: January 2006

This paper describes the combined use of controlled nanoassembly and microfabrication (photolithography) to construct multi-walled, carbon, nanotube-based fluidic devices. The nanoassembly technique utilizes dielectrophoresis to position individual nanotubes across the gap between two electrodes patterned on a wafer. The dielectrophoretic migration process was studied theoretically and experimentally. Once a tube had been trapped between a pair of electrodes, photoresist was spun over the wafer and developed to form microfluidic interfaces. Liquid condensation in and evaporation from the nanotubes were observed with optical microscopy. The nanotube-based fluidic devices can be used for studies of fluid transport under extreme confinement and as sensitive sensors.
Analysis of the Swirling Flow Downstream a Francis Turbine Runner
Authors: Susan-Resiga Romeo , Dan Ciocan Gabriel , Anton Ioan , Avellan François
Published: January 2006

An experimental and theoretical investigation of the flow at the outlet of a Francis turbine runner is carried out in order to elucidate the causes of a sudden drop in the draft tube pressure recovery coefficient at a discharge near the best efficiency operating point. Laser Doppler anemometry velocity measurements were performed for both axial and circumferential velocity components at the runner outlet. A suitable analytical representation of the swirling flow has been developed taking the discharge coefficient as independent variable. It is found that the investigated mean swirling flow can be accurately represented as a superposition of three distinct vortices. An eigenvalue analysis of the linearized equation for steady, axisymmetric, and inviscid swirling flow reveals that the swirl reaches a critical state precisely (within 1.3%) at the discharge where the sudden variation in draft tube pressure recovery is observed. This is very useful for turbine design and optimization, where a suitable runner geometry should avoid such critical swirl configuration within the normal operating range.