bluuebunny/crossref_metadata_2025 huggingface dataset:
Modulation of ingested water absorption by Palestine sunbirds: evidence for adaptive regulation
Authors: McWhorter Todd J. , del Rio Carlos Martínez , Pinshow Berry
Published: January 2003

Nectarivorous birds feed on dilute sugar solutions containing trace amounts of amino acids and electrolytes. To meet their high mass-specific energy demands they must often deal with exceptionally high proportionate water fluxes. Despite nectar intake rates that may reach more than five times body mass per day, hummingbirds appear to absorb all ingested water. Here, we report the results of experiments designed to examine the relationship between nectar intake and water turnover in nectar-feeding Palestine sunbirds(Nectarinia osea). Like hummingbirds, sunbirds ingested large amounts of water. At the lowest sucrose concentration (292 mmol l-1), food intake rates reached 2.2 times body mass. Fractional and total water turnover increased linearly with water ingestion, but the fraction of ingested water absorbed by sunbirds decreased from 100% to 36% with increasing water intake rate. Palestine sunbirds may therefore avoid absorbing, and thus having to eliminate, up to 64% of their ingested water load when feeding on dilute nectars. To our knowledge, this is the first documentation of regulation of water flux across the gastrointestinal tract to the body. Our data suggest that sunbirds regulate transepithelial water flux independently of sugar absorption. These intriguing results open the door to many questions about how water transport is regulated in the vertebrate gastrointestinal tract. We suggest that intestinal water and body water form two separate but interacting pools in nectar-feeding birds. Convergence in diet has led to the evolution of many similar traits in hummingbirds and sunbirds. The physiological traits of these two groups that allow the processing of a water and sugar diet, however,may be very different.
Proprioceptors monitoring forces in a locust hind leg during kicking form negative feedback loops with flexor tibiae motor neurons
Authors: Sasaki Ken , Burrows Malcolm
Published: January 2003

In preparation for jumping and kicking, a locust slowly generates large forces in the femoral muscles of its hind legs and stores them in elastic distortions of the tendons and femoral cuticle. At the femoro—tibial joints, the semi-lunar processes are bent, the cuticle of the dorsal distal femur is crumpled, and the femur is expanded in a mediolateral direction. We have analysed whether these distortions are monitored by sense organs and whether the information they provide is used to limit the forces generated and thus prevent structural damage to the joint.

The two sensory neurons comprising the lump receptor lie in a groove in the ventral part of the distal femur. The sensory neurons spike if force is applied to the flexor tendon when the joint is fully flexed, but not when it is extended. They also spike as the tendon of the flexor muscle slides into the ventral femoral groove when the tibia is fully flexed during the co-contraction phase of kicking. Their spike frequency correlates with the extent of bending of a semi-lunar process that provides a quantifiable measure of the joint distortions. If the tibia is not fully flexed, however, then muscle contractions still cause distortions of the joint but these are not signalled by sensory spikes from the lump receptor. The lump receptor,therefore, does not respond primarily to the joint distortions but to the movements or force in the flexor tendon.

Contractions of the flexor tibiae muscle caused by spikes in individual flexor motor neurons can evoke spikes in sensory neurons from the lump receptor when the joint is fully flexed. In turn, the sensory neurons cause a hyperpolarisation in particular flexor motor neurons in a polysynaptic negative feedback loop. The lump receptor could, therefore, regulate the output of the flexor motor neurons and, thus, limit the amount of force generated during co-contraction. It may also contribute to the inhibition of the flexors at the end of co-contraction that allows rapid kicking movements to occur.
Fretting fatigue in a spherical contact
Authors: Navarro C , García M , Domínguez J
Published: January 2003

This article describes different types of tests in fretting fatigue and different geometries for use in testing, comparing mainly cylindrical and spherical contacts and their corresponding advantages and disadvantages. Practical differences can be observed between these two types of tests regarding the alignment of the pads, the level of the loads to be applied, the rigidity in the test rig, the shape of the crack, the localization of the point of initiation of the crack, etc. The loads applied in spherical contact and stress distribution beneath the contact zone are described. As this stress state is multiaxial and non-proportional, the appropriate multiaxial criteria are needed in order to characterize the stress field. An equivalent stress is derived from the criteria and later related to the initiation of the crack. At the same time, with the use of the calculated stresses, a semi-analytical approach using a weight function is described for the determination of the stress intensity factor. The results of a series of fretting fatigue tests have been analysed. The initiation site, the orientation of the crack and, for the tests that did not fail, the length of the crack have been studied using a multiaxial fatigue criterion and linear elastic fracture mechanics (LEFM).
Influence of bulk stress on contact conditions and stresses during fretting fatigue
Authors: Tur M , Fuenmayor J , Ródenas J. J
Published: January 2003

This paper analyses the effect of bulk stress applied in the specimen on the contact conditions and stresses during a fretting fatigue test. The problem considered corresponds to that of a fretting bridge test using spherical contact pads. A three-dimensional finite element model together with a mesh subdivision h-adaptive procedure has been used. This has allowed for the evaluation of a precise solution at a reasonable computational cost. Finite element results have been compared with existing analytical models.
Prediction of crack nucleation under partial slip fretting conditions
Authors: Fouvry S , Elleuch K , Simeon G
Published: January 2003

Fretting can be defined as small-amplitude oscillatory motions between two solid surfaces in contact. It has been known for a long time that such loading introduces cracking and/or wear in many structural parts. For very small amplitudes, the contact presents a composite structure where, during the loading cycle, some domains remain stuck whereas others endure sliding. Such a partial slip condition mainly favours crack nucleation and crack propagation. The loading path imposed through the contact requires precise identification of the sliding condition and the local friction coefficient operating through the sliding domain. An energy approach is here developed combining the online identification of the sliding domain and the determination of the local friction coefficient. A normalized fretting sliding chart is introduced and discussed, taking into account the impact of plasticity on the sliding condition. Applying analytical and finite element model computations, it has been shown that crack nucleation under stabilized partial slip can be predicted by transposing multiaxial fatigue criteria (Dang Van, Crossland, McDiarmid and Smith-Watson-Topper). Such a prediction nevertheless requires that the high stress gradient imposed by the contact loading is taken into account. The loading states, which are computed to determine the risk of crack nucleation, must be averaged over an elementary volume whose length scale can be compared with the microstructure.
Mechanical analysis of coated materials undergoing fretting fatigue in a cryogenic environment
Authors: Caron I , Inglebert G , Gras R , De Monicault J. M
Published: January 2003

Fretting damage, also known as small-amplitude oscillatory sliding motion, can lead to catastrophic failure in many industrial applications. An understanding of fretting fatigue and its reproduction in laboratory tests have enabled an evaluation to be made of the fretting resistance of homogeneous substrate. To reduce the damage caused by fretting fatigue, increasing use has been made of coatings or treatments that result in inhomogeneous solids. From a theoretical point of view, ascertaining the mechanical behaviour of materials so modified is quite complex due to insufficient definition of the contact parameters. This present study seeks to analyse a layered medium undergoing fretting fatigue in a cryotechnical environment and the improvement in its fretting fatigue resistance. The first step of the analysis of a layered alloy is the determination of the new contact parameters in elasticity, and the second is the characterization of the elastic-plastic coefficients of the strain-hardening law. An evaluation of the lifetime before crack initiation will close the analysis.
Aligned and misaligned contacts of rollers to races in elastohydrodynamic finite line conjunctions
Authors: Kushwaha M , Rahnejat H , Gohar R
Published: January 2003

The paper provides a solution for finite line concentrated contact of a roller-to-race under aligned and misaligned conditions. The lubricated contact conjunction is subject to an elastohydrodynamic regime of lubrication under isothermal conditions. Of particular interest are the edge stress discontinuities, represented by large secondary pressure spikes at the side constriction and to the rear exit in the contact domain. These pressure ‘pips’ are considerably larger in magnitude than those occurring in the central exit of the contact. The presence of pressure peaks inhibits the flow of lubricant in their vicinity, causing islands of minimum lubricant film thickness at the sides of the contact, referred to as the end closure films. The paper shows that the film shape and pressure distribution at the extremities of a finite line contact are not revealed by the traditional line contact solutions usually undertaken. The flow pattern becomes more complex with roller misalignment and the edge effects described are exacerbated. The paper provides the first ever solution of misaligned roller-to-race contact for moderate to high loaded elastohydrodynamic conjunctions. The numerical predictions conform well with both experimental and numerical findings of others for the cases where similar work has been reported.
Performance of automatic ball balancers on optical disc drives
Authors: Kim W , Chung J
Published: January 2003

The balancing performance of an automatic ball balancer fitted on the spindle motor of optical disc drives, for instance CD-ROM or DVD drives, is analysed. It is well known that the operating speed of an optical disc drive with an automatic ball balancer should be higher than the translational natural frequency of the system to be balanced. However, the influence of the rotational natural frequency on the performance of automatic ball balancers has not yet been investigated. In this study, considering not only the translational but also the rotational motion of the system, the non-linear equations of motion are derived using Lagrange's equation. Applying a perturbation method to the non-linear equations, the linearized equations. Applying a perturbation method to the non-linear equations, the linearized equations in the neighbourhood of a balanced equilibrium position are obtained. These equations have time-dependent periodic coefficients, which require application of the Floquet theory for stability analysis. From the stability analysis the effects of the stiffness and damping of rubber suspensions on the automatic ball balancer's performance are evaluated for the variation of operating speed. To verify the results, time responses are also computed from the non-linear equations. It is finally found that the natural frequency of not only the translational but also the rotational motion has an influence on the balancing performance of automatic ball balancers.
Reduced-order modelling methods to assess the overall vibration response of a flexible rotor-squeeze film damper bearing assembly
Authors: Siew C C , Hill M , Holmes R , Brennan M J
Published: January 2003

This paper uses two reduced-order methods to calculate the overall non-linear vibration response of a multi-mode rotor-squeeze film damper (SFD) bearing assembly. Good agreement has been found between their computed results and those calculated by the conventional Runge-Kutta-Merson method (RKMM), yet they require less than 5 per cent of the computing time consumed by the RKMM. They are applied to compute the vibration response, three-dimensional deflection shape and the overall vibrational kinetic energy of the rotor-bearing assembly. The assembly is simulated under various operating conditions including different unbalances, oil viscosities, static misalignments and damper factors. It is concluded that effective vibration control of a flexible rotor-SFD bearing assembly can be achieved when the vertical static eccentricity ratio of the SFD is set to a certain limit.
The research on non-linear characteristics of a cracked rotor and reconstruction of the crack forces
Authors: Pu Y , Chen J , Zou J , Zhong P
Published: January 2003

The non-linear characteristics of a cracked rotor are investigated in this paper. It is known that the cracked rotor system becomes non-linear when the rotor is not in a weight-dominant condition. In the case of the crack opening, the rotor is weakened not only in the crack direction but also in the perpendicular direction of the crack, and the breathing of the crack is determined by the vibration in the crack direction. Based on these assumptions, a model of a cracked rotor can be established in rotating coordinates and the governing equations can also be obtained. The discontinuities of the governing equations make it very difficult to obtain the vibration response of this kind of model analytically, so the numerical simulation method is applied. The chaos and bifurcation phenomena is observed through the numerical results. According to the results obtained in this paper, the Lyapunov exponent calculated from the simulated time series is proven to be effective in judging the chaotic property of a system. The non-linear effect of a crack can be reconstructed by using the state space theory when considering the crack as an external force.
Robust balancing for rotating machines
Authors: Garvey S D , Friswell M I , Williams E J , Lees A W , Care I D
Published: January 2003

The balancing of rotors divides broadly into two categories: balancing in situ and balancing in a balancing machine. In the latter case, the most common practice is to arrange balance corrections on the rotor such that the net excitations of each of the four in-plane rigid-body modes of the free rotor is zero by deploying balance corrections on two independent planes. In a small proportion of cases, the net excitations of the first pair of flexural modes are also zeroed using a third correction plane.

This paper proposes that, when rotors are balanced in a balancing machine (not similar to the machine stator), substantially more utility can be gained from the balancing operation by combining a suitably weighted account of the specific balancing requirements of the machine with knowledge of the expected machine characteristics than can be achieved by ignoring this knowledge. A single cost function is established based on a numerical model of the machine. Then, depending on circumstances, either the expected value of this cost function or its worst possible value can be minimized.

The methods proposed require that relatively detailed knowledge of the distribution of residual unbalance be obtained experimentally. The paper briefly discusses some practical methods for how such information might be extracted. The definition of the cost function as a matrix quadratic form provides potentially valuable information about the necessary number and the optimal location of balance planes on a given rotor, and methods for determining an optimal set of balance planes are outlined.
Effects of pump geometry on the flow within a centrifugal impeller
Authors: Yedidiah S G
Published: January 2003

This paper briefly examines the effects of certain geometric features on the flow within the impeller of a centrifugal pump.
Separation dynamics of heat shield fairings
Authors: Lochan R , Adimurthy V , Kumar K
Published: January 2003

The dynamics associated with the jettisoning of heat shield fairings from the parent launch vehicle is investigated. The analysis considers a zip-cord based mechanism employed to jettison the fairings from the accelerating on-going stage. A scheme for characterization of the zip-cord system is described. The geometry of collision is analysed for a known, precomputed structural response. The no- collision domain is constructed in flexibility parameter space for an arbitrarily assumed impulse level. Alternatively, the minimum impulse required for a collision-free separation is also computed as a function of design parameters. Finally, the analysis enables generation of the upper limits on payload size as well as that of the external mountings for a clean separation. This methodology has led to the successful separation of shield heat fairings for the two developmental flights of the Indian launcher Polar Satellite Launch Vehicle (PSLV).
Spacecraft ion propulsion development in the United Kingdom
Authors: Fearn D G
Published: January 2003

The on-board propulsion system dominates the design of many spacecraft, particularly as regards mass and volume. However, the mass of this system, sometimes 50 per cent or more of the total, can be reduced dramatically by using ion propulsion technology, rather than the more usual chemical thrusters. In an ion thruster, the propellant is ionized in an electrical discharge, then the positive ions are extracted from the resulting plasma and accelerated to a very high velocity by an electric field, forming an extremely energetic exhaust. This paper describes the development and status of the two UK ion propulsion systems, which have thrust levels in the 1-30 mN and 50-300 mN ranges.
Surface suction on aerofoil aerodynamic characteristics at transonic speeds
Authors: Qin N , Zhu Y , Poll D I A
Published: January 2003

This paper presents a numerical study of the effects of an active flow control through surface suction on shock boundary layer interactions over transonic aerofoils. Two different aerofoils were studied. Firstly, for the purpose of validation, an NACA64A010 aerofoil with a trailing edge flap was investigated and the numerical results were compared with experimental data with and without suction for surface pressure distributions and lift and drag coefficients. Grid sensitivity has also been studied regarding the numerical accuracy. The second geometry was an RAE9647 aerofoil, which was designed for superior aerodynamic performance when applied to a helicopter rotor blade. An active surface was used to prevent or alleviate shock-induced separation. The suction strength and location were varied to determine the effect on aerodynamic performance and to provide an effective means of suppressing undesirable flow features. In both cases, increases in both lift and drag were observed when surface suction was applied. However, the benefit of suction appeared in the form of a substantial increase in the lift-drag ratio. It was also found that the shock location and strength are very sensitive to the suction location and strength. Two different mechanisms for active flow control over transonic aerofoils are discussed.
Thermodynamic analysis of the gasification of coal water slurry fuels for a circulating fluidized bed gasifier
Authors: Tsui H , Yavuzkurt S , Scaroni A
Published: January 2003

To validate the concept of coal gasification in the integrated coal gasification combined cycle (IGCC), a novel laboratory gasifier consisting of a circulating fluidized bed and a cyclone combustor has been constructed. This paper reports a thermodynamic analysis conducted to predict the maximum capacity and the condition for best operation of this circulating fluidized bed gasifier, which has an inside diameter of 0.3048m and a height of 2.5 m. The equilibrium feed rates of materials and the quality of the product gas are described as a function of the reactor temperature, the thermal capacity of the gasifier, and the water concentration in coal water slurry fuel. The results of parametric analysis show that the thermal efficiency decreases, but the efficiency of desulfurization increases as the reactor temperature increases. The thermal capacity of the gasifier has no influence on the quality of the product gas. The thermal efficiency and the efficiency of desulfurization decrease as the water concentration in the coal water slurry increases. The desulfurization in the gasifier at equilibrium conditions is very efficient and meets the EPA regulations of the USA.
Models of steam force and torque of a rotor subjected to the leakage of tip clearance
Authors: Huang D
Published: January 2003

The topic of this paper is focused on the models of steam force and torque of a rotor subjected to the leakage of tip clearance. The unsteady three-dimensional Navier-Stokes solver is used to simulate the flow field for a rotating blade row with a tip clearance. The basic algorithm time marches the fully three-dimensional unsteady equations of motion expressed in a finite volume form. The Baldwin-Lomax model is used as turbulence model. By simulating the complex flow field including oscillating tip clearance with rotor vibration, the instantaneous asymmetry and oscillatory pressures acting on the surfaces of the blade row can be acquired, and the instantaneous steam force and torque acting on the rotor can be calculated. A new nonlinear model of steam force is created in this paper, and a model of steam torque excited by leakage is also reported.
Performance-analysis-based gas turbine diagnostics: A review
Authors: Li Y. G.
Published: January 2003

Gas turbine diagnostics has a history almost as long as gas turbine development itself. Early engine fault diagnosis was carried out based on manufacturer information supplied in a technical manual combined with maintenance experience. In the late 1960s, when L. A. Urban introduced gas path analysis, gas turbine diagnostics made a big breakthrough. Since then different methods have been developed and used in both aerospace and industrial applications. To date, a substantial number of papers have been published in this area. This paper intends to give a comprehensive review of performance-analysis-based methods available thus far for gas turbine fault diagnosis in the open literature.
Transient heat transfer characteristics of an energy recovery system using a porous medium
Authors: Nassab A. S Gandjalikhan.
Published: January 2003

The transient heat transfer characteristics of an obstructing porous layer and its significant role as an effective radiative converter are analysed in this work. A basic high-temperature flow system is considered in which hot non-radiating gas flows through a homogeneous porous medium. The porous medium, in addition to its convective heat exchange with the gas, may absorb, emit, and scatter thermal radiation. It is desirable to have a large amount of radiative heat flux from the porous layer in the upstream direction (toward the system). Convection and radiation are assumed to take place simultaneously in the porous medium. In order to analyse the thermal characteristics of the porous layer, the coupled energy equations for the gas and porous medium based on a two-flux radiation model are solved numerically. The numerical simulation of the heat transfer process in the system is provide by utilization of the Crank-Nicolson method. It is found that in the initial period, the porous layer acts as an almost perfect insulator and operates efficiently to recover the energy that would otherwise be wasted. In the limited case of no scattering, the present results are compared with the reported theoretical results and also with the experimental data and good agreement is found.
Fundamental thermodynamics of fuel cell, engine, and combined heat and power system efficiencies
Authors: Barclay F J
Published: January 2003

At the 2001 Grove Symposium on Fuel Cells attended by representatives of the world-wide fuel cell industry, fuel cells and fuel cells integrated with gas turbines were discussed. Combined heat and power (CHP) aspects were also discussed. Without exception, efficiency figures were irrationally based on the measured, Carnot-limited, lower calorific value (CV) of the fuel in energy units J. The rational basis, on the other hand, is the fuel chemical exergy (work units, Ws) calculated via an equilibrium diagram. In Joules experiment 1 Ws ≫ 1 J, where the irreversible ≫ must not become an = sign. A misnomer like potential energy (exergy) fits on the left of the diagram, CV on the right. Moreover credence was also given, at the symposium, to the popular misconception that a CHP system has an efficiency of the order of 80 per cent.

An essential precursor to reading the paper is to grasp the theory of chemical equilibrium and equilibrium constants [3] as an aid to exploring the equilibrium diagram mentioned above.
Analysis of the steady-state performance of a multi-plunger hydraulic pump
Authors: Dasgupta K , Mondal S K
Published: January 2003

In this article the steady-state performance of a multi-plunger low-flow high-pressure hydraulic pump has been studied. The variation of the chamber volume with shaft rotation, along with the various flow and torque losses, demands a structured approach to arrive at its mathematical model. A bondgraph simulation technique has been used to model the system, where various losses are lumped in suitable resistive elements. Using the standard procedure, experiments have been conducted for a wide range of operating conditions. The trends of the various loss coefficients are identified. The performance of the pump has been experimentally verified.
Fuel cell power system applications in Japan
Authors: Watanabe T
Published: January 2003

Many types of fuel cells such as the PAFC (phosphoric acid fuel cell), MCFC (molten carbonate fuel cell), SOFC (solid oxide fuel cell) and PEFC (polymer electrolyte fuel cell) have been developed for utility use in Japan. Among them, the PAFC is now in the ‘plant’ stage. Several MW class large-capacity demonstration plant and many smaller-capacity units have been constructed and tested for the purpose of evaluation on their efficiency, operability and durability. Those MW class plants are expected to be used as an urban energy cenlre or a cogeneration system. Smaller units arc installed for many kinds of applications such as in hospitals, hotels, restaurants, etc. Efficient performance of fuel cell power plant has been demonstrated by those PAFC plants and units. In addition, the possibility of higher efficiency is indicated by other fuel cells in their stack level, and MCFC plant is under construction to demonstrate its performance. In the next stages, it will be required to demonstrate superior durability and cost perspective in comparison with a conventional power generation system.
A hybrid gas turbine cycle (Brayton/Ericsson): An alternative to conventional combined gas and steam turbine power plant
Authors: Frost T H , Anderson A , Agnew B
Published: January 2003

A hybrid gas turbine cycle is proposed based on the conventional Brayton cycle for the high-temperature heat addition process while adopting the Ericsson cycle for the low-temperature heat rejection process. It thus incorporates the thermodynamic advantages of a combined gas and steam turbine (CCGT) cycle without the irrevcrsibilities of the boiler and the ancillarics of the steam turbine/condenser plant. Thermodynamic analysis shows that a similar overall thermal efficiency as current CCGT plant (i.e. 0.54) would be achieved at a maximum gas temperature of 1311 °C if polytropic efficiencies of 0.90 for compression and expansion could be realized and if a maximum temperature of 77 °C was obtained during isothermal compression in the bottoming Ericsson cycle.

A novel method of achieving multistage isothermal compression using heat pipe technology is proposed.
Simulation of combustion processes in gas-fuelled diesel engines
Authors: Liu Z , Karim G A
Published: January 2003

A thermodynamic multizone model incorporating detailed kinetic schemes has been developed for the prediction of the combustion processes in dual-fuel engines and some of their performance features. The consequences of the interaction between the gaseous and the diesel fuels and the resulting modification to the combustion processes are considered. A reacting zone has been incorporated in the model to describe the partial oxidation of the gaseous fuel-air mixture. The associated formation and concentrations of exhaust emissions are correspondingly established. The model can predict the onset of knock as well as the operating features and emissions for the more demanding case of light load performance. Predicted values for methane operation show good agreement with corresponding experimental values.
A two-dimensional Navier—Stokes inverse solver for compressor and turbine blade design
Authors: Demeulenaere A , Léonard O , Van den Braembussche R
Published: January 2003

A two-dimensional viscous inverse method for the design of compressor and turbine blades is presented. It iteratively modifies an initial geometry until a prescribed pressure distribution is reached on the blade surface.

The method solves the time-dependent Navier—Stokes equations in a numerical domain of which some boundaries (the blade walls) move during the transient part of the computation. The geometry modification algorithm is based on the transpiration principle: a normal velocity distribution is computed from the difference between the actual and prescribed pressure distributions, and is used to modify the blade shape. A time iteration is then performed on this new blade shape, taking into account the grid movement during the time stepping.

A two-dimensional upwind finite-volume Navier—Stokes solver has been developed. The multiblock strategy allows for a selective concentration of the discretization points in the zones of higher gradients. Applications to turbine and compressor blade design illustrate the accuracy of the flow computation, the capabilities and efficiency of the inverse method.
On efficiency measurements for large steam turbine low-pressure stages
Authors: Krzyżanowski J
Published: January 2003

Current trends in the development of power engineering are turning attention towards modernization of the machinery and equipment. Modernization of the LP (low-pressure) part of a turbine is one of the most promising solutions. A natural question which arises concerns the effectiveness of such modernization, i.e. a comparison of the characteristics (efficiency) of this turbine part before and after modernization. Such a comparison may be based on measurements of the efficiency of the LP part and the last stage before and after the modernization, provided that the efficiency gain achieved exceeds the measuring error. This paper deals with the estimation of the accuracy of such a measurement, the principle and methodology of which have been developed at the Institute of Fluid-Flow Machinery (IFFM) for a real turbine.
The effect of gas turbine coolant modulation on the part load performance of combined cycle plants. Part 1: Gas turbines
Authors: Kim T S , Ro S T
Published: January 2003

This paper demonstrates a favourable influence of turbine coolant modulation on the part load performance of gas turbines. A general simulation programme is developed, which is capable of accurately estimating the design and part load performance of modern heavy-duty gas turbines characterized by intensive turbine blade cooling Investigations are made for a typical gas turbine and two distinct load control schemes are considered: the fuel-only control and the variable compressor geometry control. Maintaining blade temperatures as high as possible whose purpose is to minimize coolant consumption is simulated. It is found that the coolant modulation makes the part load characteristics deviate from usual behaviours and creates a considerable enhancement of part load thermal efficiency. For the fuel-only control with coolant modulation, it is predicted that efficiency can be higher than design efficiency over a wide range of part load operation.
The effect of gas turbine coolant modulation on the part load performance of combined cycle plants. Part 2: Combined cycle plant
Authors: Kim T S , Ro S T
Published: January 2003

For combined cycle plants that consist of heavy-duty gas turbine and single-pressure heat recovery steam generator, the effect of gas turbine coolant modulation on plant performance is analysed. Two distinct schemes for gas turbine load control are adopted (the fuel-only control and the variable compressor geometry control), based on the gas turbine calculation in Part 1 of this series of papers. Models for heat recovery steam generator and steam turbine are combined with the gas turbine models of Part 1 to result in a complete analysis routine for combined cycles. The purpose of gas turbine coolant modulation is to minimize coolant consumption by maintaining the turbine blade temperatures as high as possible. It is found that the coolant modulation leads to reduction in heat recovery capacity, which decreases steam cycle power. However, since the benefit of coolant modulation for the gas turbine cycle is large enough, the combine cycle efficiency is improved.
Experimental performance evaluation of a 99.0 mm radial inflow nozzled turbine with different stator throat areas
Authors: Spence S W T , Artt D W
Published: January 2003

The performance of a single-stage, radial inflow turbine was measured with seven vaned stators of different throat areas. The turbine test facility is described and details of the turbine rotor and stator geometry are provided. Efficiency, mass flowrates and rotor static pressure ratios are presented for each turbine configuration at a range of speeds and pressure ratios. The experimental data clearly shows the turbine mass flowrate increasing approximately in proportion to the stator throat area, while the maximum attainable efficiency decreased as the stator throat area decreased.
Knowledge Spillovers and Inequality
Authors: Eeckhout Jan , Jovanovic Boyan
Published: January 2003

We develop a dynamic model with knowledge spillovers in production. The model contains two opposing forces. Imitation of other firms helps followers catch up with leaders, but the prospect of doing so makes followers want to free ride. The second force dominates and creates permanent inequality. We show that the greater are the average spillovers and the easier they are to obtain, the greater is the free-riding and inequality. More directed copying raises inequality by raising the free-riding advantages of hanging back. Using Compustat and patent-citation data we find that copying is highly undirected.
Economic Status and Health in Childhood: The Origins of the Gradient
Authors: Case Anne , Lubotsky Darren , Paxson Christina
Published: January 2003

The well-known positive association between health and income in adulthood has antecedents in childhood. Not only is children's health positively related to household income, but the relationship between household income and children's health becomes more pronounced as children age. Part of the relationship can be explained by the arrival and impact of chronic conditions. Children from lower-income households with chronic conditions have worse health than do those from higher-income households. The adverse health effects of lower income accumulate over children's lives. Part of the intergenerational transmission of socioeconomic status may work through the impact of parents' income on children's health.
Airport Congestion When Carriers Have Market Power
Authors: Brueckner Jan K
Published: January 2003

This paper analyzes airport congestion when carriers are nonatomistic, showing how the results of the road-pricing literature are modified when the economic agents causing congestion have market power. The analysis shows that when an airport is dominated by a monopolist, congestion is fully internalized, yielding no role for congestion pricing under monopoly conditions. Under a Cournot oligopoly, however, carriers are shown to internalize only the congestion they impose on themselves. A toll that captures the uninternalized portion of congestion may then improve the allocation of traffic. The analysis is supported by some rudimentary empirical evidence.
Transport Costs and the Geography of Arbitrage in Eighteenth-Century China
Authors: Shiue Carol H
Published: January 2003

Trade has been considered a condition for growth and development, a view that might have merits in explaining the rise of the Western world. I use a new data set from archival sources of eighteenth-century China to revisit this question. This analysis suggests previous studies of market integration, which attribute much growth to a reduction in transport costs, have overestimated these effects. I find the overall level of market integration in China was higher than previously thought, and, intertemporal effects are important substitutes for trade. Both factors reduce the importance of trade as a unique explanation for subsequent growth.
Two-Class Voting: A Mechanism for Conflict Resolution
Authors: Maug Ernst , Yilmaz Bilge
Published: January 2003

We discuss two-class voting procedures where voters are divided into classes and a separate majority is required in each class. Examples include Chapter 11 bankruptcy proceedings and some political mechanisms. We investigate how voting mechanisms aggregate information dispersed among voters when voters have conflicts of interests as well as different information regarding a proposal. We find that two-class voting provides a significant improvement over one-class voting in all situations where voters have significant conflicts of interests, and where the voters are relatively evenly divided between interest groups. However, two-class voting is inefficient absent conflicts of interests.
Social Value of Public Information
Authors: Morris Stephen , Shin Hyun Song
Published: January 2003

What are the welfare effects of enhanced dissemination of public information through the media and disclosures by market participants with high public visibility? We examine the impact of public information in a setting where agents take actions appropriate to the underlying fundamentals, but they also have a coordination motive arising from a strategic complementarity in their actions. When the agents have no socially valuable private information, greater provision of public information always increases welfare. However, when agents also have access to independent sources of information, the welfare effect of increased public disclosures is ambiguous.
Vouchers for Private Schooling in Colombia: Evidence from a Randomized Natural Experiment
Authors: Angrist Joshua , Bettinger Eric , Bloom Erik , King Elizabeth , Kremer Michael
Published: January 2003

Colombia used lotteries to distribute vouchers which partially covered the cost of private secondary school for students who maintained satisfactory academic progress. Three years after the lotteries, winners were about 10 percentage points more likely to have finished 8th grade, primarily because they were less likely to repeat grades, and scored 0.2 standard deviations higher on achievement tests. There is some evidence that winners worked less than losers and were less likely to marry or cohabit as teenagers. Benefits to participants likely exceeded the $24 per winner additional cost to the government of supplying vouchers instead of public-school places.
School Vouchers: A Critical View
Authors: Ladd Helen F
Published: January 2003

This paper marshals available evidence from both the U.S. and other countries on the effects of private schools, peer effects, and competition to demonstrate that that any gains in overall student achievement from a large scale voucher program are at best likely to be small. Moreover, given the tendency of parents to judge schools in part by the characteristics of a school's students, a universal voucher system would undoubtedly harm large numbers of disadvantaged students. Although the case for a small means tested voucher program is somewhat stronger, it will do little to improve education for low-performing students.
Pharmaceuticals in U.S. Health Care: Determinants of Quantity and Price
Authors: Berndt Ernst R
Published: January 2003

The U.S. pharmaceutical industry has again become the focus of considerable controversy. In understanding the economics underlying this industry, distinctions between short, medium and long-run costs are critical, as is that between economic and accounting costs. Consumers' heterogeneous valuations create strong incentives for non-uniform pricing and targeted marketing. The conflict between static efficiency (price new drugs low, near short-run marginal cost) and dynamic efficiency (price new drugs high, maintain incentives for innovation) is deep and enduring. This trade-off is becoming more severe as the relative costs of bringing new drugs to market have increased sharply.
The Promise and Pitfalls of Using Imprecise School Accountability Measures
Authors: Kane Thomas J , Staiger Douglas O
Published: January 2003

In recent years, most states have constructed elaborate accountability systems using school-level test scores. However, because the median elementary school contains only 69 children per grade level, such measures are quite imprecise. We evaluate the implications for school accountability systems. For instance, rewards or sanctions for schools with scores at either extreme primarily affect small schools and provide weak incentives to large ones. Nevertheless, we conclude that accountability systems may be worthwhile. Even in states with aggressive financial incentives, the marginal reward to schools for raising student performance is a small fraction of the potential labor market value for students.
The NAIRU in Theory and Practice
Authors: Ball Laurence , Mankiw N. Gregory
Published: January 2003

This paper discusses the NAIRU—the non-accelerating inflation rate of unemployment. It first considers the role of the NAIRU concept in business cycle theory, arguing that this concept is implicit in any model in which monetary policy influences both inflation and unemployment. The exact value of the NAIRU is hard to measure, however, in part because it changes over time. The paper then discusses why the NAIRU changes and, in particular, why it fell in the United States during the 1990s. The most promising hypothesis is that the decline in the NAIRU is attributable to the acceleration in productivity growth.
Comments: Pricing and Cost of Electronics Journals
Authors: Dirkmaat Joop , Kohn Robert E , Bergstrom Theodore C
Published: January 2003

To be considered for publication in the Comments section, letters should be relatively short—generally fewer than 1000 words—and should be sent to the journal offices at the address appearing inside the front cover. The editors will choose which letters will be published. All published letters will be subject to editing for style and length. In what follows, Joop Dirkmaat of Elsevier Science and Robert E. Kohn of Southern Illinois University at Edwardsville comment on Theodore C. Bergstrom's article, “Free Labor for Costly Journals,” from the Fall 2001 issue. Bergstrom then responds.
Paediatric day-stay myringoplasty: a review of 74 consecutive cases
Authors: Ryan C. , Harris R. , Hung T. , Knight J.
Published: January 2003

Day-case surgery is particularly attractive for children, allowing post-operative recovery in the safe environment of the family home. Myringoplasty using the traditional method of underlay temporalis fasia is usually performed as an in-patient. From 1995 to 2000, 74 myringoplasties were performed in a dedicated paediatric day surgery unit at the Mayday University Hospital. We have retrospectively reviewed the outcome results of these procedures and reported them here. Only three patients required admission overnight (four per cent) and six grafts failed (8.5 per cent) complying with accepted standards. This series suggests that day-case surgery is a safe and desirable practice for children undergoing myringoplasty. However, there should be the facility for admission if required.
Histological features in routine tonsillectomy specimens: the presence and the proportion of mesenchymal tissues and seromucinous glands
Authors: Erkiliç Suna , Aydin Abdullah , Koçer N. Emrah
Published: January 2003

Tonsillectomy is a frequently performed operation but there is little information about the histological features of a tonsillectomy specimen. In this study, we re-evaluated archival materials of 1220 cases who underwent tonsillectomy because of hyperplastic tonsils. Haematoxylin and eosin sections of the cases were re-examined and the presence and the proportion of mesenchymal tissues (skeletal muscle, cartilage, fat, bone) and seromucinous glands were noted as focal, multifocal or abundant. The incidence of skeletal muscle was 89 per cent (1085 cases; 206 focal, 465 multifocal, 414 abundant), seromucinous glands 35 per cent (429 cases; 236 focal, 134 multifocal, 59 abundant), fat 21 per cent (251 cases; 208 focal, 43 multifocal), cartilage three per cent (31 cases), and bone one per cent (seven cases). Also in 165 cases (14 per cent) skeletal muscle, in 12 cases (one per cent) was seromucinous glands, in eight cases (one per cent) was cartilage, in seven cases (one per cent) fat, and in four cases (less than one per cent) bone were found between hyperplastic lymphoid tissue. It can be concluded that mesenchymal tissues (skeletal muscle, cartilage, fat, bone) and seromucinous glands may be seen in different proportions in routine tonsillectomy specimens.
Effectiveness of treatment of tonsillopharyngitis: comparative study
Authors: Abdul-Baqi Khader J. , Shakhatreh Farouk M. N.
Published: January 2003

This study was performed to find out the effectiveness of three different treatments of patients having acute tonsillopharyngitis. Three groups of patients were selected (50 patients in each group). Group I received one antibiotic according to the result of culture and sensitivity test, while groups II and III received two antibiotics or one antibiotic respectively without a culture and sensitivity test. Diagnosis was based on the acutely subjective symptoms of sore throat and positive clinical examination findings. The effectiveness of treatment in group I (96 per cent) was statistically significant (p<0.05) and higher than group II (82 per cent) and group III (74 per cent), while the difference between group II and III was not statistically significant (p>0.05).

Our findings demonstrate that a culture and sensitivity test of the throat swab is necessary before treating acute tonsillopharyngitis. It was suggested that there should be appropriate selection of antibiotics to preserve normal flora, avoidance antibiotics for viral infections, patient compliance with prescription and educational programmes directed at patients and physicians. All of these measures would assist in reducing the volume of antibiotic use, bacterial resistance and the overall cost of treatment of tonsillopharyngitis.