bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of Several Turbulence Models for Turbulent Flow in Concentric and Eccentric Annuli
Authors: Azouz I. , Shirazi S. A.
Published: February 2008

Turbulent flow in concentric and eccentric annuli is numerically simulated as part of an investigation aimed at modeling drilled cuttings transport in wellbores. A numerical code is developed to solve the time-averaged momentum equation wherein the Reynolds stresses are modeled using the eddy viscosity approach. A nonorthogonal curvilinear, boundary-fitted coordinate system is used to facilitate the implementation of boundary conditions. Several turbulence models, including a one-layer mixing length model developed as part of this study, a two-layer mixing-length model, and a low Reynolds number, two-equation (k-τ) model are used to simulate turbulent flow in several concentric and eccentric annuli. Performance of these turbulence models is evaluated by comparing numerical predictions to experimental data obtained from several sources. Results show that the proposed one-layer mixing length model performs as well as the two-layer mixing length model and the two-equation model while avoiding some of the difficulties associated with the implementation of these models.
Agglomeration and Fouling in Petroleum Coke-Fired FBC Boilers
Authors: Anthony E. J. , Preto F. , Jia L. , Iribarne J. V.
Published: February 2008

Experiments have been done subjecting ashes from industrial-scale FBC boilers to sulphating conditions in an oven for up to 105 days. These show that sulphation by itself causes agglomeration in the virtual absence of V, K, and Na, the elements normally associated with ash softening and classical fouling. In addition, it has been demonstrated that sulphation goes to completion over long periods of time and, at a specific level which differs from one ash to another, results in agglomeration. These experiments have also shown that there is a size range (75–300 μm) in which the agglomeration is worst, and particles that are smaller or larger either do not agglomerate or agglomerate more weakly. Added “inert” coal-derived ash decreases or prevents the agglomeration. However, this ash does not appear to chemically combine with the sulphate, but acts by mechanically separating the sulphating particles. Finally, if alkali metals are present they can cause agglomeration at levels lower than those at which either the alkalis or sulphation separately cause agglomeration, i.e., they operate synergistically to cause fouling. Current work is being directed at examining these phenomena at higher temperatures (900°C and above).
On the Use of Wind-Wave Spectral Formulas to Estimate Wave Energy
Authors: McCormick M. E.
Published: February 2008

It is common practice by wave energy conversion technologists to estimate long-term wave energy potentials at both offshore and coastal sites by using established wind-wave spectral formulas. It is shown that the use of these formulas can lead to both incorrect wave energy resource estimates and improperly designed conversion systems. The formulas are shown to poorly predict modal and peak spectral periods for long-term seas.
Aspect Ratio Modeling and Design Procedure for GLCC Compact Separators
Authors: Gomez L. E. , Mohan R. S. , Shoham O. , Marrelli J. D. , Kouba G. E.
Published: February 2008

The petroleum industry has recently shown interest in the development of innovative alternatives to the conventional vessel-type separator. One such alternative is the gas-liquid cylindrical cyclone (GLCC) separator, which is simple, compact, and low weight, and has low capital and operational costs. A new mechanistic model is proposed, for the first time, to predict the aspect ratio of the GLCC, based on its complex hydrodynamic multiphase flow behavior. This model incorporates an analytical solution for the gas-liquid vortex interface shape, and a unified particle trajectory model for bubbles and droplets. A simplified GLCC design methodology, based on the foregoing mechanistic model, is developed and specific design criteria are proposed as user guidelines for GLCC design. A summary of four actual field application designs is provided to demonstrate the capability of the aspect ratio modeling and the impact the GLCC technology may have on the petroleum industry.
A Mechanistic Model for Predicting Pressure Drop in Vertical Upward Two-Phase Flow
Authors: Tengesdal J. O̸. , Sarica C. , Schmidt Z. , Doty D.
Published: February 2008

A comprehensive mechanistic model is formulated to predict flow patterns, pressure drop, and liquid holdup in vertical upward two-phase flow. The model identifies five flow patterns: bubble, dispersed bubble, slug, churn, and annular. The flow pattern prediction models are the Ansari et al. (1994) model for dispersed bubble and annular flows, the Chokshi (1994) model for bubbly flow, and a new model for churn flow. Separate hydrodynamic models for each flow pattern are proposed. A new hydrodynamic model for churn flow has been developed, while Chokshi’s slug flow model has been modified. The Chokshi and Ansari et al. models have been adopted for bubbly and annular flows, respectively. The model is evaluated using the expanded Tulsa University Fluid Flow Projects (TUFFP) well data bank of 2052 well cases covering a wide range of field data. The model is also compared with the Ansari et al., (1994), Chokshi (1994), Hasan and Kabir (1994), Aziz et al. (1972), and Hagedorn and Brown (1964) methods. The comparison results show that the proposed model performs the best and agrees well with the data.
Effects of Adsorbent Conductivity and Permeability on the Performance of a Solid Sorption Heat Pump
Authors: Ellis M. W. , Wepfer W. J.
Published: February 2008

Solid sorption heat pumps can improve the effectiveness with which energy resources are used for heating and cooling. These systems operate by alternately heating and cooling beds of adsorbent material to produce a flow of refrigerant. The research presented here evaluates the effects of adsorbent thermal conductivity and permeability on the performance of a thermal wave solid sorption heat pump. In order to evaluate these effects, a numerical model of the thermal wave heat pump is developed. This model incorporates not only the effects of the conductivity and permeability, but also the effects of the adsorption equilibrium properties, refrigerant properties, application parameters, operating parameters, and bed geometry. For a typical air conditioning application, the model is used to study the influence of conductivity and permeability on the COP for systems using ammonia as a refrigerant. The results indicate that for the geometry considered, increasing the thermal conductivity of the adsorbent to 1 W/m-K can improve the COP to approximately 0.75. Further increases in conductivity do not yield improved performance. Furthermore, the reduced permeability associated with high conductivity adsorbents can impair vapor flow and lead to decreased performance.
Flow Field Prediction and Bubble Trajectory Model in Gas-Liquid Cylindrical Cyclone (GLCC) Separators
Authors: Mantilla I. , Shirazi S. A. , Shoham O.
Published: February 2008

Several mechanistic models have been already developed for predicting the onset of liquid carryover in gas-liquid cylindrical cyclone (GLCC) separators. However, currently no model is available to predict gas carryunder. A bubble trajectory model has been developed that can be used to determine the initiation of gas carryunder in the GLCC and to design GLCC for field applications. The bubble trajectory model uses a predicted flow field in GLCC that is based on swirl intensity. This paper describes the development of a general correlation to predict the decay of the swirl intensity. The correlation accounts for the effects of fluid properties (Reynolds number) as well as inlet geometry. Available experimental data as well as computational fluid dynamics (CFD) simulations were used to validate the correlation. The swirl intensity is used to calculate the local axial and tangential velocities. The flow model and improved bubble trajectory results agree with experimental observation and CFD results. Examples are provided to show how the bubble trajectory model can be used to design GLCC.
New Model to Analyze Nonlinear Leak-Off Test Behavior
Authors: Altun G. , Shirman E. , Langlinais J. P. , Bourgoyne A. T.
Published: February 2008

A leak-off test (LOT) is a verification method to estimate fracture pressure of exposed formations. After cementing each casing string, LOT is run to verify that the casing, cement and formation below the casing seat can withstand the wellbore pressure required to drill for the next casing string safely. Estimated fracture pressure from the test is used as the maximum pressure that may be imposed on that formation. Critical drilling decisions for mud weights, casing setting depths, and well control techniques are based upon the result of a LOT. Although LOT is a simple and inexpensive test, its interpretation is not always easy, particularly in formations that give nonlinear relationships between pumped volume and injection pressure. The observed shape of the LOT is primarily controlled by the local stresses. However, there are other factors that can affect and distort LOT results. Physically the LOT, indeed, reflects the total system compressibility, i.e., the compressibility of the drilling fluid, wellbore expansion, or so-called borehole ballooning, and leak (filtration) of drilling fluids into the formation. There is, however, no mathematical model explaining the nonlinear behavior. Disagreement on determining or interpreting actual leak-off pressure from the test data among the operators is common. In this paper, a mathematical model using a well-known compressibility equation is derived for total system compressibility to fully analyze nonlinear LOT behavior. This model accurately predicts the observed nonlinear behavior in a field example. The model also predicts the fracture pressure of the formation without running a test until formation fracture.
Sensitivity Analysis of Absorption Cycle Fluid Thermodynamic Properties
Authors: Palmer S. C. , Shelton S. V.
Published: February 2008

Absorption heat pump technology may be improved by new cycle configurations by new working fluids. In this study, the effect of hypothetical working fluids on performance improvement is explored. The performance of two cycles is studied using three fluid property sources for ammonia/water, i.e., curve-fit experimental data, an ideal solution model, and the Peng-Robinson equation of state model. The models require only minimal fundamental thermodynamic property data for the two pure components. This allows investigation into the influence of each fundamental property on cycle performance, providing insight into desirable properties for new absorption fluid pairs. Variations of fundamental fluid properties are used as input to the models, showing that the volatilities of the refrigerant and absorbent have the greatest effect on cycle performance.
Influence of the Parameters of a Human Triceps Surae Muscle Model on the Isometric Torque-Angle Relationship
Authors: Out Lia , Vrijkotte Tanja G. M. , van Soest Arthur J. , Bobbert Maarten F.
Published: February 2008

This study investigates the influence of parameter values of the human triceps surae muscle on the torque-angle relationship. The model used consisted of three units, each containing a contractile, a series elastic and a parallel elastic element. Parameter values were based on morphological characteristics, which made it possible to model individual units. However, for a number of parameters the values reported in the literature vary considerably. It was investigated how sensitive model results were for variation of these parameters. Slack length of the series elastic element, mean moment arm, maximum force, and length of the contractile element appeared to be the most important determinants of the behavior. For mean moment arm and contractile element length, morphology-based methods of estimation could be recommended. Slack length and maximum force were obtained through optimization. It was concluded that the model does not contain parameters on which its output depends strongly and which are difficult to estimate as well, with two exceptions: slack length of the series elastic element and maximum force.
A Poroelastic Finite Element Formulation Including Transport and Swelling in Soft Tissue Structures
Authors: Simon B. R. , Liable J. P. , Pflaster D. , Yuan Y. , Krag M. H.
Published: February 2008

A field theory is presented for the study of swelling in soft tissue structures that are modeled as poroelastic materials. As a first approximation, soft tissues are assumed to be linear isotropic materials undergoing infinitesimal strains. Material properties are identified that are necessary for the solution of initial boundary value problems where swelling and convection are significant. A finite element model is developed that includes the solid displacements, the relative fiuid displacements, and a representative concentration as the primary unknowns. A numerical example is presented based on a triphasic model. The finite model simulates a typical experimental protocol for soft tissue testing and demonstrates the interaction and coupling associated with relative fluid motion and swelling in a deforming poroelastic material. The theory and finite element model provide a starting point for nonlinear porohyperelastic transport-swelling analyses of soft tissue structures that include finite strains in anisotropic materials.
Optimal Design of Biaxial Tests for Structural Material Characterization of Flat Tissues
Authors: Lanir Y. , Lichtenstein O. , Imanuel O.
Published: February 2008

A rational methodology is developed for optimal design of biaxial stretch tests intended for estimating material parameters of flat tissues. It is applied to a structural model with a variety of constitutive equations and test protocols, and for a wide range of parameter levels. The results show nearly identical optimal designs under all circumstances. Optimality is obtained with two uniaxial stretch tests at mutually normal directions inclined by 22.5 deg to the axes of material symmetry. Protocols which include additional equibiaxial tests provide superior estimation with lower variance of estimates. Tests performed at angles 0, 45, and 90 deg to the axes of material symmetry provide unreliable estimates. The optimal sampling is variable and depends on the protocols and model parameters. In conclusion, the results indicate that biaxial tests can be improved over presently common procedures and show that this conclusion applies for a variety of circumstances.
An Investigation of the Mechanics of Tactile Sense Using Two-Dimensional Models of the Primate Fingertip
Authors: Srinivasan M. A. , Dandekar K.
Published: February 2008

Tactile information about an object in contact with the skin surface is contained in the spatiotemporal load distribution on the skin, the corresponding stresses and strains at mechanosensitive receptor locations within the skin, and the associated pattern of electrical impulses produced by the receptor population. At present, although the responses of the receptors to known stimuli can be recorded, no experimental techniques exist to observe either the load distribution on the skin or the corresponding stress-state at the receptor locations. In this paper, the role of mechanics in the neural coding of tactile information is investigated using simple models of the primate fingertip. Four models that range in geometry from a semi-infinite medium to a cylindrical finger with a rigid bone, and composed of linear elastic media, are analyzed under plane strain conditions using the finite element method. The results show that the model geometry has a significant influence on the surface load distribution as well as the subsurface stress and strain fields for a given mechanical stimulus. The elastic medium acts like a spatial low pass filter with the property that deeper the receptor location, the more blurred the tactile information. None of the models predicted the experimentally observed surface deflection profiles under line loads as closely as a simple heterogeneous waterbed model that treated the fingerpad as a membrane enclosing an incompressible fluid (Srinivasan, 1989). This waterbed model, however, predicted a uniform state of stress inside the fingertip and thus failed to explain the spatial variations observed in the neural response. For the cylindrical model indented by rectangular gratings, the maximum compressive strain and strain energy density at typical receptor locations emerged as the two strain measures that were directly related to the electrophysiologically recorded response rate of slowly adapting type I (SAI) mechanoreceptors. Strain energy density is a better candidate to be the relevant stimulus for SAIs, since it is a scalar that is invariant with respect to receptor orientations and is a direct measure of the distortion of the receptor caused by the loads imposed on the skin.
Stress and Strain Distribution in Hypertensive and Normotensive Rat Aorta Considering Residual Strain
Authors: Matsumoto Takeo , Hayashi Kozaburo
Published: February 2008

The effects of hypertension on the stress and strain distributions through the wall thickness were studied in the rat thoracic aorta. Goldblatt hypertension was induced by constricting the left renal artery for 8 weeks. Static pressure-diameter-axial force relations were determined on excised tubular segments. The segments were then sliced into thin ring specimens. Circumferential strain distributions were determined from the cross-sectional shape of the ring specimens observed before and after releasing residual stresses by radial cutting. Stress distributions were calculated using a logarithmic type of strain energy density function. The wall thickness at the systolic blood pressure, Psys, significantly correlated with Psys. The mean stress and strain developed by Psys in the circumferential direction were not significantly different between the hypertensive and control aortas, while those in the axial direction were significantly smaller in the hypertensive aorta than in the control. The opening angles of the stress-free ring specimens correlated well with Psys. The stress concentration factor in the circumferential direction was almost constant and independent of Psys, although the stress distributions were not uniform through the wall thickness. Histological observation showed that the wall thickening caused by hypertension is mainly due to the hypertrophy of the lamellar units of the media, especially in the subintimal layer where the stress increase developed by hypertension is larger than in the other layers. These results indicate that: (a) the aortic wall adapts itself to the mechanical field by changing not only the wall dimensions but also the residual stresses, (b) this adaptation is primarily related to the circumferential stress but not to the axial stress, and (c) the aortic smooth muscle cells seem to change their morphology in response to the mechanical stress.
Pulsatile Flow in the Human Left Coronary Artery Bifurcation: Average Conditions
Authors: He Xiaoyi , Ku David N.
Published: February 2008

The localization of atherosclerosis in the coronary arteries may be governed by local hemodynamic features. In this study, the pulsatile hemodynamics of the left coronary artery bifurcation was numerically simulated using the spectral element method for realistic in vivo anatomic and physiologic conditions. The velocity profiles were found to be skewed in both the left anterior descending and the circumflex coronary arteries. Velocity skewing arose from the bifurcation as well as from the curvature of the artery over the myocardial surface. Arterial wall shear stress was significantly lower in the bifurcation region, including the side walls. The greatest oscillatory behavior was localized to the outer wall of the circumflex artery. The time-averaged mean wall shear stress varied from about 3 to 98 dynes/cm2 in the left coronary artery system. The highly localized distribution of low and oscillatory shear stress along the walls strongly correlates with the focal locations of atheroma in the human left coronary artery.
Flow Rate-Pressure Drop Relation in Coronary Angioplasty: Catheter Obstruction Effect
Authors: Back L. H. , Kwack E. Y. , Back M. R.
Published: February 2008

Quantitative methods to measure the hemodynamic consequences of various endovascular interventions including balloon angioplasty are limited. Catheters measuring translesional pressure drops during balloon angioplasty procedures can cause flow blockage and thus inaccurate estimates of pre- and post-intervention flow rates. The purpose of this investigation was to examine the influence of the presence and size of an angioplasty catheter on measured mean pressure gradients across human coronary artery stenoses. Analytical flow modeling and in vitro experimental evidence, coupled with angiographic data on the dimensions and shape of stenotic vessel segments before and after angioplasty, indicated significant flow blockage effects with the catheter present.
Cavitation Dynamics of Medtronic Hall Mechanical Heart Valve Prosthesis: Fluid Squeezing Effect
Authors: Lee C. S. , Chandran K. B. , Chen L. D.
Published: February 2008

The cause of cavitation in mechanical heart valves is investigated with Medtronic Hall tilting disk valves in an in vitro flow system simulating the closing event in the mitral position. Recordings of pressure wave forms and photographs in the vicinity of the inflow surface of the valve are attempted under controlled transvalvular loading rates averaged during valve closing period. The results revealed presence of a local flow field with a very high velocity around the seat stop of mechanical heart valves that could induce pressure reduction below liquid vapor pressure and a cloud of cavitation bubbles. The analysis of the results indicates that the “fluid squeezing” between the stop and occluder as the main cause of cavitation in Medtronic Hall valves. The threshold loading rate for cavitation initiation around the stop was found to be very low (300 and 400 mmHg/s; half the predicted normal human loading rate that was estimated to be 750 mmHg/s) because even a mild impact created a high speed local flow field on the occluder surface that could induce pressure reduction below vapor pressure. The present study suggests that mechanical heart valves with stops at the edge of major orifice region are more vulnerable to cavitation, and hence, have higher potential for damage on valve components and formed elements in blood.
Particle Volumetric Residence Time Calculations in Arterial Geometries
Authors: Kunov Mads J. , Steinman D. A. , Ethier C. Ross
Published: February 2008

The quantification of particle (platelet) residence times in arterial geometries is relevant to the pathogenesis of several arterial diseases. In this manuscript, the concept of “volumetric residence time” (VRT) is introduced. The VRT takes into account where particles accumulate and how long they remain there, and is wellsuited to characterizing particle distributions in the complex geometries typical of the cardiovascular system. A technique for the calculation of volumetric residence time is described, which assumes that platelets are neutrally buoyant passive tracer particles, and which tracks small Lagrangian fluid elements containing a uniform concentration of platelets. This approach is used to quantify particle (platelet) residence times in the region of a modeled stenosis with a 45 percent area reduction. Residence time distributions are computed for a representative population of platelets, and for a subpopulation assumed to be “activated” by exposure to shear stresses above a threshold value. For activated platelets, high particle residence times were observed just distal to the apex of the stenosis throat, which can be explained by the presence of high shear stresses and low velocities in the throat immediately adjacent to the vessel wall. Interestingly, the separation zone distal to the stenosis showed only modestly elevated residence times, due to its highly mobile and transient nature. This calculation demonstrates the utility of the VRT concept for cardiovascular studies, particularly if a subpopulation of all particles is to be tracked. We conclude that the volumetric residence time is a useful tool.
Numerical Analysis of Steady Flow in Aorto-Coronary Bypass 3-D Model
Authors: Inzoli Fabio , Migliavacca Francesco , Pennati Giancarlo
Published: February 2008

Intimal hyperplasia and atherosclerosis have a predominant role in the failure of coronary artery bypass procedures. Theoretical studies and in vivo observations have shown that these pathologies are much more likely to occur in the proximity of end-to-side anastomosis, thus indicating that fluid dynamic conditions may be included in the pathogenic causes of the initiation, progression and complication of intimal hyperplasia. In order to study the fluid dynamics at the anastomosis of an aorto-coronary bypass, a three-dimensional mathematical model based on a FEM approach was developed. Steady-state simulations were studied in two different geometrical models of anastomosis which differ in their insertion angles (45 and 60 degree). Flow fields with three-dimensional helical patterns, secondary flows, and shear stresses were also investigated. The results show the presence of low shear stresses on the top wall just beyond the toe of the anastomosis and in the region of the coronary artery before the junction. A high wall shear stress region is present on the lateral wall of the coronary artery immediately downstream from the anastomosis. The influence of flow rate distribution on the secondary flows is also illustrated. These results confirm the sensitivity of flow behavior to the model’s geometrical parameters and enhance the importance of reproducing the anastomosis junction as closely as possible in order to evaluate the effective shear stress distribution.
Effects of Restressing on the Mechanical Properties of Stress-Shielded Patellar Tendons in Rabbits
Authors: Yamamoto Noritaka , Hayashi Kozaburo , Kuriyama Hiroyuki , Ohno Kazunori , Yasuda Kazunori , Kaneda Kiyoshi
Published: February 2008

We studied the effects of restressing on the mechanical properties and morphology of stress-shielded rabbit patellar tendons. After completely unloading the patellar tendon for 1 to 3 weeks, tension was again applied to the tendon for the subsequent 3 to 12 weeks. Although the stress shielding markedly decreased the tangent modulus and tensile strength of the tendon, restressing significantly increased them. However, the mechanical properties of the tendon were not completely recovered even after a prolonged period of restressing. The microstructure of the tendon was also restored by although the recovery was incomplete. These results indicate that the mechanical properties and morphology of tendinous tissue change in response to mechanical demands.
A Mathematical Expression of Three-Dimensional Configuration of the Scoliotic Spine
Authors: Kanayama M. , Tadano S. , Kaneda K. , Ukai T. , Abumi K.
Published: February 2008

Three-dimensional configuration of the scoliotic spine was mathematically expressed by a spatial curve passing through each vertebral centroid (“vertebral body line”). Three-dimensional location of the vertebral centroid was determined from digitization on the frontal and sagittal roentgenograms. Cobb angle, which is clinically used for measuring scoliosis curvature, was calculated in space to evaluate scoliosis deformity three-dimensionally. In forty-five scoliotic spines, regardless of curvature and curve patterns, the spinal configurations were excellently approximated by vertebral body lines. Vertebral body lines swerved from the sagittal plane at the end vertebrae, but aligned on a certain plane within the scoliosis region. Three-dimensional Cobb angle, which was larger than that in the frontal plane, can be utilized to evaluate the scoliosis deformity.
Normal Six-Degree-of-Freedom Motions of Knee Joint During Level Walking
Authors: Li Xue Mei , Liu Bo , Deng Bo , Zhang Shi Ming
Published: February 2008

The purpose of this study was to find an effective way for in vivo measurement of joint motion and give the normal knee motion according to this method. The joint model proposed by Grood and Suntay (1983) was chosen; the origin of the tibia was modified for the convenience of in vivo test. A computerized 6 degree-of-freedom electrogoniometer (EGM) was used in the measurement. Repeated tests on one subject were performed to establish the reliability of the method. Knee motions obtained from 42 males during level walking were expressed as flexion-extension, abduction-adduction, external-internal rotation, lateral-medial translation, anterior-posterior translation, and superior-inferior translation. The results showed that, with the proper choice of the origin of the tibia, the EGM could depend much less on external landmarks and be more effective for the joint measurement.
Flow Testing Methods to Detect and Characterize Partial Blockages in Looped Subsea Flowlines
Authors: Scott S. L. , Yi J.
Published: February 2008

Looping subsea flowlines is one method being utilized for flow assurance. Looping allows the possibility of circulating chemicals and/or tools to remediate partial flowline blockages due to wax, paraffins, asphaltenes, hydrates, sand, and scale. This study considers if flow testing can provide an a priori knowledge of the presence and/or size of the partial blockage before remedial action is taken. The techniques developed are for single-phase liquid flow and can be applied during circulation operations in looped flowlines or in single-phase oil transportation lines. Using these models, a blockage map is created to indicate what size of blockage can be detected and what size of blockage can be characterized as to length and diameter of the restriction. Field-scale verification was obtained by placing fixed length and diameter partial blockages in the 9460-ft LSU flowloop (3.64-in. i.d.). Tests were conducted with liquid rates to 11 bpm (15,840 bbl/day) and pressures to 4000 psia.
Resistive Force of Wax Deposits During Pigging Operations
Authors: Mendes P. R. Souza , Braga A. M. B. , Azevedo L. F. A. , Correa K. S.
Published: February 2008

One of the forces that act on a pig while in operation in a pipeline is that due to the layer of wax deposited on the pipeline wall. Therefore, if the motion of a pig or its cleaning efficiency are to be determined, this resistive force must be known as a function of the relevant parameters. The goal of this paper is to present a procedure for evaluating this force in situations of engineering interest. In essence, the procedure consists of the following steps: (i) determine the wax shear strength according to an experimental procedure developed for this purpose; (ii) from finite-element results, obtain the maximum stress in wax; (iii) with the foregoing information, calculate the pressure required for the pig to cause a maximum stress on wax equal to its shear strength. This pressure value is the threshold below which no wax removed can occur, according to the present simple approach. In this paper, each of these steps is described in detail and exemplified.
A New Method of Steam Quality Measurement Based on Ultrasound
Authors: Shen J. J. S.
Published: February 2008

A new method of measuring the steam quality based on ultrasound is presented in this paper. This method is based on rigorous consideration of the steam’s thermodynamic properties. It is shown in the paper that the steam’s speed of sound depends on its quality at a given saturation temperature. Thus, the quality of the wet steam can be determined if its speed of sound is known. The speed of sound of a flowing fluid can be measured using transit-time ultrasonic meters. Field testing of a pair of customdesigned ultrasonic transducers has shown that it is feasible to measure the steam’s speed of sound. An uncertainty analysis of this new method is also presented in the paper. The analysis suggests that this method is capable of achieving a measurement accuracy of better than ±3 percentage points of the steam quality under typical oil field steam injection conditions.
Effect of Coiled Tubing Initial Configuration on Buckling Behavior in Deviated Wells
Authors: Qiu W. Y. , Miska S. Z. , Volk L. J.
Published: February 2008

Current sinusoidal and helical buckling models are valid only for initially straight coiled tubing (CT). This paper stresses the effect of the pipe initial configuration (residual bending) on the sinusoidal and helical buckling behaviors in deviated wells. Using the conservation of energy and the principle of virtual work, new general equations are derived for predicting the sinusoidal and helical configurations of CT. These new equations reduce to those previously published when the CT is initially straight in deviated wells. Numerical examples are provided to show the effect of the initial amplitude, the inclination angle, and the size of a borehole on the sinusoidal and helical buckling behaviors of CT with the residual bending.
Effect of Air Preheat Temperature and Oxygen Concentration on Flame Structure and Emission
Authors: Gupta A. K. , Bolz S. , Hasegawa T.
Published: February 2008

The structure of turbulent diffusion flames with highly preheated combustion air (air preheat temperature in excess of 1150°C) has been obtained using a specially designed regenerative combustion furnace. Propane gas was used as the fuel. Data have been obtained on the global flame features, spectral emission characteristics, spatial distribution of OH, CH, and C2 species, and pollutant emission from the flames. The results have been obtained for various degrees of air preheat temperatures and O2 concentration in the air. The color of the flame was found to change from yellow to blue to bluish-green to green over the range of conditions examined. In some cases a hybrid color flame was also observed. The recorded images of the flame photographs were analyzed using color-analyzing software. The results show that thermal and chemical flame behavior strongly depends on the air preheat temperature and oxygen content in the air. The flame color was observed to be bluish-green or green at very high air preheat temperatures and low-oxygen concentration. However, at high-oxygen concentration, the flame color was yellow. The flame volume was found to increase with increase in air-preheat temperature and decrease in oxygen concentration. The flame length showed a similar behavior. The concentrations of OH, CH, and C2 increased with an increase in air preheat temperatures. These species exhibited a two-stage combustion behavior at low-oxygen concentration and single-stage combustion behavior at high-oxygen concentration in the air. Stable flames were obtained for remarkably low equivalence ratios, which would not be possible with normal combustion air. Pollutant emission, including CO2 and NOx, was much lower with highly preheated combustion air at low O2 concentration than with normal air. The results also suggest uniform flow and flame thermal characteristics with conditioned, highly preheated air. Highly preheated air combustion provides much higher heat flux than normal air, which suggests direct energy savings and a reduction of CO2 to the environment. Colorless oxidation of fuel has been observed under certain conditions.
Three-Dimensional Laser Heating Model and Entropy Generation Consideration
Authors: Yilbas B. S.
Published: February 2008

Lasers find wide applications in heat treatment of engineering parts. The modeling and energy analysis of the heating process can reduce substantially the time required for process optimization and control. In the present study, three-dimensional laser heating model is introduced using an electron kinetic theory approach, the energy analysis is carried out to predict the first and second law efficiencies, and the entropy generation number is computed during the process. The equation derived for the heat conduction is in the form of an integro-differential equation, which does not yield an analytical solution. Therefore, a numerical method employing an explicit scheme is introduced to discretize the governing heat transfer equation. It is found that the electron lattice site atom collision is the determining process for the internal energy gain of the substrate in the surface vicinity. In addition, the overall entropy generation number computed in the heating cycle is less than what occurs in the cooling cycle of the heat treatment process.
The Impact of Adding Nitrogen Substitutes to Conventional Automotive Fuels
Authors: Gouli S. , Serdari A. , Stournas S. , Lois E.
Published: February 2008

The adoption of oxygenates in gasoline was originally spurred by the oil crises of the 1970s. In more recent years, public awareness of the environmental issues constituted the main reason for the spreading of oxygenated compounds in the transportation fuels sector. This paper describes the effects of novel nitrogen compounds in gasoline and diesel fuel on ignition quality and on pollutant emissions. Our intention is to investigate the antiknock quality, as gaged by octane and cetane determinations, of organic chemical structures mostly derivable from biomass, in combination with their effectiveness in reducing exhaust emissions under various operating conditions.
Minimum Irreversibility Criteria for Heat Exchanger Configurations
Authors: Saboya F. E. M. , Costa C. E. S. M. da
Published: February 2008

From the second law of thermodynamics, the concepts of irreversibility, entropy generation, and availability are applied to counterflow, parallel-flow, and cross-flow heat exchangers. In the case of the Cross-flow configuration, there are four types of heat exchangers: I) both fluids unmixed, 2) both fluids mixed, 3) fluid of maximum heat capacity rate mixed and the other unmixed, 4) fluid of minimum heat capacity rate mixed and the other unmixed. In the analysis, the heat exchangers are assumed to have a negligible pressure drop irreversibility. The Counterflow heat exchanger is compared with the other five heat exchanger types and the comparison will indicate which one has the minimum irreversibility rate. In this comparison, only the exit temperatures and the heat transfer rates of the heat exchangers are different. The other conditions (inlet temperatures, mass flow rates, number of transfer units) and the working fluids are the same in the heat exchangers.
The Impact of the Model of the Environment in Exergy Analyses
Authors: Mun˜oz J. R. , Michaelides E. E.
Published: February 2008

The values of absolute exergy and exergetic efficiency depend on the assumed values of the “reference states,” which are essentially a model of the environment of the processes and components considered. The environmental model plays an important role in any optimization process that uses any form of exergy-based analysis. This paper examines the influence of eleven previously proposed models for the environment on the absolute values of the exergy and on the exergetic efficiencies, associated with the operation of three different power plants: a methane cogeneration plant, a coal-fired plant, and a dual-flash geothermal power plant. It is observed that although there are some common characteristics in most of the values obtained from all the models, the absolute exergies and exergetic efficiencies calculated using some of these models deviate substantially from those obtained with the others. The reasons for these deviations are also examined here.
Thermoeconomic Optimization of a Sensible-Heat Thermal-Energy-Storage System: A Complete Storage Cycle
Authors: Zubair S. M. , Al-Naglah M. A.
Published: February 2008

An analytical model for the second-law-based thermoeconomic analysis and optimization of a sensible-heat-storage system is derived and discussed, in which the storage element is both heated and cooled by flowing streams of gases. In this analysis, monetary values are attached to the irreversible losses caused by the finite temperature difference heat transfer and pressure drop in the storage system. Important dimensionless parameters are identified and the results are presented in terms of the optimum dimensionless charging time θS,opt as a function of a dimensionless temperature difference τ, as well as the optimum number of heat transfer units NTUS,opt, as a function of the dimensionless unit cost per unit heat conductance γUA and τ of the storage systems. The systems analyzed are optimized by introducing a new performance criterion described as the cost rate number, Γ*. Several example problems are also presented and the results are compared with that obtained from Krane’s analysis to illustrate the usefulness of the present approach. The influence of important unit cost parameters on NTUS,opt and θS,opt, are also studied in somewhat more detail.
Retrofitting a Geothermal Power Plant to Optimize Performance: A Case Study
Authors: Kanog˘lu M. , C¸engel Y. A.
Published: February 2008

Performance evaluation of a 12.8-MW single-flash design geothermal power plant in Northern Nevada is conducted using actual plant operating data, and potential improvement sites are identified. The unused geothermal brine reinjected back to the ground is determined to represent about 50 percent of the energy and 40 percent of the exergy available in the reservoir. The first and second-law efficiencies of the plant are determined to be 6 percent and 22 percent, respectively. Optimizing the existing single-flash system is shown to increase the net power output by up to 4 percent. Some well-known geothermal power generation technologies including double-flash, binary, and combined flash/binary designs as alternative to the existing system are evaluated and their optimum operating conditions are determined. It is found that a double-flash design, a binary design, and a combined flash/binary design can increase the net power output by up to 31 percent, 35 percent, and 54 percent, respectively, at optimum operating conditions. An economic comparison of these designs appears to favor the combined flash/binary design, followed by the double-flash design.
A 3D-LDA Study of the Relation Between Wall Shear Stress and Intimal Thickness in a Human Aortic Bifurcation
Authors: Hayashi Kozaburo , Yanai Yutaka , Naiki Takeru
Published: February 2008

A realistic model experiment on hemodynamics was performed to study correlations between wall shear stresses measured in a cast model of the aortic bifurcation and intimal thickness at each corresponding site of the native blood vessel from which the cast had been made. An elastic model of a 54 year old human aortic bifurcation was made of a polyurethane elastomer using a dipping method, and was perfused with Newtonian or non-Newtonian fluid under physiologic pulsatile flow condition. Local flow velocities were measured with an optical-fibered, 3-dimensional laser Doppler anemometer (3D-LDA) to determine wall shear stresses. Distribution of intimal thickness was determined using histological specimens of the native blood vessel. The results obtained are: 1) Non-Newtonian fluid rheology increased wall shear stresses; 2) Positive correlations were observed between intimal thickness and the maximum instantaneous wall shear stress, and 3) However, if we take only the data from the circumference at the level of the flow divider tip, there were negative correlations between them.
A Model of Fatigue and Recovery in Paraplegic’s Quadriceps Muscle Subjected to Intermittent FES
Authors: Giat Yohanan , Mizrahi Joseph , Levy Mark
Published: February 2008

The objective of this paper was to propose a mathematical model for the fatigue and recovery phases of a paraplegic’s quadriceps muscle subjected to intermittent functional electrical stimulation (FES). The model is based on in vivo, noninvasive, recording of fatigue related metabolic parameters recorded during stimulation and recovery. Records of the time variations of the muscle’s phosphorus metabolites, particularly the phosphocreatine (PCr) and inorganic phosphorus (Pi), obtained from 31P magnetic resonance spectroscopy (MRS), were used to calculate the intracellular pH level in the muscle and this latter parameter was incorporated in a musculo-tendon model. The fatigue-recovery model allows the transition from the fatiguing phase to the recovery phase as soon as the stimulation terminates and vice versa. This model was incorporated into a Huxley type muscle model expressing the dynamics of the muscle. Two ordinary differential equations describing the musculo-tendon dynamics and the dynamics of the activation were solved simultaneously and records of the force trajectory during intermittent stimulations were obtained. Study cases ranging from 5 to 30 s for each of the stimulation and recovery alternating phases were stimulated. The force and the total impulse in the modeled quadriceps muscle were computed. It was found that the greatest impulse was produced in intermittent stimulation of 40-50 s duty cycle, with a 50 percent ratio between the stimulation and recovery intervals. An additional series of six runs, including two contractions, one of 3 min and one of 1 min, separated by rest periods of 3, 6, 9, 12, 15, and 30 min was performed. From the predicted force trajectories obtained, the maximal force values served for comparison with measured values made on one patient.
A Model of Load Sharing Between Muscles and Soft Tissues at the Human Knee During Static Tasks
Authors: Lloyd D. G. , Buchanan T. S.
Published: February 2008

In this study, we had subjects voluntarily generate various forces in a transverse plane just above their ankles. The contributions of their muscles and soft tissues to the support of the total external knee joint moment were determined by analyzing the experimental data using a biomechanical model of the knee. In this model, muscle forces were estimated using the recorded EMGs. To account for subject variability, various muscle parameters were adjusted using a nonlinear least-squares fit of the model’s estimated flexion and extension joint moments to those recorded externally. Using the estimated muscle forces, the contributions from the muscles and other soft tissues to the total joint moment were obtained. The results showed that muscles were primarily used to support flexion and extension loads at the knee, but in so doing, were able to support some part of the varus or valgus loads. However, soft tissue loading was still required. Soft tissues supported up to an average maximum of 83 percent of the external load in pure varus and valgus. Soft tissue loading in pure varus and valgus was less than 100 percent of the external load as the muscles, on average, were able to support 17 percent of the external load. This muscle support was by virtue of muscle cocontraction and/or specific muscle activation.
Contact Analysis of Ultrahigh Molecular Weight Polyethylene Articular Plate in Artificial Knee Joint During Gait Movement
Authors: Ishikawa H. , Fujiki H. , Yasuda K.
Published: February 2008

To understand the wear mechanism of the ultrahigh molecular weight polyethylene (UHMWPE) articular plate used in artificial knee joints, the cyclic contact behavior of the plate during gait movement was analyzed using the constitutive equation for cyclic plasticity. In this study, two-dimensional plane strain model was employed and the contact behavior of femoral and tibial components was simulated by translating the contact stress distribution which was calculated from elasto-plastic indentation analysis of two components. For analytical model, the anatomical type artificial knee joint was employed and the effect of the shape of contact surface on the wear behavior of the plate was investigated. As a result, it was clarified that the wear of the plate should occur both from the surface and the subsurface of the plate and the wear behavior of the plate should be closely related with the shape of contact surface. Then the optimum shape of contact surface could be designed using this method.