bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Drag-Reducing Agents in Multiphase Flow Pipelines
Authors: Kang C. , Vancko R. M. , Green A. S. , Kerr H. , Jepson W. P.
Published: February 2008

The effect of drag-reducing agents (DRA) on pressure gradient and flow regime has been studied in horizontal and 2-deg upward inclined pipes. Experiments were conducted for different flow regimes in a 10-cm i.d., 18-m long plexiglass system. The effectiveness of DRA was examined for concentrations ranging from 0 to 75 ppm. Studies were done for superficial liquid velocities between 0.03 and 1.5 m/s and superficial gas velocities between 1 and 14 m/s. The results indicate that DRA was effective in reducing the pressure gradients in single and multiphase flow. The DRA was more effective for lower superficial liquid and gas velocities for both single and multiphase flow. Pressure gradient reductions of up to 42 percent for full pipe flow, 81 percent for stratified flow, and 35 percent for annular flow were achieved in horizontal pipes. In 2 deg upward inclination, the pressure gradient reduction for slug flow, with a concentration of 50 ppm DRA, was found to be 28 and 38 percent at superficial gas velocities of 2 and 6 m/s, respectively. Flow regimes maps with DRA were constructed in horizontal pipes. Transition to slug flow with addition of DRA was observed to occur at higher superficial liquid velocities.
Design and Performance of Passive Control System for Gas-Liquid Cylindrical Cyclone Separators
Authors: Mohan R. S. , Wang S. , Shoham O. , Kouba G. E.
Published: February 2008

The performance of gas-liquid cylindrical cyclone (GLCC) separators can be improved by reducing or eliminating liquid carryover into the gas stream or gas carryunder through the liquid stream, utilizing a suitable liquid level control. In this study, a new passive control system has been developed for the GLCC, in which the control is achieved by utilizing only the liquid flow energy. A passive control system is highly desirable for remote, unmanned locations operated with no external power source. Salient features of this design are presented here. Detailed experimental and modeling studies have been conducted to evaluate the improvement in the GLCC operational envelope for liquid carryover with the passive control system. The results demonstrate that a passive control system is feasible for operation in normal slug flow conditions. The advantage of a dual inlet configuration of the GLCC is quantified for comparative evaluation of the passive control system. The results of this study could form the basis for future development of active control systems using a classical control approach.
The Effect of Drillpipe Rotation on Annular Frictional Pressure Loss
Authors: Wei X. , Miska S. Z. , Takach N. E. , Bern P. , Kenny P.
Published: February 2008

Accurate predictions of annular frictional pressure losses (AFPL) are important for optimal hydraulic program design of both vertical and horizontal wells. In this study, the effects of drillpipe rotation on AFPL for laminar, helical flow of power law fluids are investigated through theoretical, study, flow models were developed for concentric and eccentric pipe configurations assuming that pipe rotates about its axis. A hybrid-analytical solution is developed for calculating AFPL in eccentric pipe configuration. Computer simulations indicate that the shear-thinning effect induced by pipe rotation results in reduction of AFPL in both concentric and eccentric pipe configurations. The pressure reduction is most significant for concentric pipe configurations. For conventional rotary drilling geometry and pipe rotary speeds, the reduction in AFPL is small. A number of laboratory experiments conducted on the full-scale TUDRP flow loop are generally in good agreement with the results of modeling. Available fileld data, however, consistently show an increase in AFPL. This behavior is explained by pipe lateral movement (swirling), which causes turbulence and eventually an increase in AFPL.
Disturbed Flow and Flow-Accelerated Corrosion in Oil and Gas Production
Authors: Efird K. D.
Published: February 2008

The effect of fluid flow on corrosion of steel in oil and gas environments involves a complex interaction of physical and chemical parameters. The basic requirement for any corrosion to occur is the existence of liquid water contacting the pipe wall, which is primarily controlled by the flow regime. The effect of flow on corrosion, or flow-accelerated corrosion, is defined by the mass transfer and wall shear stress parameters existing in the water phase that contacts the pipe wall. While existing fluid flow equations for mass transfer and wall shear stress relate to equilibrium conditions, disturbed flow introduces nonequilibrium, steady-state conditions not addressed by these equations, and corrosion testing in equilibrium conditions cannot be effectively related to corrosion in disturbed flow. The problem in relating flow effects to corrosion is that steel corrosion failures in oil and gas environments are normally associated with disturbed flow conditions as a result of weld beads, pre-existing pits, bends, flanges, valves, tubing connections, etc. Steady-state mass transfer and wall shear stress relationships to steel corrosion and corrosion testing are required for their application to corrosion of steel under disturbed flow conditions. A procedure is described to relate the results of a corrosion test directly to corrosion in an operation system where disturbed flow conditions are expected, or must be considered.
Dynamic Differential Pressure Effects on Drilling of Permeable Formations
Authors: Hoberock L. L. , Bratcher G. J.
Published: February 2008

In the mathematical modeling of bit penetration rate for tri-cone roller bits in permeable formations, virtually all of the current techniques assume that the differential pressure between the bottom-hole wellbore pressure and the formation is a “static” value. This work shows that the appropriate differential pressure is a dynamic quantity, because for overbalanced drilling, fluid filtrate from the wellbore requires a finite time to flow into the formation, producing a changing pressure gradient ahead of the bit. Moreover, this dynamic gradient is directly dependent upon the rate of drill bit penetration, which is in turn dependent upon the dynamic gradient itself. Accordingly, coupled penetration rate and dynamic gradient equations must be solved, which frequently result in the prediction of higher drilling penetration rates than when the static gradient is used. The appropriate dynamic differential pressure equations are developed and applied to an example drilling situation. It is shown that with water-based drilling fluids, for rock with permeability greater than a few microdarcies at virtually all penetration rates, and for penetration rates less than 3 m/h (9.84 ft/h) at permeabilities greater than 1 μd (microdarcy), the dynamic differential pressure is significantly less than the static differential pressure. Accordingly, using the conventional static differential pressure results in the prediction of penetration rates that are much too low. Moreover, using measured penetration rates from the field, the conventional approach yields predicted in-situ rock strength that is much too high.
A Desiccant Dehumidifier for Electric Vehicle Heating
Authors: Aceves S. M. , Smith J. R.
Published: February 2008

Vehicle heating requires a substantial amount of energy. Electric vehicles (EVs) generate little waste heat, and using battery energy for heating may consume a substantial fraction of the energy storage capacity, reducing the vehicle range, which is one of the most important parameters in determining EV acceptability. This paper analyzes the applicability of a desiccant dehumidifier for providing comfort heating in an EV with reduced energy consumption. A dehumidifier can reduce the heating load in a vehicle because it adsorbs the water vapor generated by the passengers, thereby avoiding water condensation on the windows without requiring a high external air ventilation rate or a high window temperature. The results indicate that the desiccant dehumidifier can reduce the steady-state heating load by 60 percent or more under typical conditions. The reduction in heating load is such that waste heat may be enough to provide the required heating under most ambient conditions. Desiccant system dimensions and weight appear reasonable for packaging inside an EV.
Effects of Flame Lift-Off on the Differences Between the Diffusion Flames From Circular and Elliptic Burners
Authors: Gollahalli S. R.
Published: February 2008

An experimental study conducted to determine the effects of lifting the flame base off the burner rim on the differences between the flame characteristics of diffusion flames from circular and elliptic burners is presented. The in-flame profiles of temperature, concentrations of fuel and combustion product species, and the mean and fluctuating components of axial velocity are presented. This study has shown that the effects of burner geometry in turbulent lifted flames are considerable only in the near-burner region. In the midflame and far-burner regions, the effects traceable to burner geometry are much weaker, contrary to those observed in the attached flame configuration. The observations are attributed to the turbulence and additional air entrainment into the jet prior to the flame base accompanying the lift-off process, which mitigate the effects of burner geometry.
Type Curves for Predicting Directional Tendencies of Simple Bottom-Hole Assemblies
Authors: Miska S. , Rajtar J. , Luo F.
Published: February 2008

The paper provides solutions for predicting directional tendencies of slick and single stabilizer bottom-hole assemblies (BHA) in curved boreholes. Solutions are derived in a curvilinear system of coordinates in a dimensionless form. These solutions are presented in the form of type curves. Type curves allow for quick evaluation of directional tendencies of a given BHA by providing values of side force and tilt angle at the bit. Using these values an anticipated direction of bit displacement is evaluated, using concepts of formation anisotropy index (FAI) and bit anisotropy index (BAI). Procedures are illustrated by examples of evaluation of side force and tilt at the bit for a slick and single stabilizer BHA. The resulting bit displacements are calculated assuming formation anisotropy and drill bit directional tendencies (FAI and BAI). The results derived from type curves are in agreement with computer simulated data. The comparison shows that application of graphical solutions (type curves) yields only slight error that can be ignored for practical purposes.
Numerical Simulations of Highly Preheated Air Combustion in an Industrial Furnace
Authors: Ishii T. , Zhang C. , Sugiyama S.
Published: February 2008

The numerical simulations of reactive turbulent flows and heat transfer in an industrial slab reheat furnace in which the combustion air is highly preheated have been carried out. The influence of the ratio of the air and fuel injection velocities on the NOx production rate in the furnace has also been studied numerically. A moment closure method with the assumed β probability density function (PDF) for mixture fraction was used in the present work to model the turbulent non-premixed combustion process in the furnace. The combustion model was based on the assumption of instantaneous full chemical equilibrium. The turbulence was modeled by the standard k-ε model with a wall function. The numerical simulations have provided complete information on the flow, heat, and mass transfer in the furnace. The results also indicate that a low NOx emission and high heating efficiency can be achieved in the slab reheat furnace by using low NOx regenerative burners. It is found that the air/fuel injection velocity ratio has a strong influence on the NOx production rate in the furnace.
Time-Dependent Failure Analysis of Inclined Boreholes in Fluid-Saturated Formations
Authors: Cui L. , Abousleiman Y. , Cheng A. H-D. , Roegiers J.-C.
Published: February 2008

This paper presents time-dependent poroelastic failure analyses of boreholes drilled in directions inclined to the far-field stress principal axes in fluid-saturated isotropic porous formations. The work is based on a recently derived analytical solution. The analyses include collapse as well as fracturing failures. The time-dependent failures in relation to borehole inclination, azimuth, and mud pressure are presented. The critical roles of poroelastic constants in borehole stability are also demonstrated.
Chemical Plug for Zone Isolation in Horizontal Wells
Authors: Mamora D. D. , Saavedra N. F. , Burnett D. B. , Platt F. M.
Published: February 2008

A laboratory study has been conducted on the use of chemical plugs, instead of conventional mechanical packers, to isolate water and gas-producing zones in horizontal wells. Results of experiments using horizontal wellbore models, consisting of PVC pipes internally lined with sand, indicate that slumping of the chemical plug could be avoided if the plug were spotted in a viscous brine pill. Of the three chemicals tested, a monomer, a polyacrylamide, and a plastic, only the plastic plug had a sufficiently high holding pressure. Research is being continued using a full-scale horizontal wellbore model.
Application of Flamelet Profiles to Flame Structure in Practical Burners
Authors: Krass B. J. , Zellmer B. W. , Puri I. K. , Singh S.
Published: February 2008

Partial premixing can be induced by design in combustors, occurs inadvertently during turbulent nonpremixed combustion, or arises through inadequate fuel-air mixing. Therefore, it is of interest to investigate the effect of partial premixing in a burner that mimics conditions that might occur under practice. In this investigation, we report on similitude of partially premixed flames encountered in practical complex and multi-dimensional burners with simpler, less complex flames, such as counterflow flamelets. A burner is designed to simulate the more complex multi-dimensional flows that might be encountered in practice, and includes the effects of staging, swirl, and possible quenching by introduction of secondary air. The measurements indicate that the structure of partially premixed flames in complex, practical devices can be analyzed in a manner similar to that of flamelets, even if substantial heat transfer occurs. In particular, the flame structure can be characterized in terms of a modified mixture fraction that differentiates the lean and rich zones, and identifies the spatial location of the flame.
On Total Variation Diminishing Schemes for Pressure Transients
Authors: Ibraheem S. O. , Adewumi M. A.
Published: February 2008

A higher-order numerical procedure is applied to simulate typical transient phenomena in natural gas transportation. Reliable modeling and prediction of transients features in transmission pipelines are desirable for optimal control of gas deliverability, design and implementation of active controls, and modeling of operational behavior of network peripheral equipment (e.g., chokes, valves, compressors, etc.). As an alternative to the method of characteristics (MOC) that had been widely used for modeling these systems, higher-order total variation diminishing (TVD) methods are used to model some transient problems. This class of methods has the capability of capturing fine-scale phenomena, and they do provide a better resolution of frontal discontinuities. In this study, the TVD schemes are utilized in conjunction with upwind methods. Also, in order to ensure a stable time-stepping scheme over a wide range of Courant-Friedrich-Lewy (CFL) number, a special Runge-Kutta method is employed as the base solution algorithm to integrate the highly nonlinear, hyperbolic equations which govern the transportation of natural gas in pipelines. The overall procedure is stable, robust, and accurate when applied to solve practical problems with dynamic pressure waves.
Effect of Drag-Reducing Agent on Slug Characteristics in Multiphase Flow in Inclined Pipes
Authors: Kang C. , Jepson W. P. , Gopal M.
Published: February 2008

The effect of drag-reducing agent (DRA) on multiphase flow in upward and downward inclined pipes has been studied. The effect of DRA on pressure drop and slug characteristics such as slug translational velocity, the height of the liquid film, slug frequency, and Froude number have been determined. Experiments were performed in 10-cm i.d., 18-m long plexiglass pipes at inclinations of 2 and 15 deg for 50 percent oil-50 percent water-gas. The DRA effect was examined for concentrations ranging from 0 to 50 ppm. Studies were done for superficial liquid velocities between 0.5 and 3 m/s and superficial gas velocities between 2 and 10 m/s. The results indicate that the DRA was effective in reducing the pressure drop for both upflow and downflow in inclined pipes. Pressure gradient reduction of up to 92 percent for stratified flow with a concentration of 50 ppm DRA was achieved in ±2 deg downward inclined flow. The effectiveness of DRA for slug flow was 67 percent at a superficial liquid velocity of 0.5 m/s and superficial gas velocity of 2 m/s in 15 deg upward inclined pipes. Slug translational velocity does not change with DRA concentrations. The slug frequency decreases from 68 to 54 slugs/min at superficial liquid velocity of 1 m/s and superficial gas velocity of 4 m/s in 15 deg upward inclined pipes as the concentration of 50 ppm was added. The height of the liquid film decreased with the addition of DRA, which leads to an increase in Froude number.
Investigation of Flow Regime Transitions in Large-Diameter Inclined Pipes
Authors: Cai J. Y. , Gopal M. , Jepson W. P.
Published: February 2008

Multiphase oil/water/gas flow regime transition studies are carried out in a 10-cm i.d., 18-m long pipe at inclinations of ±2 deg at system pressures between 0 to 0.79 MPa. The results are compared to those of other researchers, and the effects of pressure, inclination, and liquid viscosity are shown. The water cut of the liquid has some effects on the transition from stratified to slug flow. Increasing the water cut results in the transition occurring at higher liquid velocity at the same gas velocity. Water cut has little effect on the slug/annular transition for low viscosity oil used. The system pressure has a moderate effect on the transition from stratified to slug and slug to annular. For the transition from stratified to slug, increasing the system pressure requires higher liquid velocity. The transition from slug to annular occurs at lower liquid velocity with increasing the system pressures. The inclination of the pipe has little effect on the transition from slug to annular flow. Increasing the inclination causes the transition to occur at approximately the same gas velocity at the same liquid velocity. The experimental results show a good agreement with Wilkens’ model.
Co-Current and Countercurrent Migration of Gas Kicks in “Horizontal” Wells
Authors: Baca H. , Smith J. , Bourgoyne A. T. , Nikitopoulos D. E.
Published: February 2008

Results from experiments conducted in downward liquid-gas flows in inclined, eccentric annular pipes, with water and air as the working fluids, are presented. The gas was injected in the middle of the test section length. The operating window, in terms of liquid and gas superficial velocities, within which countercurrent gas flow occurs at two low-dip angles, has been determined experimentally. The countercurrent flow observed was in the slug regime, while the co-current one was stratified. Countercurrent flow fraction and void fraction measurements were carried out at various liquid superficial velocities and gas injection rates and correlated to visual observations through a full-scale transparent test section. Our results indicate that countercurrent flow can be easily generated at small downward dip angles, within the practical range of liquid superficial velocity for drilling operations. Such flow is also favored by low gas injection rates.
The Effects of Cardiac Infarction on Realistic Three-Dimensional Left Ventricular Blood Ejection
Authors: Taylor Tad W. , Suga Hiroyuki , Goto Yoichi , Okino Haruka , Yamaguchi Takami
Published: February 2008

The effect of cardiac infarction on the flow patterns in cardiac left ventricular ejection was studied using a realistic model which was made from the profile of the left ventricle of a dog heart in diastole. A Coordinate measuring machine was used to measure the left ventricular coordinates, and these were input into a three-dimensional flow simulation package. The left ventricular wall motion was described by having the walls moved towards the center of the aortic outlet, and in the case of infarcted tissue, the ventricular wall movement was diminished to simulate infarction flow behavior. The final ventricular volume varied from 25 percent to 54.1 percent of the initial volume in cases without and with infarction, respectively. The maximum blood ejection velocities and ventricular pressure decreased significantly in the presence of infarction. Infarcted areas showed complex blood flow vortex formation not present in the healthy ventricles. The Computational technique presented here predicts infarction flow effects which could be observed with measurement techniques such as ultrasound and magnetic resonance imaging, allowing a finer detail of understanding than using either simulation or experimental measurements alone.
Plausibility of Structural Constitutive Equations for Swelling Tissues—Implications of the C-N and S-E Conditions
Authors: Lanir Yoram
Published: February 2008

The mechanically important constituents of swelling tissues are fibers embedded in an osmotically active fluid. The tissues’ response to external loading is the sum of contribution of the axial stresses in the fibers and of the fluid pressure. The fluid osmotic properties play a key role in determining its equilibrium response. The present study examines the conditions under which the elastic response of tissues as modeled by structural constitutive equations, is thermodynamically plausible. The analysis shows that plausibility is ensured if the fibers’ axial force increases monotonically with stretch and if the fluid osmotic pressure increases convexly with concentration. Published data shows that both conditions prevail in swelling tissues. Plausibility considerations seem to pose no specific restrictions on the structure of the tissues’ fibrous network. It is thus concluded that in swelling tissues, structural constitutive formulation is compatible with thermodynamically plausible response.
Unsteady Flow in a Rigid 3-D Model of the Carotid Artery Bifurcation
Authors: Rindt C. C. M. , v. Steenhoven A. A.
Published: February 2008

In the present study, finite element calculations are performed of blood flow in the carotid artery bifurcation under physiological flow conditions. The numerical results are compared in detail with laser-Doppler velocity measurements carried out in a perspex model. It may be concluded that the numerical model as presented here is well capable in predicting axial and secondary flow of incompressible Newtonian fluids in rigid-walled three-dimensional geometries. With regard to the flow phenomena occurring, a large region with reversed axial flow is found in the carotid sinus opposite to the flow divider. This region starts to grow at peak systole, has its maximal shape at minimal flow rate and totally disappears at the start of the acceleration phase. C-shaped axial velocity contours are formed in the deceleration phase, which are highly influenced by secondary flows. These latter flows are mainly induced by centrifugal forces, flow branching, and tapering of the carotid sinus. Lowering the sinus angle, the angle between the main branch and the carotid sinus, results in a smaller region with reversed axial flow.
Biomechanical Model for the Myogenic Response in the Microcirculation: Part II—Experimental Evaluation in Rat Cremaster Muscle
Authors: Lee Shouyan , Schmid-Scho¨nbein Geert W.
Published: February 2008

In order to test experimentally a novel model for myogenic contraction of the arterioles, presented in Part I of this series, a sequence of in-vivo studies was performed on arterioles of rat cremaster muscle using a modified “Box Method.” The tissue was enclosed in a sealed chamber in which the extravascular pressure could be changed as a means of stimulating the myogenic response. The microvascular pressures were measured using an improved cremaster preparation with intact distal feeder. The experiment consists of measurements of a static myogenic response and the response to a step extravascular pressure, as well as passive viscoelastic properties of the arteriole. These measurements served to determine the parameters involved in the theoretical model. The model prediction were then compared with in-vivo observations during a ramp and during oscillatory extravascular pressure changes in the same arterioles. The results indicate that the model is capable of quantitatively predicting time dependent in-vivo changes in response to transmural pressure. The measured model parameters suggest an increase in myogenic activity from proximal arcade arterioles to more distal transverse arterioles in cremaster muscle of Wistar rats.
Flow Measurements in an Aortocoronary Bypass Graft Casting
Authors: Kwack E. Y. , Back L. H. , Ruan X. M. , Chaux A.
Published: February 2008

Flow visualization and pressure measurements were carried out in a single valve saphenous vein casting which was made from a saphenous vein segment obtained from a bypass patient at Cedars Sinai Medical Center. Dye was injected to understand the flow around the valve. The dye showed very complex flow patterns around the valve and in the valve sinus, and the cavity formed by a ligated branch. For steady flow, pressure drops across the valve were 0.72, 2.0 and 6.3 mmHg for the physiological flow rates of 45, 84, and 169 ml/min, respectively. Overall pressure drop across the casting (compared to Poiseuille flow for a straight tube) increased with the flow rate, being 130 to 290 percent higher over this flow rate range. In the case of pulsatile flow, pressure drops across the valve were 0.95 and 3.0 mmHg for the flow rates of 47 and 87 ml/min which were 26 and 43 percent higher than those of steady flow. Overall pressure drop was 220 and 360 percent higher for those flow rates compared to Poiseuille flow. The measured spatial pressure distributions along the casting and flow visualization indicated the global nature of the flow field with the accelerated flow through the valve separating and reattaching downstream along the wall in the pressure recovery region. Atherosclerosis may be prone to occur in the lower shear region along the wall beyond the valve tip in the reattachment region, as we have observed in vivo in rabbit experiments.
Investigation of Drilling Fluids Containing Blast Furnace Slag for Their Potential Impact on Formation Damage—A Laboratory Study
Authors: Tare U. A. , Takach N. E. , Miska S. Z. , Growcock F. B. , Davis N.
Published: February 2008

This work discusses the effect of incorporating blast furnace slag (BFS) as an additive in water-based drilling fluids. The intent of this treatment is rapid development of a thin, impervious, and easily removable filter cake, thereby minimizing detrimental impact of the drilling fluid on formation productivity as opposed to previous applications of BFS in universal fluids. To evaluate the impact of BFS on filter cake properties, permeability plugging apparatus (PPA) tests and dynamic formation damage (DFD) studies were conducted. Drill-in fluids and dispersed muds were tested using varying quantities of BFS. Once a steady rate of dynamic filter cake deposition was achieved, the BFS in the filter cakes was chemically activated. The results obtained from these activation studies were compared with those obtained with no BFS and with unactivated BFS. The nature of the filter cakes was examined with an environmental scanning electron microscope (ESEM). Results obtained from the PPA tests indicate substantial decreases in initial spurt loss and filtrate volume with increasing concentration of BFS. The DFD studies substantiate the aforementioned observations and show enhancement of return permeabilities with BFS activation. ESEM studies demonstrate that BFS can consolidate filter cakes.
Modeling of a Heat Pump With Evaporator Air Dehumidification for Reduced Frost Formation
Authors: Marti´nez-Fri´as J. , Aceves S. M.
Published: February 2008

This paper presents an analysis of a heat pump that uses a solid desiccant dehumidifier to reduce the humidity of the ambient air that flows into the evaporator, with the purpose of decreasing frost formation on the evaporator. The heat pump is analyzed by adding a dehumidifier model to a previously developed heat pump model that includes frost formation. The dehumidifier reduces the amount of energy required for evaporator defrosting, but introduces the need for energy for regenerating the desiccant. The purpose of the analysis is to search for operating conditions and optimum dehumidifier designs for which the use of the dehumidifier results in energy savings. The results show that the use of a dehumidifier may reduce energy consumption if the energetic cost of defrosting the evaporator is high. Other benefits of dehumidification include an increase of the time intervals between defrost cycles, a better stability in the conditions in the controlled space, and the potential for increased reliability and reduced maintenance of the heat pump.
Commercial Boiler Waste-Heat Utilization for Air Conditioning in Developing Countries
Authors: Garimella S. , Garimella V. S.
Published: February 2008

This study investigates the utilization of waste heat from commercial process steam boilers for air conditioning using absorption cooling systems. An ammonia-water generator absorber heat exchange system was developed and modeled to use waste heat from the boiler flue gases and deliver chilled water. Based on approximately 1000 process steam boilers at an average throughput of 2000 kg/h in one metropolitan area in India, the study estimates that installation of these systems could result in annual operating cost savings of $10,200,000 in this region alone. These 1000 systems would also reduce the installed electric capacity needs by 16 MW. Annual coal consumption would decrease by 87,000 tonnes, while ash production would decrease by 39,000 tonnes. Carbon-based emissions are estimated to decrease by 176,000 tonnes. Therefore, installation of these systems on a countrywide basis and also in other developing countries with high year-round cooling loads and coal-based power generation would significantly alleviate installed power capacity shortages, conserve energy resources, and reduce greenhouse gas emissions.
Plunger Lift Versus Velocity Strings
Authors: Lea J. F.
Published: February 2008

Plunger lift and velocity strings are two common methods of attacking liquid loading problems in gas wells. This paper shows how to compare the calculated performance of both methods at downhole producing conditions on one plot.
Optimal Operation of a Gogeneration Plant in Consideration of Equipment Startup/Shutdown Cost
Authors: Yokoyama R. , Ito K.
Published: February 2008

A rational method of determining the operational strategy of energy supply plants in consideration of equipment startup/shutdown cost is proposed. The operational planning problem is formulated as a large-scale mixed-integer linear programming one, in which on/off status and energy flow rates of equipment are determined so as to minimize the sum of energy supply and startup/shutdown costs over the period considered. By utilizing a special structure of the problem, an algorithm of solving the problem efficiently is proposed. Through a numerical study on the daily operational planning of a gas turbine cogeneration plant for district heating and cooling, the effectiveness of the proposed algorithm, is ascertained in terms of computation time, and the influence of equipment startup/shutdown cost on the operational strategy and cost is clarified.
Characterization of Blood Flow Turbulence With Pulsed-Wave and Power Doppler Ultrasound Imaging
Authors: Cloutier Guy , Allard Louis , Durand Louis-Gilles
Published: February 2008

Blood flow turbulence downstream of a concentric 86 percent area reduction stenosis was characterized using absolute and relative Doppler spectral broadening measurements, relative Doppler velocity fluctuation, and Doppler backscattered power. Bidi-mensional mappings of each Doppler index were obtained using a 10 MHz pulsed-wave Doppler system. Calf red cells suspended in a saline solution were used to scatter ultrasound and were circulated in an in vitro steady flow loop model. Results showed that the absolute spectral broadening was not a good index of turbulence because it was strongly affected by the deceleration of the jet and by the shear layer between the jet and the recirculation zones. Relative Doppler spectral broadening (absolute broadening divided by the frequency shift), velocity fluctuation, and Doppler power indices provided consistent mapping of the centerline axial variation of turbulence evaluated by hot-film anemometry. The best agreement between the hot-film and Doppler ultrasound methods was however obtained with the Doppler back-scattered power. The most consistent bidimensional mapping of the flow characteristics downstream of the stenosis was also observed with the Doppler power index. The relative broadening and the velocity fluctuation produced artifacts in the shear layer and in the recirculation zones. Power Doppler imaging is a new emerging technique that may provide reliable in vivo characterization of blood flow turbulence.
Constitutive Laws for Biomechanical Modeling of Refractive Surgery
Authors: Bryant Michael R. , McDonnell Peter J.
Published: February 2008

Membrane inflation tests were performed on fresh, intact human corneas using a fiber optic displacement probe to measure the apical displacements. Finite element models of each test were used to identify the material properties for four different constitutive laws commonly used to model corneal refractive surgery. Finite element models of radial keratotomy using the different best-fit constitutive laws were then compared. The results suggest that the nonlinearity in the response of the cornea is material rather than geometric, and that material nonlinearity is important for modeling refractive surgery. It was also found that linear transverse isotropy is incapable of representing the anisotropy that has been experimentally measured by others, and that a hyperelastic law is not suitable for modeling the stiffening response of the cornea.
An Analysis of Crack Propagation Paths at Implant/ Bone-Cement Interfaces
Authors: McCormack B. A. O. , Prendergast P. J.
Published: February 2008

Clinical follow-up studies of joint replacements indicate that debonding of the implant from the bone-cement is the first mechanical event of loosening. Debonding can occur due to unsustainable interface stresses, usually initiated from defects along the interface. Such defects, or flaws, are inevitably introduced during the surgical procedure and from polymerisation shrinkage. Debonding leads to increased stresses within the cement mantle. This study is concerned with modelling the propagation of a crack from the debonded region on the cement/implant interface under physiological loading conditions for different implant materials and prosthesis designs. Using the theory of linear fracture mechanics for bimaterial interfaces, the behaviour of a crack along an interface between implant materials, under various states of stress, is studied. Specifically, a model is developed to determine the conditions under which a debonded region, along an otherwise bonded interface, will either propagate along the interface or will “kink” into the cement mantle. The relationship between the stress state and the crack propagation direction at the interface is then predicted for different interface materials, and it is shown that different crack directions exist for different materials, even when the stress state is the same. Furthermore, the crack behavior is shown to be dependent on the ratio of normal stress to shear stress at the interface and this may be important for the design optimisation of load-bearing cemented prostheses. Finally, the likelihood that an interface crack will propagate into the cement mantle is explored using a suitable fracture criterion.
On the Nusselt Number in Heat Transfer Between Multiple Parallel Blood Vessels
Authors: Cousins A. K.
Published: February 2008

Arrays of two or more parallel blood vessels in a tissue matrix have been studied extensively in the context of bioheat transfer. The average vessel Nusselt number (based on the difference between the mixed-mean blood temperature and the average vessel surface temperature) is a crucial parameter in such studies. Various workers have noted that in particular cases the average Nusselt number is identical to that for fully developed flow in a single vessel in an infinite medium. In other words, the Nusselt number is unaffected by the presence of other vessels. It is proven here that this surprising result holds true for arbitrary number, size, flow direction, and velocity profile in the blood vessels, and for very general boundary conditions on the outer tissue boundary. A useful corollary is that the average wall temperature in a particular vessel may be found by evaluating the temperature fields due to the other vessels and the tissue boundaries at a single point, the center of the vessel in question.
Numerical Analysis of Three-Dimensional Bjo¨rk–Shiley Valvular Flow in an Aorta
Authors: Shim E.-B. , Chang K.-S.
Published: February 2008

Laminar vortical flow around a fully opened Bjo¨rk–Shiley valve in an aorta is obtained by solving the three-dimensional incompressible Navier–Stokes equations. Used is a noniterative implicit finite-element Navier–Stokes code developed by the authors, which makes use of the well-known finite difference algorithm PISO. The code utilizes segregated formulation and efficient iterative matrix solvers such as PCGS and ICCG. Computational results show that the three-dimensional vortical flow is recirculating with large shear in the sinus region of the valve chamber. Passing through the valve, the flow is split into major upper and lower jet flows. The spiral vortices generated by the disk are advected in the wake and attenuated rapidly downstream by diffusion. It is shown also that the shear stress becomes maximum near the leading edge of the disk valve.
Numerical Modeling of Steady Inspiratory Airflow Through a Three-Generation Model of the Human Central Airways
Authors: Wilquem F. , Degrez G.
Published: February 2008

Two-dimensional steady inspiratory airflow through a three-generation model of the human central airways is numerically investigated, with dimensions corresponding to those encountered in the fifth to seventh generations of the Weibel’s model. Wall curvatures are added at the outer walls of the junctions for physiological purposes. Computations are carried out for Reynolds numbers in the mother branch ranging from 200 to 1200, which correspond to mouth air breathing at flow rates ranging from 0.27 to 1.63 liters per second. The difficulty of generating grids in a so complex configuration is overcome using a nonoverlapping multiblock technique. Simulations demonstrate the existence of separation regions whose number, location, and size strongly depend on the Reynolds number. Consequently, four different flow configurations are detected. Velocity profiles downstream of the bifurcations are shown to be highly skewed, thus leading to an important unbalance in the flow distribution between the medial and lateral branches of the model. These results confirm the observations of Snyder et al. and Tsuda et al. and suggest that a resistance model of flow partitioning based on Kirchhoff’s laws is inadequate to simulate the flow behavior accurately within the airways. When plotted in a Moody diagram, airway resistance throughout the model is shown to fit with a linear relation of slope −0.61. This is qualitatively in good agreement with the experimental investigations of Pedley et al. and Slutsky et al.
An Active Membrane Model for Peristaltic Pumping: Part I—Periodic Activation Waves in an Infinite Tube
Authors: Carew E. O. , Pedley T. J.
Published: February 2008

A model for the coupled problem of wall deformation and fluid flow, based on thin-shell and lubrication theories, and driven by a propagating wave of smooth muscle activation, is proposed for peristaltic pumping in the ureter. The model makes use of the available experimental data on the mechanical properties of smooth muscle and accounts for the soft material between the muscle layer and the vessel lumen. The main input is the activation wave of muscular contraction. Equations for the time-dependent problem in tubes of arbitrary length are derived and applied to the particular case of periodic activation waves in an infinite tube. Mathematical (small amplitude) and numerical analyses of this case are presented. Predictions on phase-lag in wall constriction with respect to peak activation wave, lumen occlusion due to thickening lumen material with contracting smooth muscle, and the general bolus shape are in qualitative agreement with observation. Some modifications to the mechanical, elastic, and hydrodynamic properties of the ureter that will make peristalsis less efficient, due for example to disease, are identified. In particular, the flow rate-pressure rise relationship is linear for weak to moderate activation waves, but as the lumen is squeezed shut, it is seen to be nonlinear in a way that increases pumping efficiency. In every case a ureter whose lumen can theoretically be squeezed shut is the one for which pumping is most efficient.
Strain Softening in Rat Left Ventricular Myocardium
Authors: Emery J. L. , Omens J. H. , McCulloch A. D.
Published: February 2008

We investigated whether strain softening (or the Mullins effect) may explain the reduced left ventricular stiffness previously associated with the strain-history-dependent preconditioning phenomenon. Passive pressure–volume relations were measured in the isolated, arrested rat heart during LV balloon inflation and deflation cycles. With inflation to a new higher maximum pressure, the pressure–volume relation became less stiff, particularly in the low (diastolic) pressure range, without a significant change in unloaded ventricular volume. In five different loading protocols in which the maximum passive cycle pressure ranged from 10 to 120 mmHg, we measured increases at 10 mmHg in LV volume up to 350 percent of unloaded volume that depended significantly on the history (p < 0.05) and magnitude (p < 0.01) of maximum previous pressure. Although a quasi-linear viscoelastic model based on the pressure-relaxation response could produce a nonlinear pressure–volume relation with hysteresis, it was unable to show any significant change in ventricular stiffness with new maximum pressure. We incorporated a strain softening theory proposed by Johnson and Beatty (1992) into the model by modifying the elastic response with a volume-amplification factor that depended on the maximum previous pressure. This model more accurately reproduced the experimentally observed behavior. Thus, the preconditioning behavior of the myocardium is better explained by strain softening rather than viscoelasticity and may be due to damage to elastic components, rather than the effects of viscous tissue components.
Mathematical Optimization of Elastic Properties: Application to Cementless Hip Stem Design
Authors: Kuiper J. H. , Huiskes R.
Published: February 2008

The designer of a cementless hip stem in total hip replacement must find a balance between two conflicting demands. On the one hand, a stiff stem shields the surrounding bone from mechanical loading (stress shielding), which may lead to bone loss, particularly around the proximal part of the stem. Reducing the stem stiffness decreases the amount of stress shielding and hence the amount of bone loss. However, this measure inevitably promotes higher proximal interface stresses and thereby increases the risk of proximal interface failure. The designer’s task therefore is to optimize the stem stiffness in order to find the best compromise in the conflict. Yet, a better compromise might be found when the stem material was nonhomogeneous, in other words when an arbitrary distribution of the elastic properties inside the stem was allowed. The number of conceivable designs would increase enormously, making the designer’s task almost impossible. In the present paper, we develop a numerical design optimization method to determine the optimal stiffness characteristics for a hip stem. A finite element program is coupled with a numerical optimization method, thus producing a design optimization scheme. The scheme minimizes the probability for interface failure while limiting the amount of bone loss, by adapting the parameters describing the nonhomogeneous elastic modulus distribution. As an example, a simplified model of a hip stem is made, whose modulus distribution is optimized. Assuming equal long-term bone loss, the maximum interface stress can be reduced by over 50 percent when compared to a homogeneous flexible stem, thus demonstrating the value of the new approach.
The Influence of the Blunting of the Apex on the Flow in a Vertebro-Basilar Junction Model
Authors: Ravensbergen J. , Krijger J. K. B. , Verdaasdonk A. L. , Hillen B. , Hoogstraten H. W.
Published: February 2008

The apex of human vertebro-basilar junctions can be sharp-edged or blunted. In the present study, the effect of a blunted apex on the flow in vertebro-basilar junction models is investigated. We compared the flow phenomena in a series of junction models with blunted apices and confluence angles 45, 85, and 125 deg with the flow phenomena in a series of junction models with sharp-edged apices and the same range of confluence angles, studied in a previous paper (Ravensbergen et al., 1996b). The blunting of the apex appears to have an effect on the size of the local recirculation area near the apex and the prevailing low velocities. Large recirculation areas are found in the models with blunted apices, especially in those with small confluence angles. In addition, the blunting of the apex has no influence on the flow further downstream, nor on the structure and strength of the secondary flow field. Furthermore, a blunted apex appears to be a geometric risk factor for atherosclerosis. This supports the hypotheses that recirculation areas and low wall shear stress influence the development of atherosclerotic plaques.
Dynamic Contact Stress and Rolling Resistance Model for Total Knee Arthroplasties
Authors: Waldman S. D. , Bryant J. T.
Published: February 2008

Problems associated with premature failure of total knee replacements (TKR’s) include: wear, creep, and oxidation of ultrahigh-molecular-weight polyethylene (UHM-WPe) as well as adverse tissue reactions to polyethylene wear debris. These problems are associated in part with the mechanical behavior of UHMWPe. In TKR’s, contact stress analyses have been performed on the UHMWPe tibial component; however, most have employed simplified material properties and not accounted for joint kinematics. A nonlinear viscoelastic rolling model was developed for TKR’s to predict the contact stress and rolling friction for varying rolling speed, conformity, applied load, and tibial plateau thickness. Results indicated that the contact stress increased and rolling friction decreased with increasing rolling speed. Effects of conformity, applied load, and tibial plateau thickness were consistent with previous models. At large rolling speeds, predicted peak contact stresses were almost twice their static value, resulting in a compound fatigue problem in UHMWPe components due to normal cyclic loading, moving point of contact, and velocity-dependent stresses.